Re: once assigment in Python
Lorenzo Di Gregorio wrote:
Hello,
I've been using Python for some DES simulations because we don't need
full C speed and it's so much faster for writing models. During
coding I find it handy to assign a variable *unless it has been
already assigned*: I've found that this is often referred to as once
assigment.
The best I could come up with in Python is:
try:
variable
except NameError:
variable = method()
I wonder if sombody has a solution (trick, whatever ...) which looks
more compact in coding. Something like:
once(variable, method)
doesn't work, but it would be perfect. Of course I can preprocess the
Python code but an all-Python solution would be more handy.
Any suggestions?
Thx in advance,
Lorenzo
IMHO variables like what you describe are really data not program variables.
You might consider putting variables like these in a dictionary and then check
to see if the keys exist before assignment:
var_dict={}
#
# See if 'varname' initialized, if not it needs to be
#
if 'varname' not in var_dict:
var_dict[varname]=somevalue
-Larry
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Re: once assigment in Python
On 17 Sep., 16:54, Larry Bates [EMAIL PROTECTED] wrote:
IMHO variables like what you describe are really data not program variables.
You might consider putting variables like these in a dictionary and then check
to see if the keys exist before assignment:
var_dict={}
#
# See if 'varname' initialized, if not it needs to be
#
if 'varname' not in var_dict:
var_dict[varname]=somevalue
This is a good point!
I could write something like:
instantiate('component',function)
and have instantiate() to conditionally do:
instance['component']=Module(function)
Of course this means using:
instance['component'].method()
instead of just:
component.method()
A more annoying difficulty is with the fact that, while the scope of
ordinary variables is limited (function, package), the scope of
'component' would be global (the whole instance[]): having global
names could be pretty annoying in modeling a hierarchy. Anyway, some
scoping mechanism could be implemented over the global dictionary and
this could be a good price to pay to avoid other tricks.
Further suggestions?
Thank you!
Lorenzo
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Re: once assigment in Python
Lorenzo Di Gregorio wrote:
On 17 Sep., 16:54, Larry Bates [EMAIL PROTECTED] wrote:
IMHO variables like what you describe are really data not program variables.
You might consider putting variables like these in a dictionary and then
check
to see if the keys exist before assignment:
var_dict={}
#
# See if 'varname' initialized, if not it needs to be
#
if 'varname' not in var_dict:
var_dict[varname]=somevalue
This is a good point!
I could write something like:
instantiate('component',function)
and have instantiate() to conditionally do:
instance['component']=Module(function)
Of course this means using:
instance['component'].method()
instead of just:
component.method()
Well, you could do that. Or you could define a class, with a __getattr__
that returns the required default value in the case where no setting had
been made.
Then you can assign components (with their own hierarchies) to the
attributes of your top-level design ...
A more annoying difficulty is with the fact that, while the scope of
ordinary variables is limited (function, package), the scope of
'component' would be global (the whole instance[]): having global
names could be pretty annoying in modeling a hierarchy. Anyway, some
scoping mechanism could be implemented over the global dictionary and
this could be a good price to pay to avoid other tricks.
Further suggestions?
It's not actually that hard to create objects that allow you to access
their attributes using *either* attribute or indexing notation.
regards
Steve
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Steve Holden+1 571 484 6266 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
Sorry, the dog ate my .sigline
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once assigment in Python
Hello, I've been using Python for some DES simulations because we don't need full C speed and it's so much faster for writing models. During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. The best I could come up with in Python is: try: variable except NameError: variable = method() I wonder if sombody has a solution (trick, whatever ...) which looks more compact in coding. Something like: once(variable, method) doesn't work, but it would be perfect. Of course I can preprocess the Python code but an all-Python solution would be more handy. Any suggestions? Thx in advance, Lorenzo -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
This is one of the things that I often see people trying to do in Python, where the best solution is simply to understand how Python works and craft the code to work with the language. The problem, in my view, is not that you don't have a good way to do this once assignment operation, but that you are in a position where you don't know if a name has been assigned to or not. Yes, Python is a very dynamic language, but that doesn't mean you should exploit this to be so dynamic that you don't know the state of your own code at a given point. If you don't know that a name has been assigned to or not, there are only a few kinds of structures that lead to such a lack of state knowledge and all of them are avoidable. A common idiom might go along the lines of: if A: B = value Now, if A is false we might not have B defined at all, turning a condition into an error (a raised NameError if we try to use B. What we need to do is either find an alternative value, a default value to assign before all this, or do everything with the value inside the if block, and not conditionally create names. There is no good way to do your once assignment, and that isn't an accident. Your code is fragile if you don't know what variables you have. Depending on the type, there are some good solutions. if the value cannot be None or if its only numbers, for example, you can do this: B = B or value Now it only changes it if B is None or 0, depending on the case. This may or may not help you. If you use a default None, you can also just check for that identity explicitly. On 9/14/07, Lorenzo Di Gregorio [EMAIL PROTECTED] wrote: Hello, I've been using Python for some DES simulations because we don't need full C speed and it's so much faster for writing models. During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. The best I could come up with in Python is: try: variable except NameError: variable = method() I wonder if sombody has a solution (trick, whatever ...) which looks more compact in coding. Something like: once(variable, method) doesn't work, but it would be perfect. Of course I can preprocess the Python code but an all-Python solution would be more handy. Any suggestions? Thx in advance, Lorenzo -- http://mail.python.org/mailman/listinfo/python-list -- Read my blog! I depend on your acceptance of my opinion! I am interesting! http://ironfroggy-code.blogspot.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
In message [EMAIL PROTECTED], Lorenzo Di Gregorio wrote: During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. Why not just assign to it once at the beginning and be done with it? -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
Lorenzo Di Gregorio wrote: Hello, I've been using Python for some DES simulations because we don't need full C speed and it's so much faster for writing models. During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. The best I could come up with in Python is: try: variable except NameError: variable = method() I wonder if sombody has a solution (trick, whatever ...) which looks more compact in coding. Something like: once(variable, method) doesn't work, but it would be perfect. Of course I can preprocess the Python code but an all-Python solution would be more handy. Any suggestions? Without being fatuous, I would suggest you rethink your approach to the problem. Do your variables have default values? If not, what happens if the user does not set them and your code tries to refer to them? You seem to be trying to apply defaults if the user hasn't set a value, when the correct things to do is to apply the defaults *before the user gets a chance to do anything at all*. Then any changes made by the user will overwrite the defaults. Even better, if its practical, is to provide your functionality as one or more functions, whose definitions can set default values for keyword arguments. regards Steve -- Steve Holden+1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden Sorry, the dog ate my .sigline -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
Agree that what you are looking for may not be a good idea. So make sure you don't shoot yourself in the foot with it. You should probably look into your problem some more. def once(obj,attrname,value): ... if hasattr(obj,attrname): ... return ... else: ... setattr(obj,attrname,value) ... class Foo: ... pass ... foo = Foo() once(foo,a,1) foo.a 1 once(foo,b,2) foo.a 1 def m1(self): ... print i am m1 ... def m2(self): ... print i am m2 ... once(Foo,mx,m1) foo.mx() i am m1 once(Foo,mx,m2) foo.mx() i am m1 This is very generic code, but you could make it specific to a class and an attribute. If so look into properties. class Foo2: def _setMyAttr(self,value): if hasattr(self,_myAttr): return self._myAttr = value def _getMyAttr(self): if not hasattr(self,_myAttr): return somedefaultvalue return self._myAttr myAttr = property(_getMyAttr,_setMyAttr) Note also : stay away from __setattr__ until you know what you are doing and you know when to use self.__dict__ assignments. Painful personal experiences for me. -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
Lorenzo Di Gregorio wrote: I've been using Python for some DES simulations because we don't need full C speed and it's so much faster for writing models. During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. The best I could come up with in Python is: try: variable except NameError: variable = method() I wonder if sombody has a solution (trick, whatever ...) which looks more compact in coding. Something like: once(variable, method) doesn't work, but it would be perfect. Of course I can preprocess the Python code but an all-Python solution would be more handy. Any suggestions? You can use properties to implement lazy evaluation. Or you can rely on a naming convention: class Once(object): ... def __getattr__(self, name): ... if name.startswith(_calc_): ... raise AttributeError(No method to calculate attribute %r % name[6:]) ... value = getattr(self, _calc_ + name)() ... setattr(self, name, value) ... return value ... class A(Once): ... def _calc_foo(self): ... print calculating foo ... return 42 ... a = A() a.foo calculating foo 42 a.foo 42 a.bar Traceback (most recent call last): File stdin, line 1, in module File stdin, line 5, in __getattr__ File stdin, line 4, in __getattr__ AttributeError: No method to calculate attribute 'bar' a._calc_bar = lambda: bar-value a.bar 'bar-value' Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
On Fri, 14 Sep 2007 06:16:56 +, Lorenzo Di Gregorio wrote: Hello, I've been using Python for some DES simulations because we don't need full C speed and it's so much faster for writing models. During coding I find it handy to assign a variable *unless it has been already assigned*: I've found that this is often referred to as once assigment. I could see reasons for doing something like this at a module level or for attributes; such as setting default values when defaults are expensive to calculate. You could, as others have said, initialize the variable to a trivial value and then test whether it still held the trivial value later, but what's the point? The best I could come up with in Python is: try: variable except NameError: variable = method() I wonder if sombody has a solution (trick, whatever ...) which looks more compact in coding. Something like: once(variable, method) doesn't work, but it would be perfect. For module level variables you can do something like this: def once(symbol,method): g = globals() if symbol not in g: g[symbol] = method() You'd have to pass a symbol as a string, but that's no big deal. For local variables you're stuck with trying to catch UnboundLocalError. There's a way to do it by examining stack frames, but I don't really recommend it: it's inefficient, and the once assignment doesn't make as much sense for local variables. Carl Banks -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
Thank you very much for your suggestions! I'll try in the next days to elaborate a bit on the last two ones. By the way, the once assignment is not that evil if you use it for hardware modeling. Most hardware models look like: wire1 = function() instance component(input=wire1,output=wire2) result = function(wire2) When employing Python it's pretty straightforward to translate the instance to an object. instance = Component(input=wire1,output=wire2) Then you don't use instance *almost* anymore: it's an object which gets registered with the simulator kernel and gets called by reference and event-driven only by the simulator kernel. We might reuse the name for calling some administrative methods related to the instance (e.g. for reporting) but that's a pretty safe thing to do. Of course all this can be done during initialization, but there are some good reasons (see Verilog vs VHDL) why it's handy do be able to do it *anywhere*. The annoying problem was that every time the program flow goes over the assignment, the object gets recreated. Indeed Python itself is not a hardware modeling language, but I built some infrastructure to fill what I was missing and for quickly building up a functional prototype and testing some ideas Python is really excellent. Best Regards, Lorenzo -- http://mail.python.org/mailman/listinfo/python-list
Re: once assigment in Python
Lorenzo Di Gregorio [EMAIL PROTECTED] wrote: When employing Python it's pretty straightforward to translate the instance to an object. instance = Component(input=wire1,output=wire2) Then you don't use instance *almost* anymore: it's an object which gets registered with the simulator kernel and gets called by reference and event-driven only by the simulator kernel. We might reuse the name for calling some administrative methods related to the instance (e.g. for reporting) but that's a pretty safe thing to do. Of course all this can be done during initialization, but there are some good reasons (see Verilog vs VHDL) why it's handy do be able to do it *anywhere*. The annoying problem was that every time the program flow goes over the assignment, the object gets recreated. If you originally set, e.g., instance = None then using in your later code: instance = instance or Component(...) will stop the multiple creations. Other possibilities include using a compound name (say an.instance where 'an' is an instance of a suitable container class) and overriding the __new__ method of class Component so that it will not return multiple distinct objects with identical attributes. Has this *plain* name ever been previously assigned to anything at all is simply not a particularly good condition to test for (you COULD probably write a decorator that ensures that all uninitialized local variables of a function are instead initialized to None, but I'd DEFINITELY advise against such black magic). Alex -- http://mail.python.org/mailman/listinfo/python-list
