Why doesn't Python remember the initial directory?
As far as I've been able to determine, Python does not remember (immutably, that is) the working directory at the program's start-up, or, if it does, it does not officially expose this information. Does anyone know why this is? Is there a PEP stating the rationale for it? Thanks! -- http://mail.python.org/mailman/listinfo/python-list
Re: Why doesn't Python remember the initial directory?
In roy-ca6d77.17031119082...@news.panix.com Roy Smith r...@panix.com writes:
In article k0rj38$2gc$1...@reader1.panix.com, kj no.em...@please.post
wrote:
As far as I've been able to determine, Python does not remember
(immutably, that is) the working directory at the program's start-up,
or, if it does, it does not officially expose this information.
Why would you expect that it would? What would it (or you) do with this
information?
More to the point, doing a chdir() is not something any library code
would do (at least not that I'm aware of), so if the directory changed,
it's because some application code did it. In which case, you could
have just stored the working directory yourself.
This means that no library code can ever count on, for example,
being able to reliably find the path to the file that contains the
definition of __main__. That's a weakness, IMO. One manifestation
of this weakness is that os.chdir breaks inspect.getmodule, at
least on Unix. If you have some Unix system handy, you can try
the following. First change the argument to os.chdir below to some
valid directory other than your working directory. Then, run the
script, making sure that you refer to it using a relative path.
When I do this on my system (OS X + Python 2.7.3), the script bombs
at the last print statement, because the second call to inspect.getmodule
(though not the first one) returns None.
import inspect
import os
frame = inspect.currentframe()
print inspect.getmodule(frame).__name__
os.chdir('/some/other/directory') # where '/some/other/directory' is
# different from the initial directory
print inspect.getmodule(frame).__name__
...
% python demo.py
python demo.py
__main__
Traceback (most recent call last):
File demo.py, line 11, in module
print inspect.getmodule(frame).__name__
AttributeError: 'NoneType' object has no attribute '__name__'
I don't know of any way to fix inspect.getmodule that does not
involve, directly or indirectly, keeping a stable record of the
starting directory.
But, who am I kidding? What needs fixing, right? That's not a
bug, that's a feature! Etc.
By now I have learned to expect that 99.99% of Python programmers
will find that there's nothing wrong with behavior like the one
described above, that it is in fact exactly As It Should Be, because,
you see, since Python is the epitome of perfection, it follows
inexorably that any flaw or shortcoming one may *perceive* in Python
is only an *illusion*: the flaw or shortcoming is really in the
benighted programmer, for having stupid ideas about programming
(i.e. any idea that may entail that Python is not *gasp* perfect).
Pardon my cynicism, but the general vibe from the replies I've
gotten to my post (i.e. if Python ain't got it, it means you don't
need it) is entirely in line with these expectations.
--
http://mail.python.org/mailman/listinfo/python-list
How to get initial absolute working dir reliably?
What's the most reliable way for module code to determine the absolute path of the working directory at the start of execution? (By module code I mean code that lives in a file that is not meant to be run as a script, but rather it is meant to be loaded as the result of some import statement. In other words, module code is code that must operate under the assumption that it can be loaded at any time after the start of execution.) Functions like os.path.abspath produce wrong results if the working directory is changed, e.g. through os.chdir, so it is not terribly reliable for determining the initial working directory. Basically, I'm looking for a read-only variable (or variables) initialized by Python at the start of execution, and from which the initial working directory may be read or computed. Thanks! -- http://mail.python.org/mailman/listinfo/python-list
Re: Official reason for omitting inspect.currentcallable() ?
In mailman.3221.1344847903.4697.python-l...@python.org Chris Angelico ros...@gmail.com writes: I'm not familiar with it by that name, but Pike's this_function is what the OP's describing. You got it. It's a useful construct in theory when you want to write in recursion, which was part of the rationale behind PEP 3130 Thank you! kj -- http://mail.python.org/mailman/listinfo/python-list
Official reason for omitting inspect.currentcallable() ?
Is there an *explicitly stated* reason (e.g. in a PEP, or in some python dev list message) for why the inspect module (at least for Python 2.7) does not include anything like a currentcallable() function that would *stably*[1] return the currently executing callable object? (It seems unlikely that the absence in the inspect module of anything even remotely like such a currentcallable is merely an oversight, considering how many introspection facilities the inspect module provides. It seems far more likely that this absence is either due to some fundamental limitation of Python that makes it impossible to fully specify such a function, or it is the result of a deliberate policy against including such a function in inspect.) Thanks! [1] By stably above I mean, e.g., that the value returned by the top-level function (object) defined by def spam(): return inspect.currentcallable() is *invariant*, in contrast to the value returned by the top-level function (object) defined by def ham(): return ham which is whatever the current value of the 'ham' global happens to be. -- http://mail.python.org/mailman/listinfo/python-list
sick of distribute, setup, and all the rest...
it's an all-out disgrace. when is python going to get a decent module distribution system??? and don't tell me to do it myself: it's clear that the sorry situation we have now is precisely that too many programmers without the requisite expertise or policy-making authority have decided to pitch in. This is something for GvR and his top Python core library team to do, because the problems are as much policy and institutional ones as they are technical (programming) ones. -- http://mail.python.org/mailman/listinfo/python-list
Java is killing me! (AKA: Java for Pythonheads?)
*Please* forgive me for asking a Java question in a Python forum. My only excuse for this no-no is that a Python forum is more likely than a Java one to have among its readers those who have had to deal with the same problems I'm wrestling with. Due to my job, I have to port some Python code to Java, and write tests for the ported code. (Yes, I've considered finding myself another job, but this is not an option in the immediate future.) What's giving me the hardest time is that the original Python code uses a lot of functions with optional arguments (as is natural to do in Python). As far as I can tell (admittedly I'm no Java expert, and have not programmed in it since 2001), to implement a Java method with n optional arguments, one needs at least 2**n method definitions. Even if all but one of these definitions are simple wrappers that call the one that does all the work, it's still a lot of code to wade through, for nothing. That's bad enough, but even worse is writing the unit tests for the resulting mountain of fluffCode. I find myself writing test classes whose constructors also require 2**n definitions, one for each form of the function to be tested... I ask myself, how does the journeyman Python programmer cope with such nonsense? For the sake of concreteness, consider the following run-of-the-mill Python function of 3 arguments (the first argument, xs, is expected to be either a float or a sequence of floats; the second and third arguments, an int and a float, are optional): def quant(xs, nlevels=MAXN, xlim=MAXX): if not hasattr(xs, '__iter__'): return spam((xs,), n, xlim)[0] if _bad_quant_args(xs, nlevels, xlim): raise TypeError(invalid arguments) retval = [] for x in xs: # ... # elaborate acrobatics that set y # ... retval.append(y) return retval My Java implementation of it already requires at least 8 method definitions, with signatures: short[] quant (float[], int, float) short[] quant (float[], int ) short[] quant (float[], float) short[] quant (float[]) short quant (float , int, float) short quant (float , int ) short quant (float , float) short quant (float ) Actually, for additional reasons, too arcane to go into, I also need four more: short quant (Float , Integer, Float) short quant (Float , Integer ) short quant (Float , Float) short quant (Float ) Writing JUnit tests for these methods is literally driving me INSANE. Some advice on implementing and testing functions with optional arguments in Java would be appreciated. TIA! kj -- http://mail.python.org/mailman/listinfo/python-list
Python app dev tools for Gnome?
There's a zillion utility apps that I've had kicking around in my head for years, but I've never implemented because I absolutely hate GUI programming. But I'm increasingly impressed by the quality, stability, and sheer number, of Gnome apps that I keep coming across that use Python under the hood. This gives me hope that maybe programming GUI Python apps for Gnome these days is no longer the traumatizing experience it used to be when I last tried it. Can someone recommend some good tools to speed up the development of Python apps[1] for Gnome? E.g. is there anything like Xcode for Gnome+Python? TIA! ~kj [1] Needless to say, when I write apps I mean full-blown GUI apps: windows, menus, events, threads, clickable icon, the whole ball of wax. As opposed to cli apps, panel widgets, etc. -- http://mail.python.org/mailman/listinfo/python-list
Re: __delitem__ feature
In 4d181afb$0$30001$c3e8da3$54964...@news.astraweb.com Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes: We know it because it explains the observable facts. So does Monday-night quarterbacking... -- http://mail.python.org/mailman/listinfo/python-list
__delitem__ feature
When I execute this file:
#--
def nodelfactory(klass):
class nodel(klass):
def _delitem(self, _):
raise TypeError(can't delete)
# __delitem__ = _delitem
def __init__(self, *a, **k):
klass.__init__(self, *a, **k)
self.__delitem__ = self._delitem
nodel.__name__ = 'nodel%s' % klass.__name__
return nodel
if __name__ == '__main__':
import traceback as tb
d = nodelfactory(dict)([('k1', 'v1'), ('k2', 'v2')])
try: d.__delitem__('k1')
except TypeError: tb.print_exc()
print d
try: del d['k1']
except TypeError: tb.print_exc()
print d
l = nodelfactory(list)([1, 2, 3, 4])
try: l.__delitem__(0)
except TypeError: tb.print_exc()
print l
try: del l[0]
except TypeError: tb.print_exc()
print l
#--
...the output I get is:
Traceback (most recent call last):
File /tmp/delbug.py, line 20, in module
try: d.__delitem__('k1')
File /tmp/delbug.py, line 4, in _delitem
raise TypeError(can't delete)
TypeError: can't delete
{'k2': 'v2', 'k1': 'v1'}
{'k2': 'v2'}
Traceback (most recent call last):
File /tmp/delbug.py, line 30, in module
try: l.__delitem__(0)
File /tmp/delbug.py, line 4, in _delitem
raise TypeError(can't delete)
TypeError: can't delete
[1, 2, 3, 4]
[2, 3, 4]
It means that, for both subclasses, del fails to trigger the
dynamically installed instance method __delitem__.
If I replace dict with UserDict, *both* deletion attempts lead to
a call to the dynamic __delitem__ method, and are thus blocked.
This is the behavior I expected of dict (and will help me hold on
to my belief that I'm not going insane when inevitably I'm told
that there's no bug in dict or list).
Interestingly enough, if I replace list with UserList, I see no
change in behavior. So maybe I am going insane after all.
~kj
P.S. If you uncomment the commented-out line, and comment out the
last line of the __init__ method (which installs self._delitem as
self.__delitem__) then *all* the deletion attempts invoke the
__delitem__ method, and are therefore blocked. FWIW.
--
http://mail.python.org/mailman/listinfo/python-list
Re: type(d) != type(d.copy()) when type(d).issubclass(dict)
John O'Hagan resea...@johnohagan.com writes: IMO one of the benefits of subclassing is that you can just bolt on additional behaviour without having to know all the inner workings of the superclass, a benefit that is somewhat defeated by this behaviour of builtins. I agree. I've read the old post/articles by GvR and other over how great it will be now that one can subclass Python builtin types like any other class (GvR even gives explicit examples of this luscious possibility in his paper on type/class unification). But now I'm discovering so many caveats, exceptions, and gotchas about subclassing builtins that I have to conclude that this much celebrated new capability is basically useless... Just like readability counts, it is also true that conceptual clarity counts, and treating builtins as classes in Python is the most obfuscated design I've ever seen. UserDict, come back, all is forgotten! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: type(d) != type(d.copy()) when type(d).issubclass(dict)
In xns9e59a27def178duncanbo...@127.0.0.1 Duncan Booth duncan.bo...@invalid.invalid writes: kj no.em...@please.post wrote: Watch this: class neodict(dict): pass ... d = neodict() type(d) class '__main__.neodict' type(d.copy()) type 'dict' Bug? Feature? Genius beyond the grasp of schlubs like me? Feature. In (almost?) all cases any objects constructed by a subclass of a builtin class will be of the original builtin class. What I *really* would like to know is: how do *you* know this (and the same question goes for the other responders who see this behavior of dict as par for the course). Can you show me where it is in the documentation? I'd really appreciate it. TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: __delitem__ feature
In mailman.302.1293387041.6505.python-l...@python.org Ian Kelly ian.g.ke...@gmail.com writes: On 12/26/2010 10:53 AM, kj wrote: P.S. If you uncomment the commented-out line, and comment out the last line of the __init__ method (which installs self._delitem as self.__delitem__) then *all* the deletion attempts invoke the __delitem__ method, and are therefore blocked. FWIW. Because subclasses of builtins only check the class __dict__ for special method overrides, not the instance __dict__. How do you know this? Is this documented? Or is this a case of Monday-night quarterbacking? ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: How to pop the interpreter's stack?
In mailman.280.1293287106.6505.python-l...@python Robert Kern robert.k...@gmail.com writes: Except that the *caller* never gets the traceback (unless if it deliberately inspects the stack for some metaprogramming reason). It gets the exception, and that is the same no matter what you do. The developer/user gets the traceback, and those implementation details *are* often important to them. Just look at what Python shows you if you pass the wrong number of arguments to a function: def spam(x, y, z): pass ... spam(1, 2) Traceback (most recent call last): File stdin, line 1, in module TypeError: spam() takes exactly 3 arguments (2 given) That's it. The traceback stops at the point of the error. Python doesn't show you all the underlying C-coded machinery that went into detecting the error and emitting the error message. *No one* needs this information at this point. All I'm saying is that I want to do the same thing with my argument validation code as Python does with its argument validation code: keep it out of sight. When my argument validation code fires an exception ***there's no bug in **my** code***. It's doing exactly what it's supposed to do. Therefore, there's no need for me to debug anything, and certainly no need for me to inspect the traceback all the way to the exception. The bug is in the code that called my function with the wrong arguments. The developer of that code has no more use for seeing the traceback all the way to where my code raises the exception than I have for seeing the traceback of Python's underlying C code when I get an error like the one shown above. ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: How to pop the interpreter's stack?
In mailman.301.1293383804.6505.python-l...@python.org Ethan Furman et...@stoneleaf.us writes: You failed to mention that cleverness is not a prime requisite of the python programmer -- in fact, it's usually frowned upon. That's the party line, anyway. I no longer believe it. I've been crashing against one bit of cleverness after another in Python's unification of types and classes... -- http://mail.python.org/mailman/listinfo/python-list
How can a function find the function that called it?
I want to implement a frozen and ordered dict. I thought I'd implement it as a subclass of collections.OrderedDict that prohibits all modifications to the dictionary after it has been initialized. In particular, calling this frozen subclass's update method should, in general, trigger an exception (object is not mutable). But OrderedDict's functionality *requires* that its __init__ be run, and this __init__, in turn, does part of its initialization by calling the update method. Therefore, the update method of the new subclass needs to be able to identify the calling function in order to make a special allowance for calls coming from OrderedDict.__init__. (Better yet, it should be able to allow calls coming from its own class's __init__, via OrderedDict.__init__.) The best I've been able to do is to use inspect to get the name of the calling function. For the case I'm trying to identify, this name is simply __init__. But Python code is awash in __init__'s... Is it possible to achieve a more precise identification? Specifically, I want to know the *class* (not the file) where this '__init__' is defined. (BTW, I don't understand why inspect doesn't provide something as basic as the *class* that the method belongs to, whenever applicable. I imagine there's a good reason for this coyness, but I can't figure it out.) TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: How can a function find the function that called it?
In mailman.272.1293215190.6505.python-l...@python.org Daniel Urban urban.d...@gmail.com writes: On Fri, Dec 24, 2010 at 17:24, kj no.em...@please.post wrote: (BTW, I don't understand why inspect doesn't provide something as basic as the *class* that the method belongs to, whenever applicable. I imagine there's a good reason for this coyness, but I can't figure it out.) One function object can belong to (be in the namespace of) more than one class, so there is no the class. There are many other properties that inspect reports on (e.g. filename) that may not apply to an individual case. For 99.9% of methods, the class in which it was lexically defined would be good enough. -- http://mail.python.org/mailman/listinfo/python-list
type(d) != type(d.copy()) when type(d).issubclass(dict)
Watch this: class neodict(dict): pass ... d = neodict() type(d) class '__main__.neodict' type(d.copy()) type 'dict' Bug? Feature? Genius beyond the grasp of schlubs like me? ~kj -- http://mail.python.org/mailman/listinfo/python-list
How to order base classes?
Suppose that I want to write a subclass C of base classes A and B. What considerations should go into choosing the ordering of A and B in C's base class list? Since any order one chooses can be overridden on a per-method basis, by assigning the desired parent's method to the appropriate class attribute, like this: class C(B, A) # override methods spam, ham, and eggs from B spam = A.spam; ham = A.ham; eggs = A.eggs; ... ...it is difficult for me to see a strong compelling reason for picking an ordering over another. But may be just ignorance on my part. How should one go about deciding the ordering of base classes? TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: How to order base classes?
In 4d14209d$0$3$c3e8da3$54964...@news.astraweb.com Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes: The question you ask can only be answered in reference to a specific class with specific methods. There is no general principle, it depends entirely on the problem being solved. Thanks! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: Partition Recursive
In be6f45b2-5a5a-4461-b5de-000487a46...@w2g2000yqb.googlegroups.com macm
moura.ma...@gmail.com writes:
url =
'http://docs.python.org/dev/library/stdtypes.html?highlight=partition#str.partition'
So I want convert to
myList =
['http',':','//','docs','.','python','.','org','/','dev','/','library','/','stdtypes','.','html','?','highlight','=','partition','#','str','.','partition']
The reserved char are:
specialMeaning = [//,;,/, ?, :, @, = , ,#]
You forgot '.'.
import re # sorry
sp = re.compile('(//?|[;?:@=#.])')
filter(len, sp.split(url))
['http', ':', '//', 'docs', '.', 'python', '.', 'org', '/', 'dev', '/',
'library', '/', 'stdtypes', '.', 'html', '\
?', 'highlight', '=', 'partition', '#', 'str', '.', 'partition']
~kj
--
http://mail.python.org/mailman/listinfo/python-list
issubclass(dict, Mapping)
In a message (4cf97c94$0$30003$c3e8da3$54964...@news.astraweb.com) on a different thread, Steven D'Aprano tells me: I suspect you're trying to make this more complicated than it actually is. You keep finding little corner cases that expose implementation details (such as the heap-types issue above) and leaping to the erroneous conclusion that because you didn't understand this tiny little corner of Python's class model, you didn't understand any of it. Python's object model is relatively simple, but it does occasionally expose a few messy corners. I disagree with your assessment. What you call little corner cases I call fundamental, as in you can't really call yourself competent with Python if you're ignorant about them. To use a term I first saw in an article by Joel Spolsky (http://is.gd/je42O), Python's object model is a rather leaky abstraction. This refers to the situation in which a user is not shielded from the implementation details. When an abstraction leaks, implementation details are no longer negligible, they cease to be little corner cases. Here's another example, fresh from today's crop of wonders: (v. 2.7.0) from collections import Mapping issubclass(dict, Mapping) True dict.__bases__ (type 'object',) [issubclass(b, Mapping) for b in dict.__bases__] [False] So dict is a subclass of Mapping, even though none of the bases of dict is either Mapping or a subclass of Mapping. Great. I suspect this is another abstraction leak (dict is *supposed* to be a Python class like all others, but in fact it's not *really*. You see, once upon a time...). I conclude that, for me to understand Python's (rather leaky) object model abstraction, I have to understand its underlying implementation. Unfortunately, as far as I know, there's no other choice but to study the source code, since there's no other more readable description of this implementation. Maybe there are fewer abstraction leaks in 3.0... ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: How to pop the interpreter's stack?
In mailman.65.1292517591.6505.python-l...@python.org Robert Kern
robert.k...@gmail.com writes:
Obfuscating the location that an exception gets raised prevents a lot of
debugging...
The Python interpreter does a lot of that obfuscation already, and I
find the resulting tracebacks more useful for it.
An error message is only useful to a given audience if that audience
can use the information in the message to modify what they are
doing to avoid the error. It is of no use (certainly no *immediate*
use) to this audience to see tracebacks that go deep into code that
they don't know anything about and cannot change.
For example, consider this:
#-
def foo(x, **k): pass
def bar(*a, **k):
if len(a) 1: raise TypeError('too many args')
def baz(*a, **k): _pre_baz(*a, **k)
def _pre_baz(*a, **k):
if len(a) 1: raise TypeError('too many args')
if __name__ == '__main__':
from traceback import print_exc
try: foo(1, 2)
except: print_exc()
try: bar(1, 2)
except: print_exc()
try: baz(1, 2)
except: print_exc()
#-
(The code in the if __name__ == '__main__' section is meant to
simulate the general case in which the functions defined in this file
are called by third-party code.) When you run this code the output is
this (a few blank lines added for clarity):
Traceback (most recent call last):
File /tmp/ex2.py, line 5, in module
try: foo(1, 2)
TypeError: foo() takes exactly 1 argument (2 given)
Traceback (most recent call last):
File /tmp/ex2.py, line 7, in module
try: bar(1, 2)
File /tmp/example.py, line 4, in bar
if len(a) 1: raise TypeError('too many args')
TypeError: too many args
Traceback (most recent call last):
File /tmp/ex2.py, line 9, in module
try: baz(1, 2)
File /tmp/example.py, line 6, in baz
def baz(*a, **k): _pre_baz(*a, **k)
File /tmp/example.py, line 9, in _pre_baz
if len(a) 1: raise TypeError('too many args')
TypeError: too many args
In all cases, the *programming* errors are identical: functions called
with the wrong arguments. The traceback from foo(1, 2) tells me this
very clearly, and I'm glad that Python is not also giving me the
traceback down to where the underlying C code throws the exception: I
don't need to see all this machinery.
In contrast, the tracebacks from bar(1, 2) and baz(1, 2) obscure the
fundamental problem with useless detail. From the point of view of
the programmer that is using these functions, it is of no use to know
that the error resulted from some raise TypeError statement
somewhere, let alone that this happened in some obscure, private
function _pre_baz.
Perhaps I should have made it clearer in my original post that the
tracebacks I want to clean up are those from exceptions *explicitly*
raised by my argument-validating helper function, analogous to
_pre_baz above. I.e. I want that when my spam function is called
(by code written by someone else) with the wrong arguments, the
traceback looks more like this
Traceback (most recent call last):
File /some/python/code.py, line 123, in module
spam(some, bad, args)
TypeError: the second argument is bad
than like this:
Traceback (most recent call last):
File /some/python/code.py, line 123, in module
spam(some, bad, args)
File /my/niftymodule.py, line 456, in niftymodule
_pre_spam(*a, **k)
File /my/niftymodule.py, line 789, in __pre_spam
raise TypeError('second argument to spam is bad')
TypeError: the second argument is bad
In my opinion, the idea that more is always better in a traceback
is flat out wrong. As the example above illustrates, the most
useful traceback is the one that stops at the deepest point where
the *intended audience* for the traceback can take action, and goes
no further. The intended audience for the errors generated by my
argument-checking functions should see no further than the point
where they called a function incorrectly.
~kj
--
http://mail.python.org/mailman/listinfo/python-list
Re: How to pop the interpreter's stack?
In 1f47c36d-a509-4d05-ba79-62b4a534b...@j19g2000prh.googlegroups.com Carl Banks pavlovevide...@gmail.com writes: On Dec 22, 8:52=A0am, kj no.em...@please.post wrote: In mailman.65.1292517591.6505.python-l...@python.org Robert Kern rober= t.k...@gmail.com writes: Obfuscating the location that an exception gets raised prevents a lot of debugging... The Python interpreter does a lot of that obfuscation already, and I find the resulting tracebacks more useful for it. An error message is only useful to a given audience if that audience can use the information in the message to modify what they are doing to avoid the error. =A0It is of no use (certainly no *immediate* use) to this audience to see tracebacks that go deep into code that they don't know anything about and cannot change. So when the audience files a bug report it's not useful for them to include the whole traceback? Learn to read, buster. I wrote *immediate* use. ~kj -- http://mail.python.org/mailman/listinfo/python-list
general problem when subclassing a built-in class
Suppose that you want to implement a subclass of built-in class, to
meet some specific design requirements.
Where in the Python documentation can one find the information
required to determine the minimal[1] set of methods that one would
need to override to achieve this goal?
In my experience, educated guesswork doesn't get one very far with
this question.
Here's a *toy example*, just to illustrate this last point.
Suppose that one wants to implement a subclass of dict, call it
TSDict, to meet these two design requirements:
1. for each one of its keys, an instance of TSDict should keep a
timestamp (as given by time.time, and accessible via the new method
get_timestamp(key)) of the last time that the key had a value
assigned to it;
2. other than the added capability described in (1), an instance
of TSDict should behave *exactly* like a built-in dictionary.
In particular, we should be able to observe behavior like this:
d = TSDict((('uno', 1), ('dos', 2)), tres=3, cuatro=4)
d['cinco'] = 5
d
{'cuatro': 4, 'dos': 2, 'tres': 3, 'cinco': 5, 'uno': 1}
d.get_timestamp('uno')
1293046586.644436
OK, here's one strategy, right out of OOP 101:
#---
from time import time
class TSDict(dict):
def __setitem__(self, key, value):
# save the value and timestamp for key as a tuple;
# see footnote [2]
dict.__setitem__(self, key, (value, time()))
def __getitem__(self, key):
# extract the value from the value-timestamp pair and return it
return dict.__getitem__(self, key)[0]
def get_timestamp(self, key):
# extract the timestamp from the value-timestamp pair and return it
return dict.__getitem__(self, key)[1]
#---
This implementation *should* work (again, at least according to
OOP 101), but, in fact, it doesn't come *even close*:
d = TSDict((('uno', 1), ('dos', 2)), tres=3, cuatro=4)
d['cinco'] = 5
d
{'cuatro': 4, 'dos': 2, 'tres': 3, 'cinco': (5, 1293059516.942985), 'uno': 1}
d.get_timestamp('uno')
Traceback (most recent call last):
File stdin, line 1, in module
File /tmp/tsdict.py, line 23, in get_timestamp
return dict.__getitem__(self, key)[1]
TypeError: 'int' object is not subscriptable
From the above you can see that TSDict fails at *both* of the design
requirements listed above: it fails to add a timestamp to all keys in
the dictionary (e.g. 'uno', ..., 'cuatro' didn't get a timestamp), and
get_timestamp bombs; and it also fails to behave in every other
respect exactly like a built-in dict (e.g., repr(d) reveals the
timestamps and how they are kept).
So back to the general problem: to implement a subclass of a built-in
class to meet a given set of design specifications. Where is the
documentation needed to do this without guesswork?
Given results like the one illustrated above, I can think of only two
approaches (other than scrapping the whole idea of subclassing a
built-in class in the first place):
1) Study the Python C source code for the built-in class in the hopes
of somehow figuring out what API methods need to be overridden;
2) Through blind trial-and-error, keep trying different implementation
strategies and/or keep overriding additional built-in class methods
until the behavior of the resulting subclass approximates
sufficiently the design specs.
IMO, both of these approaches suck. Approach (1) would take *me*
forever, since I don't know the first thing about Python's internals,
and even if I did, going behind the documented API like that would
make whatever I implement very likely to break with future releases of
Python. Approach (2) could also take a very long time (probably much
longer than the implementation would take if no guesswork was
involved), but worse than that, one would have little assurance that
one's experimentation has truly uncovered all the necessary details;
IME, programming-by-guesswork leads to numerous and often nasty bugs.
Is there any other way?
TIA!
~kj
[1] The minimal bit in the question statement is just another way of
specifying a maximal reuse of the built-in's class code.
[2] For this example, I've accessed the parent's methods directly
through dict rather than through super(TSDict, self), just to keep
the code as uncluttered as possible, but the results are the same
if one uses super.
--
http://mail.python.org/mailman/listinfo/python-list
Re: issubclass(dict, Mapping)
In 4d127d5e$0$29997$c3e8da3$54964...@news.astraweb.com Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes: On Wed, 22 Dec 2010 14:20:51 +, kj wrote: Here's another example, fresh from today's crop of wonders: (v. 2.7.0) from collections import Mapping issubclass(dict, Mapping) True dict.__bases__ (type 'object',) [issubclass(b, Mapping) for b in dict.__bases__] [False] So dict is a subclass of Mapping, even though none of the bases of dict is either Mapping or a subclass of Mapping. Great. Yes. So what? That's being deliberately obtuse. The situation described goes smack against standard OOP semantics, which would be fine if all this stuff was documented clearly and reasonably, i.e. in one (preferably official) place rather than scattered over a bazillion separate documents, PEP this, module that, GvR musing #42, etc. Let's just say that I'm looking forward to the end to these surprises. ~kj -- http://mail.python.org/mailman/listinfo/python-list
How to pop the interpreter's stack?
Consider this code: def spam(*args, **kwargs): args, kwargs = __pre_spam(*args, **kwargs) # args kwargs are OK: proceed # ... def __pre_spam(*args, **kwargs): # validate args kwargs; # return canonicalized versions of args kwargs; # on failure, raise some *informative* exception # ... return canonicalized_args, canonicalized_kwargs I write functions like __pre_spam for one reason only: to remove clutter from a corresponding spam function that has a particularly complex argument-validation/canonicalization stage. In effect, spam outsources to __pre_spam the messy business of checking and conditioning its arguments. The one thing I don't like about this strategy is that the tracebacks of exceptions raised during the execution of __pre_spam include one unwanted stack level (namely, the one corresponding to __pre_spam itself). __pre_spam should be completely invisible and unobtrusive, as if it had been textually inlined into spam prior to the code's interpretation. And I want to achieve this without in any way cluttering spam with try/catches, decorators, and whatnot. (After all, the whole point of introducing __pre_spam is to declutter spam.) It occurs to me, in my innocence (since I don't know the first thing about the Python internals), that one way to achieve this would be to have __pre_spam trap any exceptions (with a try/catch around its entire body), and somehow pop its frame from the interpreter stack before re-raising the exception. (Or some clueful/non-oxymoronic version of this.) How feasible is this? And, if it is quite unfeasible, is there some other way to achieve the same overall design goals described above? TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Assigning to __class__ attribute
I have a couple of questions regarding assigning to an instance's __class__ attribute. The first is illustrated by the following interaction. First I define an empty class: class Spam(object): pass ... Now I define an instance of Spam and an instance of Spam's superclass: x = Spam() y = Spam.__mro__[1]() # (btw, is there a less uncouth way to do this???) [z.__class__.__name__ for z in x, y] ['Spam', 'object'] Now I define a second empty class: class Ham(object): pass ... Next, I attempt to assign the value Ham to x.__class__: x.__class__ = Ham [isinstance(x, z) for z in Spam, Ham] [False, True] This was the first surprise for me: assigning to the __class__ attribute not only isn't vetoed, but in fact changes the instances class: Oh-ky... First question: how kosher is this sort of class transmutation through assignment to __class__? I've never seen it done. Is this because it considered something to do only as a last resort, or is it simply because the technique is not needed often, but it is otherwise perfectly ok? The second, and much bigger, surprise comes when I attempt to do the same class-switching with y: y.__class__ = Ham Traceback (most recent call last): File stdin, line 1, in module TypeError: __class__ assignment: only for heap types (If you recall, y's class is object, the superclass of x.) Apparently Spam is a heap type (whatever that is) but its superclass, object, isn't. This definitely rattles my notions of inheritance: since the definition of Spam was empty, I didn't expect it to have any significant properties that are not already present in its superclass. What's going on here? Is this a bug, or a feature? I can see no logical justification for allowing such class switching for only some class and not others. One last question: as the questions above make clear, I have a colossal muddle inside my head regarding Python's model of classes and inheritance. This is not for lack of trying to understand it, but, rather, for exactly the opposite reason: in my zeal to gain the fullest understanding of this topic, I think I have read too much that is incorrect, or obsolete, or incomplete... What is the most complete, definitive, excruciatingly detailed exposition of Python's class and inheritance model? I'm expressly avoiding Google to answer this question, and instead asking it here, because I suspect that there's some connection between my state of utter confusion on this topic and the ease with which the complete/definitive/highest-quality information can get lost among a huge number of Google hits to popular-but-only-partially-correct sources of information. (In fact, I *may* have already read the source I'm seeking, but subsequent readings of incorrect stuff may have overwritten the correct information in my brain.) TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Comparing floats
I'm defining a class (Spam) of objects that are characterized by three parameters, A, B, C, where A and C are n-tuples of floats and B is an n*n tuple-of-tuples of floats. I'm defining a limited multiplication for these objects, like this: Spam(A, B, C) * Spam(D, E, F) = Spam(A, dot(B, E), F) if and only if C == D. (Here dot(B, E) represents the matrix multiplication of B and E). In other words, this multiplication is defined only for the case where the last parameter of the first object is equal to first parameter of the second object. An exception should be thrown if one attempts to multiply two objects that fail to meet this requirement. Therefore, to implement this multiplication operation I need to have a way to verify that the float tuples C and D are equal. Certainly, I want to do this in in a way that takes into account the fact machine computations with floats can produce small differences between numbers that are notionally the same. E.g. (in Python 2.6.1 at least): 49.0 * (1.0/49.0) 0.99989 1.0 == 49.0 * (1.0/49.0) False The only approach I know of is to pick some arbitrary tolerance epsilon (e.g. 1e-6) and declare floats x and y equal iff the absolute value of x - y is less than epsilon. Does the Python standard library provide any better methods for performing such comparisons? I understand that, in Python 2.7 and 3.x = 3.1, when the interactive shell displays a float it shows the shortest decimal fraction that rounds correctly back to the true binary value. Is there a way to access this rounding functionality from code that must be able to run under version 2.6? (The idea would be to implement float comparison as a comparison of the rounded versions of floats.) Absent these possibilities, does Python provide any standard value of epsilon for this purpose? TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: Why flat is better than nested?
In mailman.278.1288129342.2218.python-l...@python.org Steve Holden st...@holdenweb.com writes: On 10/26/2010 2:44 PM, kj wrote: In mailman.258.1288104186.2218.python-l...@python.org Steve Holden st...@holdenweb.com writes: The answer is probably the same as you will see if you try from __future__ import braces That feature *is* available in Python 2.6 ;-) Now, that's hilarious. See, there *is* a place for humor :) I have nothing against humor. The reason why I find import braces funny is that it is so obviously a joke. But I do find it mildly annoying (and just mildly) that a joke/hoax/farce like ZoP/this.py is built into the standard lib, because a lot of people (not just me) don't realize it's a joke. (In fact, the reason I learned about ZoP/this.py was that in a reply to some post of mine in some Python forum [maybe c.l.py], the responder simply told me to run import this, with the implication that it would answer whatever it was that I was asking about. Either this person took ZoP seriously, or was just having fun at a noob's expense. Either way, I don't like it.) Learning a new programming language (which entails becoming familiar not only with some new syntax, but new libraries, new general ideas, new ways of doing stuff), is already disorienting enough as it is. I don't see the point of making the task any harder than it already is by injecting additional *gratuitous confusion* in the form pseudo-rogramming advice that apparently no experienced Python program really believes/takes seriously anyway. I just don't understand the need of having this.py in the std lib of all places. It's not like there's any risk of losing the ZoP if it were removed from it. Zillions of copies of it would be floating around in the web. But it would not be confused as something that is somehow endorsed by those who put together the distribution. I know. Not a chance. :) ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: Why flat is better than nested?
In mailman.241.1288036400.2218.python-l...@python.org Terry Reedy tjre...@udel.edu writes: On 10/25/2010 3:11 PM, kj wrote: Well, it's pretty *enshrined*, wouldn't you say? No. After all, it is part of the standard distribution, So is 'import antigravity' Are you playing with my feelings? % python Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) [GCC 4.2.1 (Apple Inc. build 5646)] on darwin import antigravity Traceback (most recent call last): File stdin, line 1, in module ImportError: No module named antigravity Too bad, I was looking forward to that. has an easy-to-remember invocation, etc. *Someone* must have taken it seriously enough to go through all this bother. If it is as trivial as you suggest (and for all I know you're absolutely right), then let's knock it off its pedestal once and for all, and remove it from the standard distribution. If you are being serious, you are being too serious (as in humorless). Guilty as charged, both in the too serious and the humorless counts. :/ Blame it on the Asperger's. My only defense is that, while learning Python over the past year, I've had *many* you've got to be joking moments while reading what's ostensible serious Python documents (e.g. PEP 8, PEP 257) as well as assorted threads featuring GvR and others involved in the design of Python, to the point that sometimes I do have a hard time gauging the seriousness of what's considered good programming / best practice in the Python world. Plus, I think it's fair to say that the Python community as a whole (or at least its more vocal members) are more concerned with correctness (for lack of a better term) and code aesthetics than, say, the Perl community. E.g., only in Python-related threads I've seen the adjective icky used routinely to indicate that some code is unacceptable on (more or less) aesthetic grounds. My point is that, even if one detects some levity in ZoP, given everything else one runs into in the Python world, for the uninitiated like me it is still hard to distinguish between what's in jest and what's in earnest. Perhaps the disconnect here is that you're seeing the whole thing from an insider's point of view, while I'm still enough of an outsider not to share this point of view. (I happen to think that one the hallmarks of being an initiate to a discipline is an almost complete loss of any memory of what that discipline looked like when the person was a complete novice. If this is true, then it's easy to understand the difference in our perceptions.) Anyway, thanks for letting me in on the joke. I'll pass it on. (Though, humorless as it is of me, I still would prefer the ZoP out of the standard library, to save myself having to tell those who are even newer to Python than me not to take it seriously.) ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: Why flat is better than nested?
In mailman.258.1288104186.2218.python-l...@python.org Steve Holden st...@holdenweb.com writes: The answer is probably the same as you will see if you try from __future__ import braces That feature *is* available in Python 2.6 ;-) Now, that's hilarious. kj -- http://mail.python.org/mailman/listinfo/python-list
Why flat is better than nested?
In The Zen of Python, one of the maxims is flat is better than nested? Why? Can anyone give me a concrete example that illustrates this point? TIA! ~kj PS: My question should not be construed as a defense for nested. I have no particular preference for either flat or nested; it all depends on the situation; I would have asked the same question if the maxim had been nested is better than flat. -- http://mail.python.org/mailman/listinfo/python-list
Re: Why flat is better than nested?
In f8b6c925-ca3b-4be4-8851-6b18c6465...@j18g2000yqd.googlegroups.com rantingrick rantingr...@gmail.com writes: On Oct 25, 5:07=A0am, kj no.em...@please.post wrote: In The Zen of Python, one of the maxims is flat is better than nested? =A0Why? =A0Can anyone give me a concrete example that illustrate= s this point? Simple. This commandment (endowed by the anointed one, GvR) is directed directly at lisp and those filthy lispers. If you don't know what lisp is then Google it. Then try to program with it for one hour. Very soon after your head will explode from the nested bracket plague and then you shall be enlightened! Some of the earliest programming I ever did was in Lisp. Scheme actually. In good ol' 6.001, back in '82. I loved it. I have no problem whatsoever with it. Benightedly yours, ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: Why flat is better than nested?
In mailman.232.1288020268.2218.python-l...@python.org Steve Holden st...@holdenweb.com writes: On 10/25/2010 10:47 AM, rantingrick wrote: On Oct 25, 5:07 am, kj no.em...@please.post wrote: In The Zen of Python, one of the maxims is flat is better than nested? Why? Can anyone give me a concrete example that illustrates this point? Simple. This commandment (endowed by the anointed one, GvR) is directed directly at lisp and those filthy lispers. If you don't know what lisp is then Google it. Then try to program with it for one hour. Very soon after your head will explode from the nested bracket plague and then you shall be enlightened! And everyone taking the Zen too seriously should remember that it was written by Tim Peters one night during the commercial breaks between rounds of wrestling on television. So while it can give useful guidance, it's nether prescriptive nor a bible ... Well, it's pretty *enshrined*, wouldn't you say? After all, it is part of the standard distribution, has an easy-to-remember invocation, etc. *Someone* must have taken it seriously enough to go through all this bother. If it is as trivial as you suggest (and for all I know you're absolutely right), then let's knock it off its pedestal once and for all, and remove it from the standard distribution. ~kj -- http://mail.python.org/mailman/listinfo/python-list
How to optimize and monitor garbage collection?
I'm designing a system that will be very memory hungry unless it is garbage-collected very aggressively. In the past I have had disappointing results with the gc module: I noticed practically no difference in memory usage with and without it. It is possible, however, that I was not measuring memory consumption adequately. What's the most accurate way to monitor memory consumption in a Python program, and thereby ensure that gc is working properly? Also, are there programming techniques that will result in better garbage collection? For example, would it help to explicitly call del on objects that should be gc'd? TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
MATLAB:matplotlib::Mathematica::???
Is there anything that does for Mathematica what matplotlib does for MATLAB? matplotlib, even in its underlying so-called OO mode, follows MATLAB's graphics model, which, in my very subjective opinion, is vastly inferior to Mathematica's. The latter allows for a clean separation between the textual specification of a graphic object (which can be very complex), and its graphic representation. Furthermore, it is general enough to allow for the composition of graphic objects within other graphic objects, to arbitrary depth levels. This readily allows for the representation of complex composite figures that are common in scientific publishing today, where figures not only routinely consist of several subfigures, but the subfigures themselves contain mutliple sub-subfigures, and so on. (In contrast, matplotlib supports at most two levels of composition [a two-dimensional array of sub-plots], which is both too inflexible and too limited.) More generally, despite its usefulness, I find MATLAB in the end to be one big ugly hack, so, as a developer, I would prefer to stay clear of anything that is modeled after MATLAB, however loosely. Any pointers to something more Mathematica-like in Python would be appreciated. TIA! kj -- http://mail.python.org/mailman/listinfo/python-list
Re: python shell silently ignores termios.tcsetattr()
In i9o8jc$9p...@lust.ihug.co.nz Lawrence D'Oliveiro l...@geek-central.gen.new_zealand writes: In message i9n4ph$d7...@reader1.panix.com, kj wrote: I tried to fix the problem by applying the equivalent of stty -echo within a python interactive session, but discovered that this setting is immediately (and silently) overwritten. That seems reasonable behaviour; the command loop is resetting the terminal to a reasonable state to allow it to read the next command. But the command you execute can do anything it likes in-between. What’s wrong with that? What's wrong with it is that what python thinks is a reasonable state is actually wrong in this case (it differs from the default setting established by the Emacs shell). I had hoped that there was a way to tell python what to regard as a reasonable state. I gather that there is no way to do this? ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: annoying CL echo in interactive python / ipython
In mailman.29.1287515736.2218.python-l...@python.org Jed Smith j...@jedsmith.org writes: On Tue, Oct 19, 2010 at 2:35 PM, kj no.em...@please.post wrote: In mailman.24.1287510296.2218.python-l...@python.org Jed Smith j...@jed= smith.org writes: On Tue, Oct 19, 2010 at 1:37 PM, kj no.em...@please.post wrote: % stty -echo That doesn't do what you think it does. Gee, thanks. =A0That really helped. =A0I'll go talk to my guru now, and meditate over this. You're right, I could have been more clear. I was nudging you to go read the man page of stty(1), but since you won't and want to get snarky instead, I will for you: echo (-echo) Echo back (do not echo back) every character typed. I read that, and it did not add anything new to what I already knew about stty -echo. I'm going to guess that the percent sign in your prompt indicates that you're using zsh(1). With my minimally-customized zsh, the echo option is reset every time the prompt is displayed. That means you can type stty -echo, push CR, the echo option is cleared, then zsh immediately sets it before you get to type again. Wrong guess. After I run stty -echo, the echoing stays disabled: % stty -echo % date Wed Oct 20 10:01:46 EDT 2010 % date Wed Oct 20 10:01:47 EDT 2010 % date Wed Oct 20 10:01:48 EDT 2010 % date Wed Oct 20 10:01:49 EDT 2010 As to the guess about readline, I only observe this problem with python (interactive) and ipython, but not with, say, the Perl debugger, which uses readline as well. FWIW. ~kj -- http://mail.python.org/mailman/listinfo/python-list
python shell silently ignores termios.tcsetattr()
This post is a continuation of an earlier thread called annoying CL echo in interactive python / ipython I found some more clues to the problem, although no solution yet. First, I found a post from 2009.05.09 that describes exactly the same situation I've observed (although it got no responses): http://groups.google.com/group/comp.emacs/browse_thread/thread/72973892717d0bfa I tried to fix the problem by applying the equivalent of stty -echo within a python interactive session, but discovered that this setting is immediately (and silently) overwritten. The following interaction illustrates what I mean. (I've added my comments, preceded by ###.) First I start the barest possible instance of Emacs (only the bare mininum of environment settings, and loading no configurations from an ~/.emacs file): % env -i HOME=$HOME DISPLAY=$DISPLAY TERM=$TERM /opt/local/bin/emacs -Q Within Emacs, the first command I execute is M-x shell, to bring up an Emacs shell. What follows is the interaction within this shell: sh-3.2$ stty -a ### first I get the initial settings for the terminal speed 9600 baud; 0 rows; 0 columns; lflags: icanon isig iexten -echo echoe -echok echoke -echonl echoctl -echoprt -altwerase -noflsh -tostop -flusho -pendin -nokerninfo -extproc iflags: -istrip icrnl -inlcr -igncr ixon -ixoff ixany imaxbel -iutf8 -ignbrk brkint -inpck -ignpar -parmrk oflags: opost -onlcr -oxtabs -onocr -onlret cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb -crtscts -dsrflow -dtrflow -mdmbuf cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = undef; eol2 = undef; erase = undef; intr = ^C; kill = undef; lnext = ^V; min = 1; quit = ^\; reprint = ^R; start = ^Q; status = ^T; stop = ^S; susp = ^Z; time = 0; werase = ^W; ### note the echo setting under lflags and the onlcr setting under ### oflags; also note that the stty -a command above was not echoed sh-3.2$ python ### next I start an interactive python session Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) [GCC 4.2.1 (Apple Inc. build 5646)] on darwin Type help, copyright, credits or license for more information. import termios import termios ### note the echoing old = termios.tcgetattr(1) old = termios.tcgetattr(1) new = termios.tcgetattr(1) new = termios.tcgetattr(1) new[3] = new[3] ~termios.ECHO ~termios.ONLCR new[3] = new[3] ~termios.ECHO old[3], new[3] old[3], new[3] (536872395, 536872385) (termios.tcsetattr(1, termios.TCSANOW, new), termios.tcgetattr(1)[3], old[3], new[3]) (termios.tcsetattr(1, termios.TCSANOW, new), termios.tcgetattr(1)[3], old[3], new[3]) (None, 536872385, 536872395, 536872385) ### The output above shows that the setting attempted through ### tcsetattr took momentarily... termios.tcgetattr(1) == old termios.tcgetattr(1) == old True ### ...but by this point it has already been reset back to its original value. termios.tcgetattr(1)[3], old[3], new[3] termios.tcgetattr(1)[3], old[3], new[3] (536872395, 536872395, 536872385) ^D sh-3.2$ stty -a ### after quitting python, the echo and onlcr settings have been reversed stty -a speed 9600 baud; 0 rows; 0 columns; lflags: icanon isig iexten echo echoe -echok echoke -echonl echoctl -echoprt -altwerase -noflsh -tostop -flusho -pendin -nokerninfo -extproc iflags: -istrip icrnl -inlcr -igncr ixon -ixoff ixany imaxbel -iutf8 -ignbrk brkint -inpck -ignpar -parmrk oflags: opost onlcr -oxtabs -onocr -onlret cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb -crtscts -dsrflow -dtrflow -mdmbuf cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = undef; eol2 = undef; erase = ^?; intr = ^C; kill = ^U; lnext = ^V; min = 1; quit = ^\; reprint = ^R; start = ^Q; status = ^T; stop = ^S; susp = ^Z; time = 0; werase = ^W; sh-3.2$ Does anyone understand what's going on? Is there any way to prevent python from resetting the settings made with tcgetattr? (I realize that this is relatively low-level stuff, so it is unlikely that there's a clean solution; I'm hoping, however, that there may be a way to fool python into doing the right thing; after all, this strange behavior only happens under the Emacs shell; I don't observe it under, e.g., Terminal or xterm.) TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
annoying CL echo in interactive python / ipython
Under some parent shells, both my interactive python as well as ipython, produce an unwanted echoing of the input line. E.g. 1 + 1 1 + 1 2 What's worse, upon exiting the interactive python/ipython session, the terminal is left in echo mode: EOF % date date Tue Oct 19 13:27:47 EDT 2010 % stty -echo % date Tue Oct 19 13:27:50 EDT 2010 It's basically the same story for ipython. (If I run stty -echo before running either python or ipython, I still get the echo when I'm in them. So the problem is not a pre-existing terminal setting.) (As I said, this happens only under some shells (e.g. emacs shell), so YMMV.) Does anyone know how can I suppress this annoying feature? TIA! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: annoying CL echo in interactive python / ipython
In mailman.24.1287510296.2218.python-l...@python.org Jed Smith j...@jedsmith.org writes: On Tue, Oct 19, 2010 at 1:37 PM, kj no.em...@please.post wrote: % stty -echo That doesn't do what you think it does. Gee, thanks. That really helped. I'll go talk to my guru now, and meditate over this. -- http://mail.python.org/mailman/listinfo/python-list
docstring that use globals?
Consider the following Python snippet:
SEPARATOR = '+'
def spam(ham, eggs):
Return a hash of ham and eggs.
The variables ham and eggs are tuples of strings. The returned
hash is a dict made from the pairs returned by zip(ham, eggs).
If ham contains repeated keys, the corresponding values in eggs
are concatenated using the string SEPARATOR.
For example, spam(('a', 'b', 'a'), ('x', 'y', 'z')) returns
{'a': 'xSEPARATORz', 'b': 'y'}.
# implementation follows...
Of course, as written, the docstring above is no good. All
occurrences of the string SEPARATOR in it should be replaced with
the actual value of the global variable SEPARATOR, which in this
case is the string '+'.
My first attempt at achieving this effect was the following:
SEPARATOR = '+'
def spam(ham, eggs):
Return a hash of ham and eggs.
The variables ham and eggs are tuples of strings. The returned
hash is a dict made from the pairs returned by zip(ham, eggs).
If ham contains repeated keys, the corresponding values in eggs
are concatenated using the string %(SEPARATOR)s.
For example, spam(('a', 'b', 'a'), ('x', 'y', 'z')) returns
{'a': 'x%(SEPARATOR)sz', 'b': 'y'}. % globals()
# implementation follows...
...which, of course (in retrospect), does not work, since only a
string literal, and not any ol' string-valued expression, at the
beginning of a function's body can serve as a docstring.
What's the best way to achieve what I'm trying to do?
(Of course, all these acrobatics are hardly worth the effort for
the simple example I give above. In the actual situation I'm
dealing with, however, there are a lot more docstrings and global
variables in the mix, and at this early stage of the development,
the values of the globals are still fluid. I'm trying to minimize
the effort required to keep the docstrings current, while still
retaining the freedom to adjust the values of the globals. Also,
FWIW, most of these globals are merely default values that can be
overridden at runtime.)
TIA!
~kj
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Re: docstring that use globals?
MRAB, Peter: thanks for the decorator idea! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: docstring that use globals?
In i9clfa$mk...@reader1.panix.com kj no.em...@please.post writes: MRAB, Peter: thanks for the decorator idea! As an afterthought, is there any way to extend this general idea to other docstrings beyond function docstrings? I imagine that the decorator idea works well for method docstrings too (though I have not tried any of this yet). But, IIRC, decorators don't work for classes, so class docstrings would need to be expanded expressly. The hardest case is module docstrings. In particular, the docstring for a script. Is there any way to apply the globals expansion idea to a script's toplevel docstring? (I imagine the answer to this last question is no, but I thought I'd ask. :) ) ~kj -- http://mail.python.org/mailman/listinfo/python-list
dynamic loading error (Symbol not found)
The following interaction (in OS X) summarizes the situation: % echo $DYLD_LIBRARY_PATH /Applications/MATLAB_R2010a.app/bin/maci64 % grep -r _engClose $DYLD_LIBRARY_PATH Binary file /Applications/MATLAB_R2010a.app/bin/maci64/libeng.dylib matches % export DYLD_LIBRARY_PATH % python Python 2.6.5 (r265:79063, May 22 2010, 18:34:46) [GCC 4.0.1 (Apple Inc. build 5465)] on darwin Type help, copyright, credits or license for more information. from mlabwrap import mlab Traceback (most recent call last): File stdin, line 1, in module File mlabwrap.py, line 188, in module import mlabraw ImportError: dlopen(/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mlabrawmodule.so, 2): Symbol not found: _engClose Referenced from: /opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/mlabrawmodule.so Expected in: dynamic lookup In summary, 1) dlopen produces a Symbol not found: _engClose error; 2) according to grep, a file accessible through the variable DYLD_LIBRARY_PATH matches the string _engClose; The permissions of the file in question are all OK (0555); likewise, the permissions of all the prefix subpaths leading to this file are fine. (For all I know, it is possible that, even though the libeng.dylib file matches _engClose, this is only a fragment of a longer symbol name.) Can anyone suggest a way to fix this error? Thanks! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: dynamic loading error (Symbol not found)
In mailman.1782.1287266502.29448.python-l...@python.org Emile van Sebille em...@fenx.com writes: On 10/16/2010 2:15 PM kj said... The following interaction (in OS X) summarizes the situation: % echo $DYLD_LIBRARY_PATH /Applications/MATLAB_R2010a.app/bin/maci64 % grep -r _engClose $DYLD_LIBRARY_PATH Binary file /Applications/MATLAB_R2010a.app/bin/maci64/libeng.dylib matches % export DYLD_LIBRARY_PATH % python Python 2.6.5 (r265:79063, May 22 2010, 18:34:46) [GCC 4.0.1 (Apple Inc. build 5465)] on darwin Type help, copyright, credits or license for more information. from mlabwrap import mlab I'd start by checking the version numbers and compatibility specifications on mlabwrap which, per their site, claims mlabwrap should work with python=2.4. I don't get your point. As the fragment you quoted shows, I'm using Python 2.6.5. -- http://mail.python.org/mailman/listinfo/python-list
Re: docstring that use globals?
In 8hujfsfb9...@mid.individual.net Gregory Ewing greg.ew...@canterbury.ac.nz writes: kj wrote: The hardest case is module docstrings. Actually, that one's quite easy, just assign to __doc__. __doc__ = This is a %s docstring % made-up D'oh! Thanks. -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In 87hbgu8irb@gmail.com Arnaud Delobelle arno...@gmail.com writes: Steven D'Aprano st...@remove-this-cybersource.com.au writes: On Sat, 09 Oct 2010 21:39:51 +0100, Arnaud Delobelle wrote: 1. hash() is an idempotent function, i.e. hash(hash(x)) == hash(x) hold for any hashable x (this is a simple consequence of the fact that hash(x) == x for any int x (by 'int' I mean 2.X int)). It's a beautiful theory, but, alas, it is not the case. hash(-1) == -1 False hash(2**64) == 2**64 False to give only two of an infinite number of counter-examples. (Infinite???) And, in fact, (-1) is the only int such that hash(x) != x. Arnaud, how did you determine that -1 is the only such int? I can't imagine any other approach other than a brute-force check of all ints... When I tried this I ran into unforeseen limitations in xrange, etc. It's all very doable, but still, it would take at least about 3 hours on my laptop. In can only guess that (-1) is a value that has a special meaning when hashing. Try this (Python 2.6): class A(object): ... def __hash__(self): return -1 ... a = A() hash(a) -2 Very cool. BTW, thank you for the explanation in your previous post. It makes a lot of sense. I find it interesting (as Hrvoje pointed out) that the hash function is (or appears to be) idempotent on integers (long or not), even though it is not the identity on the integers. Thanks to Steven for the counterexamples to show the latter. I've learned tons from this exchange. ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In 4cb1dc9a$0$29970$c3e8da3$54964...@news.astraweb.com Steven D'Aprano st...@remove-this-cybersource.com.au writes: Reading the source code is also a good approach. Every time I have attempted to answer a question by looking at the Python C source, all I've achieved was wasting time, sometimes a *lot* of time, so by now I've developed what can only be described as a phobia to it. I probably need professional help at this point. ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In 87y6a9lqnj@gmail.com Arnaud Delobelle arno...@gmail.com writes:
You could do something like this:
deep_methods = {
list: lambda f, l: tuple(map(f, l)),
dict: lambda f, d: frozenset((k, f(v)) for k, v in d.items()),
set: lambda f, s: frozenset(map(f, s)),
# Add more if needed
}
def apply_method(f, obj):
try:
method = deep_methods[type(obj)]
except KeyError:
return obj
return method(f, obj)
def deepfreeze(obj):
Return a 'hashable version' of an object
return apply_method(deepfreeze, obj)
def deephash(obj):
Return hash(deepfreeze(obj)) without deepfreezing
return hash(apply_method(deephash, obj))
# Example of deepfreezable object:
obj = [1, foo, {(2, 4): {7, 5, 4}, bar: baz}]
^ ^
| |
`---`--- what's this?
deepfreeze(obj)
(1, 'foo', frozenset({('bar', 'baz'), ((2, 4), frozenset({4, 5, 7}))}))
deephash(obj)
1341422540
hash(deepfreeze(obj))
1341422540
After fixing the missing in deepfreeze this code works as
advertised, but I'm mystified by the identity between hash(deepfreeze(...))
and deephash(...). Without some knowledge of the Python internals,
I don't see how this follows.
More specifically, it is not obvious to me that, for example,
hash(frozenset((whatever,)))
would be identical to
hash(frozenset((hash(whatever),)))
but this identity has held every time I've checked it. Similarly
for other more complicated variations on this theme.
Anyway, thanks for the code. It's very useful.
~kj
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Re: frozendict (v0.1)
In 4cae667c$0$29993$c3e8da3$54964...@news.astraweb.com Steven D'Aprano st...@remove-this-cybersource.com.au writes: On Fri, 08 Oct 2010 00:23:30 +, kj wrote: Because it's always better to use a well-written, fast, efficient, correct, well-tested wheel than to invent your own slow, incorrect wheel :) IOW, don't you worry your little head about why. -- http://mail.python.org/mailman/listinfo/python-list
Re: frozendict (v0.1)
In 878w29kxjp@gmail.com Arnaud Delobelle arno...@gmail.com writes:
E.g., try with {1:'a', 1j:'b'}
I see. Thanks for this clarification. I learned a lot from it.
I guess that frozenset must have some way of canonicalizing the
order of its elements that is dependent on their Python values but
not on their comparability. My first thought was that they are
ordered according to their hash values, but this theory doesn't
hold:
abc = ('a', 'b', 'c')
sorted(map(hash, abc))
[-468864544, -340864157, -212863774]
map(hash, frozenset(abc))
[-468864544, -212863774, -340864157]
I.e. the ordering of the elements in the frozenset does not correspond
to the ordering of their hashes in either direction. Hmmm.
I tried to understand this by looking at the C source but I gave
up after 10 fruitless minutes. (This has been invariably the
outcome of all my attempts at finding my way through the Python C
source.)
I guess the take-home message is that frozenset is a more general
way to canonicalize an iterable object than sorting, even though
the reasons for this still remain a mystery to me... Then again,
just looking at the voodoo that goes into algorithms for computing
hashes fills me with despair. As much as I dislike it, sooner or
later I'll have to go on faith.
~kj
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Re: frozendict (v0.1)
In i8loa2$3o...@reader1.panix.com kj no.em...@please.post writes:
At any rate, using your [i.e. Arnaud's] suggestions in this and
your other post, the current implementation of frozendict stands
at:
class frozendict(dict):
for method in ('__delitem__ __setitem__ clear pop popitem setdefault '
'update').split():
exec
def %s(self, *a, **k):
cn = self.__class__.__name__
raise TypeError('%%s' object is not mutable %% cn)
% method
def __hash__(self):
return hash(frozenset(self.items()))
...which is a lot nicer!
As a side comment on my own post, this is the second time in less
than a week that I find myself having to resort to exec'ing some
code generated from a template. This one is worse, because there's
nothing runtime-dependent about the code being exec'd. It sticks
in my craw somehow. It just doesn't seem quite right that at the
time of doing something as bread-and-butter as implementing a
subclass, one has to choose between explicitly writing a whole
bunch of identical methods, and exec-based hacks like what I'm
doing above. There's got to be a better to tell Python: override
all these superclass methods with this one.
Or maybe the problem here is that, in a perfect world, frozendict
would be in the standard library, as a superclass of dict lacking
all of dict's destructive methods. :)
~kj
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Re: frozendict (v0.1)
In mailman.1461.1286539843.29448.python-l...@python.org Jonas H. jo...@lophus.org writes: On 10/08/2010 02:23 AM, kj wrote: Here's my implementation suggestion: class frozendict(dict): def _immutable_error(self, *args, **kwargs): raise TypeError(%r object is immutable % self.__class__.__name__) __setitem__ = __delitem__ = clear = pop \ = popitem = setdefault = update = _immutable_error def __hash__(self): return hash(frozenset(self.iteritems())) Only 9 lines :-) Thanks, you just answered the question I just posted, while I was still writing it! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: frozendict (v0.1)
In mailman.1463.1286546684.29448.python-l...@python.org Jonas H. jo...@lophus.org writes: Hope this helps :-) It did! Thanks! For one thing now I see that I was barking up the wrong tree in focusing on a canonical order, when, as the code you posted shows, it is actually not required for hashing. In fact, I'd come to the conclusion that frozensets had a consistent order (i.e. frozensets that were equal according to '==' would be iterated over in the same order), but now I'm not sure that this is the case. (Granted, semantically, there's nothing in the definition of a frozenset that would imply a consistent iteration order.) Thanks again! ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In mailman.1415.1286438617.29448.python-l...@python.org Terry Reedy tjre...@udel.edu writes: If these two attributes, and hence the dicts, are public, then your instances are mutable. I guess I should have written immutable among consenting adults. As far as I know, Python does not support private attributes, so I guess the dicts are public no matter what I do. I suppose that I can implement frozendict, but I can't think of any way to enforce the immutability of these frozendicts that would not be trivial to circumvent (it better be, in fact, otherwise I wouldn't be able to initialize the damn things). If you had something else in mind, please let me know. ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In m2fwwjazs2@web.de de...@web.de (Diez B. Roggisch) writes: kj no.em...@please.post writes: The short version of this question is: where can I find the algorithm used by the tuple class's __hash__ method? Surprisingly, in the source: http://google.com/codesearch/p?hl=de#-2BKs-LW4I0/trunk/python/src/Objects/tupleobject.cq=python%20tuplehashsa=Ncd=1ct=rc Thanks to you, and to all who pointed me to the place in the source where this is. How exactly did you search for this? Taking a hint from the url above, I went to Google Code Search and searched for python tuple hash (minus the quotes, of course), which produced a ton of irrelevant stuff (almost 80K hits). Searching for python tuple hash lang:c cut down the number of hits to ~8K, but still too much to wade through. Clearly I need a smarter search strategy (one that does not include foreknowledge of the name of the actual function in the C source, of course). ~kj -- http://mail.python.org/mailman/listinfo/python-list
Re: hashkey/digest for a complex object
In 87pqvmp611@web.de de...@web.de (Diez B. Roggisch) writes: I tried codesearch first. Not satisfied after 30 seconds with the results, I did the most straight forward thing - I downloaded and un-packed the python source... and took a look. From that I learned the tuplehash function name. You must be at least somewhat familiar with the Python source. Everytime I've peeked into it I just feel lost, but it's clearly something I need to master sooner or later... I can't wait for the next one of those occasional introductions to the Python internals at our local Python club. Thanks, ~kj -- http://mail.python.org/mailman/listinfo/python-list
how to test for atomicity/mutability/hashability?
I want to implement a test t() that will return True if its two arguments are completely different. By this I mean that they don't share any non-atomic component. E.g., if a = [0, 1] b = [0, 1] c = [2, 3] d = [2, 3] A = (a, c, 0) B = (a, d, 1) C = (b, d, 0) The desired test t() would yield: t(A, B) - False (A and B share the mutable component a) t(A, C) - True (a =!= c, b =!= d, and 0 is not mutable) t(B, C) - False (B and C share the mutable component d) (=!= is shorthand with is not identical to.) It would facilitate the implementation of t() to have a simple test for mutability. Is there one? Thanks! ~kj -- http://mail.python.org/mailman/listinfo/python-list
frozendict (v0.1)
Following a suggestion from MRAB, I attempted to implement a
frozendict class. My implementation took a lot more work than
something this simple should take, and it still sucks. So I'm
hoping someone can show me a better way. Specifically, I'm hoping
that there is a recipe for building off standard classes that
cover all the bases with a minimum of tedious repetitive work.
Here's my little monster:
ass frozendict():
_DESTRUCTIVE = set(('__delitem__ __setitem__ clear pop popitem setdefault '
'update').split())
_NON_DESTRUCTIVE = set(('__contains__ __format__ __getitem__ __hash__ '
'__init__ __iter__ __len__ __repr__ __sizeof__ '
'__str__ copy fromkeys get has_key items iteritems '
'iterkeys itervalues keys values'.split()))
_COMPARISONS = set(('__cmp__ __eq__ __ge__ __gt__ __le__ __lt__ '
'__ne__').split())
def __init__(self, iterable=(), **kwargs):
self._dict = dict(iterable, **kwargs)
def __hash__(self):
return hash(tuple(self.items()))
def __getattr__(self, attrib):
class_ = self.__class__
dict_ = self._dict
if attrib in class_._COMPARISONS:
return lambda x: dict_.__getattribute__(attrib)(x._dict)
elif attrib in class_._NON_DESTRUCTIVE:
return dict_.__getattribute__(attrib)
else:
if attrib in class_._DESTRUCTIVE:
raise TypeError('%s' object is not mutable % class_.__name__)
else:
raise AttributeError('%s' object has no attribute '%s' %
(class_.__name__, attrib))
I didn't implement this as a subclass of dict to avoid having to
write a dumb little blocking method for every destructive dict
method. (I couldn't figure out how to write a loop to define these
overriding methods programmatically, because their signatures are
all over the place.)
I didn't implement it as a subclass of object with an internal dict
delegate, because I couldn't figure a reasonable way to pass certain
object methods to the delegate (since in this case frozendict.__getattr__
wouldn't be called).
The handling of comparison methods is particularly horrific and
inefficient.
If Beautiful is better than ugly, I sure how there's another way
that is a lot more beautiful than this one.
TIA!
~kj
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Re: frozendict (v0.1)
In 87hbgxlk67@gmail.com Arnaud Delobelle arno...@gmail.com writes:
A simple fix is to use hash(frozenset(self.items())) instead.
Thanks for pointing out the hash bug. It was an oversight: I meant
to write
def __hash__(self):
return hash(sorted(tuple(self.items(
I imagine that frozenset is better than sorted(tuple(...)) here,
but it's not obvious to me why.
At any rate, using your suggestions in this and your other post,
the current implementation of frozendict stands at:
class frozendict(dict):
for method in ('__delitem__ __setitem__ clear pop popitem setdefault '
'update').split():
exec
def %s(self, *a, **k):
cn = self.__class__.__name__
raise TypeError('%%s' object is not mutable %% cn)
% method
def __hash__(self):
return hash(frozenset(self.items()))
...which is a lot nicer!
Thanks!
~kj
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hashkey/digest for a complex object
The short version of this question is: where can I find the algorithm
used by the tuple class's __hash__ method?
Now, for the long version of this question, I'm working with some
complext Python objects that I want to be able to compare for
equality easily.
These objects are non-mutable once they are created, so I would
like to use a two-step comparison for equality, based on the
assumption that I can compute (either at creation time, or as needed
and memoized) a hashkey/digest for each object. The test for
equality of two of these objects would first compare their hashkeys.
If they are different, the two objects are declared different; if
they match, then a more stringent test for equality is performed.
So the problem is to come up with a reasonable hashkey for each of
these objects. The objects have two significant attributes, and
two of these objects should be regarded as equal if these attributes
are the same in both. The first attribute is a simple dictionary
whose keys are integers and values are strings. The second attribute
is more complicated. It is a tree structure, represented as a
dictionary of dictionaries of dictionaries... until we get to the
leaf elements, which are frozensets of strings. The keys at every
level of this structure are strings. E.g. a simple example of such
an attribute would look like:
{'A': {'a': set(['1', '2', '3']),
'b': set(['4', '5'])},
'B': set(['6', '7', '8'])}
I'm looking for a good algorithm for computing a hash key for
something like this? (Basically I'm looking for a good way to
combine hashkeys.)
Thanks!
kj
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How parametrize classes by class data?
I want to implement a class of classes, so that, instead of the usual: spam = MyClass(eggs) ...I can write spam = MyClass(ham)(eggs) ...where ham is a parameter that will end up as the value of a class variable of the class returned by MyClass(ham). In other words, MyClass is a metaclass: a class whose instances are themselves classes. In the immediate use I have for such a metaclass, the parameter is going to be a list of lists of headers, which is used by the __init__ of the generated class to interpret its inputs. The standard library's collections.namedtuple needs to do something similar to this, so I thought I could learn how to do it in Python by studying its source code. I was surprised to discover that collections.namedtuple achieves this class parametrization by generating some source code on the fly, from a template, and exec'ing it. This looked to me like a rather un-Pythonic hack, but seeing there in the venerable collections module suggested to me that maybe this is actually the best way to achieve this effect in Python. Is this so? If not, please let me know of a better way. TIA! kj -- http://mail.python.org/mailman/listinfo/python-list
namespace hacking question
This is a recurrent situation: I want to initialize a whole bunch
of local variables in a uniform way, but after initialization, I
need to do different things with the various variables.
What I end up doing is using a dict:
d = dict()
for v in ('spam', 'ham', 'eggs'):
d[v] = init(v)
foo(d['spam'])
bar(d['ham'])
baz(d['eggs'])
This is fine, but I'd like to get rid of the tedium of typing all
those extra d['...']s.
I.e., what I would *like* to do is something closer to this:
d = locals()
for v in ('spam', 'ham', 'eggs'):
d[v] = init(v)
foo(spam)
bar(ham)
baz(eggs)
...but this results in errors like NameError: global name 'spam' is
not defined.
But the problem is deeper than the fact that the error above would
suggest, because even this fails:
spam = ham = eggs = None
d = locals()
for v in ('spam', 'ham', 'eggs'):
d[v] = init(v)
foo(spam) # calls foo(None)
bar(ham) # calls bar(None)
baz(eggs) # calls baz(None)
In other words, setting the value of locals()['x'] does not set
the value of the local variable x.
I also tried a hack using eval:
for v in ('spam', 'ham', 'eggs'):
eval %s = init('%s') % (v, v)
but the = sign in the eval string resulted in a SyntaxError:
invalid syntax.
Is there any way to use a loop to set a whole bunch of local
variables (and later refer to these variables by their individual
names)?
TIA!
kj
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Supplementing the std lib (Was: partial sums problem)
In mailman.1142.1285722789.29448.python-l...@python.org Terry Reedy tjre...@udel.edu writes: Do not try to do a reduction with a comprehension. Just write clear, straightforward code that obviously works. s=[1,2,3,4,5,6] def cusum(s): t = 0 for i in s: t += i yield t print(list(cusum(s))) [1, 3, 6, 10, 15, 21] Actually, this is just fine. Thank you! But it brings up a new question, of an entirely different nature. It's a recurrent conundrum, actually, and I have not found a good solution. Your cusum function is one that I would like to see somewhere in the standard library (in itertools maybe?). Maybe some future version of the standard library will have it, or something like it (I'm thinking of a generalized form which, like reduce, takes a function and an initial value as arguments). But in the immediate term, cusum is not part of the standard library. Where would you put it if you wanted to reuse it? Do you create a module just for it? Or do you create a general stdlib2 module with all those workhorse functions that have not made it to the standard library? Or something else entirely? (I'm not expecting to get *the* solution from anyone reply; rather, I'm interested in reading people's take on the question and their way of dealing with those functions they consider worthy of the standard library.) kj -- http://mail.python.org/mailman/listinfo/python-list
partial sums problem
The following attempt to get a list of partial sums fails: s = 0 [((s += t) and s) for t in range(1, 10)] File stdin, line 1 [((s += t) and s) for t in range(1, 10)] ^ SyntaxError: invalid syntax What's the best way to get a list of partial sums? TIA! kj -- http://mail.python.org/mailman/listinfo/python-list
audio time-stretching?
Does anyone know of a Python module for *moderate* time-stretching[1] an MP3 (or AIFF) file? FWIW, the audio I want to time-stretch is human speech. TIA! ~K [1] By moderate time stretching I mean, for example, taking an audio that would normally play in 5 seconds, and stretch it so that it plays in 7.5 seconds, keeping the pitch unchanged. A lot of software out there does this badly; e.g. the time-stretched audio springs extraneous beats of intensity that are very obtrusive and annoying; I guess it's some weird wave self-interference effect. Also, I stress *moderate* time stretching to explicitly rule out the extreme (~50X) time-stretching that software like PaulStretch is designed to accomplish. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to convert (unicode) text to image?
In mailman.123.1282955703.29448.python-l...@python.org Benjamin Kaplan benjamin.kap...@case.edu writes: On Fri, Aug 27, 2010 at 8:01 PM, kj no.em...@please.post wrote: Hi! =A0Does anyone know of an easy way to convert a Unicode string into a= n image file (either jpg or png)? Do you mean you have some text and you want an image containing that text? PIL's ImageDraw module can do that. Thanks for the pointer, but... RANT The documentation I have found for PIL (at http://www.pythonware.com/library/pil/handbook) is beyond atrocious. If this is the only way to learn how to use this library, then I really don't understand how anyone who is not clairvoyant can do it. Example: I went to the docs page for ImageDraw. There I find that the constructor for an ImageDraw.Draw object takes an argument, but *what* this argument should be (integer? object? string?) is left entirely undefined. From the examples given I *guessed* that it was an object of class Image, so I repeated the exercise: I consulted the docs for the Image module. There I learn that the constructor for the Image class takes among its parameters one called mode and one called color, but, here again, what these parameters are is left completely undefined. (mode is left both syntactically and semantically undefined; color is left syntactically undefined, though the documentation includes a bit by way of semantic definition of this parameter.) What's up with this practice of leaving parameters undefined like this??? Wasn't it obvious to the person writing the Image module docs that without explaining what these parameters should be the documentation is nearly useless? Is such poor documentation an unintended consequence of duck typing??? Sorry for the outburst, but unfortunately, PIL is not alone in this. Python is awash in poor documentation. The number two complaint I've heard from those who dislike Python is the poor quality of its documentation, and in particular the fact that function parameters are typically left undefined, as is the case in the PIL docs. I like Python a lot, but I have to agree with this criticism. (The number one complaint has to do with the syntactic significance of whitespace; of course, I find *this* criticism silly.) What is most frustrating about such poor documentation is that it is exactly the opposite from what one would expect from the carefulness and thoroughness found in the PEPs... I have been using Python as my primary scripting language for about one year, after many years of programming in Perl, and now Python is my language of choice. But I must say that the documentation standards I found in the Perl world are *well above* those in the Python world. This is not to say that Perl documentation is always excellent; it certainly has its gaps, as one would expect from volunteer-contributed software. But I don't recall being frustrated by Perl module docs anywhere nearly as often as I am by Python module docs. I have to conclude that the problem with Python docs is somehow systemic... /RANT -- http://mail.python.org/mailman/listinfo/python-list
How to convert (unicode) text to image?
Hi! Does anyone know of an easy way to convert a Unicode string into an image file (either jpg or png)? TIA! ~k -- http://mail.python.org/mailman/listinfo/python-list
Re: shelf-like list?
In mailman.2125.1281849995.1673.python-l...@python.org Chris Rebert c...@rebertia.com writes: On Sat, Aug 14, 2010 at 5:13 PM, kj no.em...@please.post wrote: In af7fdb85-8c87-434e-94f3-18d8729bf...@l25g2000prn.googlegroups.com Ra= ymond Hettinger pyt...@rcn.com writes: On Aug 12, 1:37=3DA0pm, Thomas Jollans tho...@jollybox.de wrote: On Tuesday 10 August 2010, it occurred to kj to exclaim: I'm looking for a module that implements persistent lists: objects that behave like lists except that all their elements are stored on disk. =3DA0IOW, the equivalent of shelves, but for lists rather than a dictionaries. . . . You could simply use pickle to save the data every once in a while. That is a very reasonable solution. Sorry I don't follow. =C2=A0Some sample code would be helpful. I would assume something along the lines of (untested): from pickle import dump MOD_THRESHOLD =3D 42 class PersistentList(list): def __init__(self, filepath): self.filepath =3D filepath self._mod_count =3D 0 def save(self): with open(self.filepath, 'w') as f: dump(self, f) self._mod_count =3D 0 def append(self, *args, **kwds): super(PersistentList, self).append(*args, **kwds) self._mod_count +=3D 1 if self._mod_count =3D MOD_THRESHOLD: # obviously time-since-last-dump or other # more sophisticated metrics might be used instead self.save() Even though it is saved periodically to disk, it looks like the whole list remains in memory all the time? (If so, it's not what I'm looking for; the whole point of saving stuff to disk is to keep the list's memory footprint low.) ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: shelf-like list?
In i486al$b...@online.de Martin v. Loewis mar...@v.loewis.de writes: Does anyone know of such a module? ZODB supports persistent lists. Thanks; I'll check it out. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: shelf-like list?
In af7fdb85-8c87-434e-94f3-18d8729bf...@l25g2000prn.googlegroups.com Raymond Hettinger pyt...@rcn.com writes: On Aug 12, 1:37=A0pm, Thomas Jollans tho...@jollybox.de wrote: On Tuesday 10 August 2010, it occurred to kj to exclaim: I'm looking for a module that implements persistent lists: objects that behave like lists except that all their elements are stored on disk. =A0IOW, the equivalent of shelves, but for lists rather than a dictionaries. . . . You could simply use pickle to save the data every once in a while. That is a very reasonable solution. Sorry I don't follow. Some sample code would be helpful. TIA, ~K -- http://mail.python.org/mailman/listinfo/python-list
How to add silent stretches to MP3 using Python?
Here's the problem: I have about 25,000 mp3 files, each lasting, *on average*, only a few seconds, though the variance is wide (the longest one lasts around 20 seconds). (These files correspond to sample sentences for foreign language training.) The problem is that there is basically no padding before and after the sound signal. I want to prepend about 2 seconds of silence to each file, and append another silent stretch at the end lasting either 2 seconds or some multiplier of the duration of the original file, whichever is greater. I know that manipulating MP3 audio programmatically is usually not easy, but this has got to be one of the simplest manipulations possible, so I'm hoping I'll be able to pull it off with Python. But I have not had much luck finding a Python library to do this. If someone knows how to do this, and could give me some pointers, I'd appreciate it. TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
shelf-like list?
I'm looking for a module that implements persistent lists: objects that behave like lists except that all their elements are stored on disk. IOW, the equivalent of shelves, but for lists rather than a dictionaries. Does anyone know of such a module? (I suppose that I could slap together a crude implementation of such a thing by wrapping a shelf with suitable methods to simulate the list interface. But I'd rather not roll my own if a tested implementation already exist.) TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: Unicode error
In 4c5d4ad9$0$28666$c3e8...@news.astraweb.com Steven D'Aprano st...@remove-this-cybersource.com.au writes: On Sat, 07 Aug 2010 19:28:56 +1200, Gregory Ewing wrote: Steven D'Aprano wrote: No memory? No disk space? No problem! Just a flesh wound! What's the point of that? +1 QOTW While I'm always happy to be nominated for QOTW, in this case I didn't say it, and the nomination should go to KJ. (The ol' insert Monty Python reference move: it never fails...) -- http://mail.python.org/mailman/listinfo/python-list
Re: Unicode error
In pan.2010.07.23.21.46.03.547...@nowhere.com Nobody nob...@nowhere.com writes: On Fri, 23 Jul 2010 10:42:26 +, Steven D'Aprano wrote: Don't write bare excepts, always catch the error you want and nothing else. That advice would make more sense if it was possible to know which exceptions could be raised. In practice, that isn't possible, as the documentation seldom provides this information. Even for the built-in classes, the documentation is weak in this regard; for less important modules and third-party libraries, it's entirely absent. I don't get your point. Even when I *know* that a certain exception may happen, I don't necessarily catch it. I catch only those exceptions for which I can think of a suitable response that is *different* from just letting the program fail. (After all, my own code raises its own exceptions with the precise intention of making the program fail.) If an unexpected exception occurs, then by definition, I had no better response in mind for that situation than just letting the program fail, so I'm happy to let that happen. If, afterwards, I think of a different response for a previously uncaught exception, I'll modify the code accordingly. I find this approach far preferable to the alternative of knowing a long list of possible exceptions (some of which may never happen in actual practice), and think of ways to keep the program still alive no-matter-what. No memory? No disk space? No problem! Just a flesh wound! What's the point of that? (If I want the final error message to be something other than a bare stack trace, I may wrap the whole execution in a global/top-level try/catch block so that I can fashion a suitable error message right before calling exit, but that's just softening the fall: the program still will go down.) -- http://mail.python.org/mailman/listinfo/python-list
XML parsing: SAX/expat yield
I want to write code that parses a file that is far bigger than the amount of memory I can count on. Therefore, I want to stay as far away as possible from anything that produces a memory-resident DOM tree. The top-level structure of this xml is very simple: it's just a very long list of records. All the complexity of the data is at the level of the individual records, but these records are tiny in size (relative to the size of the entire file). So the ideal would be a parser-iterator, which parses just enough of the file to yield (in the generator sense) the next record, thereby returning control to the caller; the caller can process the record, delete it from memory, and return control to the parser-iterator; once parser-iterator regains control, it repeats this sequence starting where it left off. The problem, as I see it, is that SAX-type parsers like expat want to do everything with callbacks, which is not readily compatible with the generator paradigm I just described. Is there a way to get an xml.parsers.expat parser (or any other SAX-type parser) to stop at a particular point to yield a value? The only approach I can think of is to have the appropriate parser callbacks throw an exception wherever a yield would have been. The exception-handling code would have the actual yield statement, followed by code that restarts the parser where it left off. Additional logic would be necessary to implement the piecemeal reading of the input file into memory. But I'm not very conversant with SAX parsers, and even less with generators, so all this may be unnecessary, or way off. Any other tricks/suggestions for turning a SAX parsers into a generator, please let me know. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: XML parsing: SAX/expat yield
In i3c7lc$e6v$0...@news.t-online.com Peter Otten __pete...@web.de writes: How about http://effbot.org/zone/element-iterparse.htm#incremental-parsing Exactly! Thanks! ~K -- http://mail.python.org/mailman/listinfo/python-list
how to pretty-print Python dict with unicode?
Is there a simple way to get Python to pretty-print a dict whose
values contain Unicode? (Of course, the goal here is that these
printed values are human-readable.)
If I run the following simple script:
from pprint import pprint
x = u'\u6c17\u304c\u9055\u3046'
print '{%s: %s}' % (u'x', x)
print {u'x': x}
pprint({u'x': x})
The first print statement produces perfectly readable Japanese,
but the remaining statements both produce the line
{u'x': u'\u6c17\u304c\u9055\u3046'}
I've tried everything I can think of (including a lot of crazy
stuff) short of re-writing pprint from scratch (which I think would
be faster than grokking it and hacking at it).
Isn't there an easier way to do this?
Thanks!
~K
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Re: ctypes' c_longdouble: underflow error (bug?)
In mailman.778.1279213534.1673.python-l...@python.org Thomas Jollans tho...@jollans.com writes: On 07/15/2010 06:41 PM, kj wrote: In mailman.733.1279124991.1673.python-l...@python.org Thomas Jollans tho...@jollans.com writes: http://docs.python.org/library/ctypes.html#fundamental-data-types c_longdouble maps to float Thanks for pointing this out! ~K (Does it make *any difference at all* to use c_longdouble instead of c_double? If not, I wonder what's the point of having c_longdouble at all.) they're still different types (in C). i understand that doubles and long doubles are different in C. on my machine, a double is 64 bits wide, while a long double is 129 bits wide. This means that a function that expects a long double argument will expect 16 bytes, but ctypes will only pass 8 bytes if you tell it to pass double. The same applies to return values. This is extremely confusing. From my naive reading of the documentation, I would have expected that the following two blocks would produce identical results (expl is one of the standard C math library exponential functions, with signature long double expl(long double)): MATH.expl.argtypes = [c_longdouble] MATH.expl.restype = c_longdouble print MATH.expl(0) MATH.expl.argtypes = [c_double] MATH.expl.restype = c_double print MATH.expl(0) ...but no, they don't: the first one prints out the correct result, 1.0, while the second one prints out 0.0, of all things. (In fact, with the second (mis)configuration, the value returned by MATH.expl is always equal to its argument, go figure.) I find these results perplexing because, based on the docs, I expected that they *both* would be analogous to doing the following in C: printf(%f\n, (double) expl((double) 0.0)); /* prints out 1.00 */ i.e., in *both* cases, expl would get passed a double (which gets automatically cast into a long double), and in both cases its returned value would be cast into a double. Clearly, this is not what's happening, but I can't figure out the correct interpreation of what's going on based on the documentation... -- http://mail.python.org/mailman/listinfo/python-list
Re: ctypes' c_longdouble: underflow error (bug?)
In mailman.733.1279124991.1673.python-l...@python.org Thomas Jollans tho...@jollans.com writes: http://docs.python.org/library/ctypes.html#fundamental-data-types c_longdouble maps to float Thanks for pointing this out! ~K (Does it make *any difference at all* to use c_longdouble instead of c_double? If not, I wonder what's the point of having c_longdouble at all.) -- http://mail.python.org/mailman/listinfo/python-list
Q for Emacs users: code-folding (hideshow)
This is a question _for Emacs users_ (the rest of you, go away :) ). How do you do Python code-folding in Emacs? Thanks! ~K -- http://mail.python.org/mailman/listinfo/python-list
ctypes' c_longdouble: underflow error (bug?)
I have a C library function hg that returns a long double, so when I import it using C types I specify this return type like this: MYLIB.hg.restype = ctypes.c_longdouble But certain non-zero values returned by hg appear as zero Python-side. If I modify hg so that it prints out its value right before returning it, I get stuff like the following: 0 == MYLIB.hg(100, 200, 100, 6000) from hg: 2.96517e-161 False 0 == MYLIB.hg(200, 200, 200, 6000) from hg: 5.28791e-380 True So, although the value returned by hg in the second invocation above is 5.28791e-380, Python sees it as 0. What am I doing wrong? TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
Numerics question
I define ninv = 1.0/n ...where n is some integer, and I want to write some function f such that f(m * ninv) returns the smallest integer that is = m * ninv, where m is some other integer. And, in particular, if m is p*n for some integer p, then f((p*n) * ninv) should return the integer p. The first solution that comes to mind is something like def f(x): return int(math.ceil(x)) At first this seems to work: f((7*2) * (1.0/2)) 7 f((7*3) * (1.0/3)) 7 ...but there are values of n for which it fails: f((7*75) * (1.0/75)) 8 The problem here is that, due to numerical error, the expression ((7*75) * (1.0/75)) evaluates to a number *just* above 7. The surrounding math.ceil then turns this into 8.0, etc. Is there a way to define f so that it behaves as expected? TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: Numerics question
Please disregard my ineptly posed question. ~K In i0l9f3$7d...@reader1.panix.com kj no.em...@please.post writes: I define ninv = 1.0/n ...where n is some integer, and I want to write some function f such that f(m * ninv) returns the smallest integer that is = m * ninv, where m is some other integer. And, in particular, if m is p*n for some integer p, then f((p*n) * ninv) should return the integer p. The first solution that comes to mind is something like def f(x): return int(math.ceil(x)) At first this seems to work: f((7*2) * (1.0/2)) 7 f((7*3) * (1.0/3)) 7 ...but there are values of n for which it fails: f((7*75) * (1.0/75)) 8 The problem here is that, due to numerical error, the expression ((7*75) * (1.0/75)) evaluates to a number *just* above 7. The surrounding math.ceil then turns this into 8.0, etc. Is there a way to define f so that it behaves as expected? TIA! ~K -- http://mail.python.org/mailman/listinfo/python-list
tallying occurrences in list
Task: given a list, produce a tally of all the distinct items in
the list (for some suitable notion of distinct).
Example: if the list is ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b',
'c', 'a'], then the desired tally would look something like this:
[('a', 4), ('b', 3), ('c', 3)]
I find myself needing this simple operation so often that I wonder:
1. is there a standard name for it?
2. is there already a function to do it somewhere in the Python
standard library?
Granted, as long as the list consists only of items that can be
used as dictionary keys (and Python's equality test for hashkeys
agrees with the desired notion of distinctness for the tallying),
then the following does the job passably well:
def tally(c):
t = dict()
for x in c:
t[x] = t.get(x, 0) + 1
return sorted(t.items(), key=lambda x: (-x[1], x[0]))
But, of course, if a standard library solution exists it would be
preferable. Otherwise I either cut-and-paste the above every time
I need it, or I create a module just for it. (I don't like either
of these, though I suppose that the latter is much better than the
former.)
So anyway, I thought I'd ask. :)
~K
--
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Re: tallying occurrences in list
Thank you all! ~K -- http://mail.python.org/mailman/listinfo/python-list
atexit/signal for non-interactive jobs
I want to implement clean-up functions for scripts to be run on a
Linux cluster (through LSF). The goal is to make sure that a
minimal wrap-up sequence (print diagnostic info, flush buffers,
etc.) gets executed if the job is terminated for some reason. (The
most common reason for premature termination is a SIGUSR2 signal,
sent to the process by LSF when the job has taken longer than the
time limit for the job's LSF queue.)
No matter what I try, I can't get this wrap-up sequence to work
when the script runs non-interactively.
The latest I have looks something like this:
import signal
import atexit
def set_handlers(handler):
atexit.register(handler)
for sig in dir(signal):
if sig.startswith('SIG') and not '_' in sig:
signal(getattr(signal, sig), handler)
def wrapup(*args):
# etc, etc
def main():
# ...
set_handlers(wrapup)
run()
main()
If I start the script interactively and after a few seconds (i.e.
before it terminates) I hit Ctrl-C (which sends a TERM signal to
the process), the wrapup function gets called as desired (although
this action appears to be triggered by atexit, and not by the
handler for SIGTERM, which apparently never gets called).
But if instead I background the script after starting it, and then
use kill to explicitly send a TERM signal to its process, the script
terminates, but the wrapup code does not execute.
It makes no sense to me. If someone cares to explain it, I'd be
very grateful. (Is my code doing something wrong?)
But more to the point, what must I do to get this to work in the
non-interactive case?
TIA!
~k
--
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Re: Limitation of os.walk
In mailman.86.1273631889.32709.python-l...@python.org Tim Chase writes: 05/11/2010 09:07 PM, Terry Reedy wrote: If os.walk were rewritten, it should be as an iterator (generator). Directory entry and exit functions could still be added as params. It *is* an iterator/generator. However, I suspect you mean that it should slurp the dirs/files iteratively instead of using listdir() as was discussed on c.l.p a few months back. Thanks for mentioning this thread. Very interesting stuff. Apropos the implementability of an iterative listdir, I wonder if some variation of glob.iglob() would fit the bill. (Maybe it's too slow, though.) I suspect if I thought about it much longer, only one would really be needed, the other accommodated by the topdown parameter. Yeah, I think one only needs a post hook. The fact that it's a generator obviates need for a pre hook, since the yield returns control to the calling function right around where the pre-hook would run anyway. For the same reason, the post hook is needed only for the case topdown=True. Another useful callback would be one that replaced listdir in the generation of the current directory's contents. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: Limitation of os.walk
In mailman.82.1273630064.32709.python-l...@python.org Terry Reedy tjre...@udel.edu writes: On 5/11/2010 3:49 PM, kj wrote: PS: I never understood why os.walk does not support hooks for key events during such a tree traversal. Either 1) it is intentionally simple, with the expectation that people would write there own code for more complicated uses or 2) no one has submitted a 'full-featured' version or 3) both. I hope it's not (1): I want the language I use to include more batteries not fewer. petpeeveIt seems that a similar simplicity argument was invoked to strip the cmp option from sort in Python 3. G. Simplicity is great, but when the drive for it starts causing useful functionality to be thrown out, then it is going too far. Yes, I know that it is possible to do everything that sort's cmp option does through clever tricks with key, but grokking and coding this maneuver requires a *lot* more Python-fu than cmp did, which makes this functionality a lot less accessible to beginners that the intrinsic complexity of the problem warrants. And for what? To get rid of an *option* that could be easily disregarded by anyone who found it too complex? It makes no sense to me./petpeeve -- http://mail.python.org/mailman/listinfo/python-list
Limitation of os.walk
I want implement a function that walks through a directory tree and performs an analsysis of all the subdirectories found. The task has two essential requirements that, AFAICT, make it impossible to use os.walk for this: 1. I need to be able to prune certain directories from being visited. 2. The analysis on each directory can be performed only after it has been performed on all its subdirectories. Unless I'm missing something, to do (1), os.walk must be run with topdown=True, whereas to do (2) it must be run with topdown=False. Is there a work around that I'm missing? TIA! ~K PS: I never understood why os.walk does not support hooks for key events during such a tree traversal. -- http://mail.python.org/mailman/listinfo/python-list
Re: Limitation of os.walk
In mailman.75.1273614838.32709.python-l...@python.org Tim Chase python.l...@tim.thechases.com writes: That said, the core source for os.walk() is a whole 23 lines of code, it's easy enough to just clone it and add what you need... Thanks, that was a good idea. ~K -- http://mail.python.org/mailman/listinfo/python-list
Inheritable computed class attributes?
I want to define a class attribute that is computed from other class attributes. Furthermore, this attribute should be inheritable, and its value in the subclasses should reflect the subclasses values of the attributes used to compute the computed attribute. I tried the following: class Spam(object): X = 3 @property @classmethod def Y(cls): return cls.X * 3 ...but Spam.Y returns property object at 0x.., rather than 9. How can I define a class property? Is it possible at all? Ultimately, I'd like to be able to define multiple subclasses of Spam, e.g. class Ham(Spam): X = 7 class Eggs(Spam): X = '.' and have Ham.Y and Eggs.Y evaluate to 21 and '...', respectively. Thanks! ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: Inheritable computed class attributes?
In 4bdb4e4...@dnews.tpgi.com.au Lie Ryan lie.1...@gmail.com writes: class MetaSpam(type): @property def Y(cls): return cls.X * 3 class Spam(object): __metaclass__ = MetaSpam and there we go: class Ham(Spam): ... X = 7 ... class Eggs(Spam): ... X = '.' ... Ham.Y; Eggs.Y 21 '...' !!!hmmm!!! That's very interesting! I did not know about metaclasses; I need to learn more about them. Thanks for the pointer! ~K -- http://mail.python.org/mailman/listinfo/python-list
Looking for registration package
I'm looking for a Python-based, small, self-contained package to hand out API keys, in the same spirit as Google API keys. The basic specs are simple: 1) enforce the one key per customer rule; 2) be robot-proof; 3) be reasonably difficult to circumvent even for humans. (This is for a web service we would like to implement; the goal is to be able to control the load on our servers. Therefore, if the package includes an automated log-analysis component, all the better, but this is not necessary.) Any suggestions would be appreciated. Thanks! ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: How to access args as a list?
In 4bb802f7$0$8827$c3e8...@news.astraweb.com Steven D'Aprano
st...@remove-this-cybersource.com.au writes:
On Sat, 03 Apr 2010 22:58:43 +, kj wrote:
Suppose I have a function with the following signature:
def spam(x, y, z):
# etc.
Is there a way to refer, within the function, to all its arguments as a
single list? (I.e. I'm looking for Python's equivalent of Perl's @_
variable.)
Does this help?
def spam(a, b, c=3, d=4):
... pass
...
spam.__code__.co_varnames
('a', 'b', 'c', 'd')
That's very handy. Thanks!
The hardest part is having the function know its own name.
Indeed. Why Python does not provide this elmentary form of
introspection as a built-in variable is extremely puzzling to me
(even--no, *more so*--after reading PEP 3130).
I see that you are already using the inspect module. That almost
certainly is the correct approach. I'd be surprised if inspect is too
heavyweight, but if it is, you can pull out the bits you need into your
own function.
That's a good idea. Thanks!
~K
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How to access args as a list?
Suppose I have a function with the following signature: def spam(x, y, z): # etc. Is there a way to refer, within the function, to all its arguments as a single list? (I.e. I'm looking for Python's equivalent of Perl's @_ variable.) I'm aware of locals(), but I want to preserve the order in which the arguments appear in the signature. My immediate aim is to set up a simple class that will allow me to iterate over the arguments passed to the constructor (plus let me refer to these individual arguments by their names using an instance.attribute syntax, as usual). The best I have managed looks like this: class _Spam(object): def __init__(self, x, y, z): self.__dict__ = OrderedDict(()) for p in inspect.getargspec(_Spam.__init__).args[1:]: self.__dict__[p] = locals()[p] def __iter__(self): return iter(self.__dict__.values()) but rolling out inspect.getargspec for this sort of thing looks to me like overkill. Is there a more basic approach? P.S. this is just an example; the function I want to implement has more parameters in its signature, with longer, more informative names. -- http://mail.python.org/mailman/listinfo/python-list
Re: How to access args as a list?
In hp8h73$k1...@reader1.panix.com kj no.em...@please.post writes: Suppose I have a function with the following signature: def spam(x, y, z): # etc. Is there a way to refer, within the function, to all its arguments as a single list? (I.e. I'm looking for Python's equivalent of Perl's @_ variable.) I'm aware of locals(), but I want to preserve the order in which the arguments appear in the signature. My immediate aim is to set up a simple class that will allow me to iterate over the arguments passed to the constructor (plus letS me refer to these individual arguments by their names using an ^^^ instance.attribute syntax, as usual). ^ The underlined portion explains why __init__(self, *args) fails to fit the bill. P.S. this is just an example; the function I want to implement has more parameters in its signature, with longer, more informative names. Andreas, perhaps this paragraph explains why I find your solution unappealing: it requires typing the same thing over and over, which increases the chances of bugs. That's the reason I avoid such repetitiveness, not laziness, as you so were so quick to accuse me of. ~K -- http://mail.python.org/mailman/listinfo/python-list
Re: How to access args as a list?
In hp8kc9$dg...@reader1.panix.com kj no.em...@please.post writes: In hp8h73$k1...@reader1.panix.com kj no.em...@please.post writes: Suppose I have a function with the following signature: def spam(x, y, z): # etc. Is there a way to refer, within the function, to all its arguments as a single list? (I.e. I'm looking for Python's equivalent of Perl's @_ variable.) I'm aware of locals(), but I want to preserve the order in which the arguments appear in the signature. My immediate aim is to set up a simple class that will allow me to iterate over the arguments passed to the constructor (plus letS me refer to these individual arguments by their names using an ^^^ instance.attribute syntax, as usual). ^ The underlined portion explains why __init__(self, *args) fails to fit the bill. The minute I hit send I realized that this is wrong. Sorry. Thanks. -- http://mail.python.org/mailman/listinfo/python-list
