Newbie questions on Python
Hi all, i'm programming in python for the first time (usually i use C as programming language). I don't understand these results: a=[1,2,3,4,5] a[:-1] [1, 2, 3, 4] a[::-1] [5, 4, 3, 2, 1] a[2::-1] [3, 2, 1] what does a[2::-1] means? Thanks -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie questions on Python
On Wed, Apr 17, 2013 at 1:20 AM, idkfaidkfaid...@gmail.com wrote: Hi all, i'm programming in python for the first time (usually i use C as programming language). I don't understand these results: a=[1,2,3,4,5] a[:-1] [1, 2, 3, 4] a[::-1] [5, 4, 3, 2, 1] a[2::-1] [3, 2, 1] what does a[2::-1] means? That's taking a slice. This page has something to say on the subject: http://docs.python.org/3.3/tutorial/introduction.html By the way, regarding your email address: there are no cheat codes in Python... either that, or Python *is* a cheat code. :) ChrisA [[ VERY HAPPY CODING ADDED ]] -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie questions on Python
When slicing: l[start:end:step] In your example of a[2::-1] you are reversing the list by using a step of -1, then you are slicing at index 2 (third element). *Matt Jones* On Tue, Apr 16, 2013 at 10:30 AM, Chris Angelico ros...@gmail.com wrote: On Wed, Apr 17, 2013 at 1:20 AM, idkfaidkfaid...@gmail.com wrote: Hi all, i'm programming in python for the first time (usually i use C as programming language). I don't understand these results: a=[1,2,3,4,5] a[:-1] [1, 2, 3, 4] a[::-1] [5, 4, 3, 2, 1] a[2::-1] [3, 2, 1] what does a[2::-1] means? That's taking a slice. This page has something to say on the subject: http://docs.python.org/3.3/tutorial/introduction.html By the way, regarding your email address: there are no cheat codes in Python... either that, or Python *is* a cheat code. :) ChrisA [[ VERY HAPPY CODING ADDED ]] -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie questions on Python
On 2013-04-16, idkfaidkfaid...@gmail.com idkfaidkfaid...@gmail.com wrote:
Hi all,
i'm programming in python for the first time (usually i use C as programming
language). I don't understand these results:
a=[1,2,3,4,5]
a[:-1]
[1, 2, 3, 4]
a[::-1]
[5, 4, 3, 2, 1]
a[2::-1]
[3, 2, 1]
The third item is the step. The default value is 1. If you
provide a negative step, your slice will be in reverse. So you
are getting item 2 through 0 in reverse order in your result
slice.
Imagine something like the following for loop taking place
somewhere:
for (int i = 2; i = 0; --i) {
fprintf(a[i]);
}
--
Neil Cerutti
--
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Re: Newbie questions on Python
Neil Cerutti ne...@norwich.edu writes:
Imagine something like the following for loop taking place
somewhere:
for (int i = 2; i = 0; --i) {
fprintf(a[i]);
}
Neil most probably meant
for (int i = 2; i = 0; --i) {
fprintf(a[i]);
}
where fprintf is actually a fictitious do_something function.
ciao, lele.
--
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
l...@metapensiero.it | -- Fortunato Depero, 1929.
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Re: Newbie questions on Python
On 2013-04-16, Lele Gaifax l...@metapensiero.it wrote:
Neil Cerutti ne...@norwich.edu writes:
Imagine something like the following for loop taking place
somewhere:
for (int i = 2; i = 0; --i) {
fprintf(a[i]);
}
Neil most probably meant
for (int i = 2; i = 0; --i) {
fprintf(a[i]);
}
where fprintf is actually a fictitious do_something function.
ciao, lele.
Thanks for the correction.
--
Neil Cerutti
--
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Re: Newbie questions on Python
On Wed, 17 Apr 2013 01:30:03 +1000, Chris Angelico wrote: By the way, regarding your email address: there are no cheat codes in Python ROFLMAO. Incidentally, my son used to use IDDQD rather than IDKFA. I of course spurned all such, since I preferred to do it the hard way. Thus I was Doomed. -- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie questions on Python
On Wed, Apr 17, 2013 at 7:40 AM, Walter Hurry walterhu...@lavabit.com wrote: On Wed, 17 Apr 2013 01:30:03 +1000, Chris Angelico wrote: By the way, regarding your email address: there are no cheat codes in Python ROFLMAO. Incidentally, my son used to use IDDQD rather than IDKFA. I of course spurned all such, since I preferred to do it the hard way. Thus I was Doomed. I'd sometimes use IDDQD/IDFA (no K) and then see how quickly I could blitz the levels, with proper navigation. There's something inherently fun about blasting through everything with the rocket launcher. Mind you, I was really cheap, so I never bought Doom... and was just playing through the demo over and over. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
Peter Pearson wrote: On Sat, 09 Oct 2010 19:30:16 -0700, Ethan Furman et...@stoneleaf.us wrote: Steven D'Aprano wrote: [snip] But that doesn't mean that the list comp is the general purpose solution. Consider the obvious use of the idiom: def func(arg, count): # Initialise the list. L = [arg for i in range(count)] # Do something with it. process(L, some_function) def process(L, f): # Do something with each element. for item in enumerate(L): f(item) Looks good, right? But it isn't, because it will suffer the exact same surprising behaviour if f modifies the items in place. Using a list comp doesn't save you if you don't know what the object is. I've only been using Python for a couple years on a part-time basis, so I am not aquainted with this obvious use -- could you give a more concrete example? Also, I do not see what the list comp has to do with the problem in process() -- the list has already been created at that point, so how is it the list comp's fault? Well, here's a worked example of Steven D's code (Python 2.5.2): def func(arg, count): ... L = [arg for i in range(count)] ... process(L, some_function) ... def process(L, v): ... for item in L: ... v(item) ... def some_function(x): ... x.append(1) ... print x ... func([], 3) [1] [1, 1] [1, 1, 1] Is that the output you expected? Probably not: the unwary reader (including me, not too long ago) expects that L = [arg for i in range(count)] will be equivalent to L = [[], [], []] but it's not, because the three elements in the first L are three references to the *same* list. Observe: arg = [] L = [arg for i in range(3)] L [[], [], []] L[0].append(1) L [[1], [1], [1]] ... as opposed to ... L = [ [] for i in range(3)] L [[], [], []] L[0].append(1) L [[1], [], []] After that many replies and dozen of solutions I wonder if the OP is puzzeled or satisfied. Just saying ... :D JM -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Sun, 10 Oct 2010 18:36:27 -0700, Ethan Furman et...@stoneleaf.us wrote: [snip] My question is more along the lines of: a mutable object was passed in to func()... what style of loop could be used to turn that one object into /n/ distinct objects? A list comp won't do it, but neither will a for loop, nor a while loop. Ah. Quite right. So, Yes, what you want is more objects that in some sense are copies of the original object. This is getting way over my head, because there are situations in which it's hard to say what would constitute a copy of an object, such as when the object in question is a chat session with a specific interlocutor. -- To email me, substitute nowhere-spamcop, invalid-net. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Sat, 09 Oct 2010 19:30:16 -0700, Ethan Furman et...@stoneleaf.us wrote: Steven D'Aprano wrote: [snip] But that doesn't mean that the list comp is the general purpose solution. Consider the obvious use of the idiom: def func(arg, count): # Initialise the list. L = [arg for i in range(count)] # Do something with it. process(L, some_function) def process(L, f): # Do something with each element. for item in enumerate(L): f(item) Looks good, right? But it isn't, because it will suffer the exact same surprising behaviour if f modifies the items in place. Using a list comp doesn't save you if you don't know what the object is. I've only been using Python for a couple years on a part-time basis, so I am not aquainted with this obvious use -- could you give a more concrete example? Also, I do not see what the list comp has to do with the problem in process() -- the list has already been created at that point, so how is it the list comp's fault? Well, here's a worked example of Steven D's code (Python 2.5.2): def func(arg, count): ... L = [arg for i in range(count)] ... process(L, some_function) ... def process(L, v): ... for item in L: ... v(item) ... def some_function(x): ... x.append(1) ... print x ... func([], 3) [1] [1, 1] [1, 1, 1] Is that the output you expected? Probably not: the unwary reader (including me, not too long ago) expects that L = [arg for i in range(count)] will be equivalent to L = [[], [], []] but it's not, because the three elements in the first L are three references to the *same* list. Observe: arg = [] L = [arg for i in range(3)] L [[], [], []] L[0].append(1) L [[1], [1], [1]] ... as opposed to ... L = [ [] for i in range(3)] L [[], [], []] L[0].append(1) L [[1], [], []] -- To email me, substitute nowhere-spamcop, invalid-net. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
Peter Pearson wrote: On Sat, 09 Oct 2010 19:30:16 -0700, Ethan Furman et...@stoneleaf.us wrote: Steven D'Aprano wrote: [snip] But that doesn't mean that the list comp is the general purpose solution. Consider the obvious use of the idiom: def func(arg, count): # Initialise the list. L = [arg for i in range(count)] # Do something with it. process(L, some_function) def process(L, f): # Do something with each element. for item in enumerate(L): f(item) Looks good, right? But it isn't, because it will suffer the exact same surprising behaviour if f modifies the items in place. Using a list comp doesn't save you if you don't know what the object is. I've only been using Python for a couple years on a part-time basis, so I am not aquainted with this obvious use -- could you give a more concrete example? Also, I do not see what the list comp has to do with the problem in process() -- the list has already been created at that point, so how is it the list comp's fault? Well, snip the unwary reader (including me, not too long ago) expects that L = [arg for i in range(count)] will be equivalent to L = [[], [], []] but it's not, because the three elements in the first L are three references to the *same* list. Observe: snip My question is more along the lines of: a mutable object was passed in to func()... what style of loop could be used to turn that one object into /n/ distinct objects? A list comp won't do it, but neither will a for loop, nor a while loop. Further, Steven stated it will suffer the exact same surprising behaviour if f modifies the items in place -- the damage has already been done by that point, as L has however many copies of the *same* object. Seems to me you would have to use copy.copy or copy.deepcopy to be safe in such a circumstance, and the loop style is irrelevant. If I've missed something, somebody please enlighten me. ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Thu, 07 Oct 2010 18:34:58 -0700, alex23 wrote: On Oct 8, 10:27 am, Steven D'Aprano st...@remove-this- cybersource.com.au wrote: v = [0 for i in range(20)] Absolutely not. Such a code snippet is very common, in fact I've done it myself, but it is a hammer solution -- to a small boy with a hammer, everything looks like a nail that needs hammering. Writing such a list comprehension is a list comp solution. v = [0] * 20 Yes, this is the solution. But the list comp version will work as expected for mutable types, whereas the 'solution' only works for immutable types. Yes, that is a good point. Repeating the same mutable object may give surprising results. If anything, I feel like the list comp version is the correct solution because of its reliability, whereas the multiplication form feels like either a lucky naive approach or relies on the reader to know the type of the initialising value and its mutability. And how often do you have an list that you are creating where you don't know what items you have to initialise the list with? There are three common use-cases for the idiom the OP was describing: initialising a list with zeroes (or sometimes some other numeric value), initialising it with None, or creating a list-of-lists (or more rarely, a list-of-dicts). The first one is *far* more common than the next two. You are right to point out that the third case is a Python gotcha: [[]]*n doesn't behave as expected by the naive or inexperienced Python programmer. I should have mentioned it, and pointed out that in that case you do want a list comp [[] for i in range(n)]. But that doesn't mean that the list comp is the general purpose solution. Consider the obvious use of the idiom: def func(arg, count): # Initialise the list. L = [arg for i in range(count)] # Do something with it. process(L, some_function) def process(L, f): # Do something with each element. for item in enumerate(L): f(item) Looks good, right? But it isn't, because it will suffer the exact same surprising behaviour if f modifies the items in place. Using a list comp doesn't save you if you don't know what the object is. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
alex23 wuwe...@gmail.com writes: If anything, I feel like the list comp version is the correct solution because of its reliability, whereas the multiplication form feels like either a lucky naive approach or relies on the reader to know the type of the initialising value and its mutability. Other than list comp being slower, I'd like to point out that in some cases the multiplication is far from being naive. Consider this pattern: def in_groups_of(n, iterable): Yield items of iterable packed in tuples of size n. return itertools.izip(*[iter(iterable)] * n) for a, b, c in in_groups_of(3, xrange(9)): ... print a, b, c ... 0 1 2 3 4 5 6 7 8 In the call to itertools.izip we are actually counting on list repetition to refer to the same object. Rewriting it as the list comprehension would break it: def in_groups_of(n, iterable): return itertools.izip(*[iter(iterable) for _ in xrange(n)]) for a, b, c in in_groups_of(3, xrange(9)): ... print a, b, c ... 0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
Steven D'Aprano wrote: And how often do you have an list that you are creating where you don't know what items you have to initialise the list with? [snip] You are right to point out that the third case is a Python gotcha: [[]]*n doesn't behave as expected by the naive or inexperienced Python programmer. I should have mentioned it, and pointed out that in that case you do want a list comp [[] for i in range(n)]. But that doesn't mean that the list comp is the general purpose solution. Consider the obvious use of the idiom: def func(arg, count): # Initialise the list. L = [arg for i in range(count)] # Do something with it. process(L, some_function) def process(L, f): # Do something with each element. for item in enumerate(L): f(item) Looks good, right? But it isn't, because it will suffer the exact same surprising behaviour if f modifies the items in place. Using a list comp doesn't save you if you don't know what the object is. I've only been using Python for a couple years on a part-time basis, so I am not aquainted with this obvious use -- could you give a more concrete example? Also, I do not see what the list comp has to do with the problem in process() -- the list has already been created at that point, so how is it the list comp's fault? ~Ethan~ -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
Rogério Brito wrote: class C: f = 1 def g(self): return f I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. If I make g return self.f instead, things work as expected, but the code loses some readability. Is there any way around this or is that simply a matter of life? class C: f =1 creates the 'C.f ' name. When 'f' is used in g, you'll get then an error. class C: f = 1 def g(self): return C.f is the obvious solution. However it can be slightly improved. f is a class attribute, meaning it's common to all instances of the C class. Thus g would be a class method, and is here declared liek a instance method (the instance being self). class C: f = 1 @classmethod def g(cls): return cls.f c1 = C() c2 = C() print c1.f, c2.f # f is not an attribute of c1 nor c2, thus the lookup will try in the class and find C.f 1 1 c1.f = 10 # this create the c1 instance attribute f != class attribute f c2.f = 20 # this create the c2 instance attribute f != class attribute f print c1.f, c2.f, c1.g(), c2.g(), C.f 10 20 1 1 1 Cheers, JM -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
Rogério Brito rbr...@ime.usp.br wrote in message news:i8lk0n$g3...@speranza.aioe.org... My first try to write it in Python was something like this: v = [] for i in range(20): v[i] = 0 Unfortunately, this doesn't work, as I get an index out of bounds when trying to index the v list. Python can't grow a list by assigning to out-of-bound elements (although, being the language it is, there is probably a way of achieving that by redefining how [] works...) What is the Pythonic way of writing code like this? So far, I have found many v = [0 for i in range(20)] v = [0] * 20 v = [] for i in range(20): v.append(0) What should I prefer? Any other alternative? v=[0,0,0,0,0, 0,0,0,0,0, 0,0,0,0,0, 0,0,0,0,0] will also work. But none of these appeal too much. I would probably do: def newlist(length,init=0): return [init]*length ... v=newlist(1000) (with the proviso that someone mentioned when the init value is complex: you might not get unique copies of each). If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). A sparse array? Even if an array could be constructed by assigning to arbitrary elements, the gaps created would still need filling in with None or Unassigned. 2 - If I declare a class with some member variables, is is strictly necessary This is where I bail out... -- Bartc -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 2010-10-08, BartC b...@freeuk.com wrote: Rogério Brito rbr...@ime.usp.br wrote in message news:i8lk0n$g3...@speranza.aioe.org... If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). A sparse array? Even if an array could be constructed by assigning to I agree, what the OP seems to want (whether he actually needs it or not) is a sparse array. A sparse array? Even if an array could be constructed by assigning to arbitrary elements, the gaps created would still need filling in with None or Unassigned. That is only true when attempting to address elements based on their position rather then something like rank. You could create an object with some kind of index attribute that you could search for built in. When you needed to access the element, you simply walked the list until you find the matching identifier. The spaces no longer need filling since you are not matching based on absolute position in the list. This is of course inefficient for large lists, and while there are several methods of making this method more efficient (indexing, skip linking, b-tree, etc), you can get what you want very efficiently most of the time by simply using the built in dictionary with integer keys instead of a list. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 2010-10-07, Rog??rio Brito rbr...@ime.usp.br wrote: If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). At first you say you want a list, later you say you want an array. They're two different things. Arrays are variable-length and can be heterogeneous. If what you really want is a fixed-length, homogeneous array, then use an array instead of a list: http://docs.python.org/library/array.html If I declare a class with some member variables, is is strictly necessary for me to qualify those members in a method in that class? Yes. I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. If I make g return self.f instead, things work as expected, but the code loses some readability. That's a matter of opinion. Some of us _like_ self.f since it explicitly shows the reader that f isn't a global or local but a class or instance variable. Any time you make the reader/maintainer guess what something is, that's a bug waiting to happen. Is there any way around this or is that simply a matter of life? Well, that's how Python works. I won't comment on life. -- Grant Edwards grant.b.edwardsYow! I feel like a wet at parking meter on Darvon! gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 2010-10-08, Grant Edwards inva...@invalid.invalid wrote: On 2010-10-07, Rog??rio Brito rbr...@ime.usp.br wrote: If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). At first you say you want a list, later you say you want an array. They're two different things. Arrays are variable-length and can be heterogeneous. I meant _Lists_ are fixed-length and homogeneous If what you really want is a fixed-length, homogeneous array, then use an array instead of a list: http://docs.python.org/library/array.html Actually, that's not the right link either. I was thinking more of NumPy arrays, where you can create an arbitrary sized homogeneous array of a desired type (either uninitialized or filled with zeros or ones): http://www.scipy.org/Tentative_NumPy_Tutorial#head-d3f8e5fe9b903f3c3b2a5c0dfceb60d71602cf93 If you're crunching so many numbers that initializing a list is a problem, then you probably ought to be using NumPy. -- Grant Edwards grant.b.edwardsYow! for ARTIFICIAL at FLAVORING!! gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 2010-10-08, Grant Edwards inva...@invalid.invalid wrote: On 2010-10-08, Grant Edwards inva...@invalid.invalid wrote: On 2010-10-07, Rog??rio Brito rbr...@ime.usp.br wrote: If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). At first you say you want a list, later you say you want an array. They're two different things. Arrays are variable-length and can be heterogeneous. I meant _Lists_ are fixed-length and homogeneous Damn. I should give up and go golfing. _Lists_ are variable-length and can be heterogenous. _Arrays_ are homogenous and sort-of fixed length. [...] I was thinking more of NumPy arrays, where you can create an arbitrary sized homogeneous array of a desired type (either uninitialized or filled with zeros or ones): http://www.scipy.org/Tentative_NumPy_Tutorial#head-d3f8e5fe9b903f3c3b2a5c0dfceb60d71602cf93 If you're crunching so many numbers that initializing a list is a problem, then you probably ought to be using NumPy. -- Grant Edwards grant.b.edwardsYow! I'm shaving!! at I'M SHAVING!! gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 10/8/2010 10:15 AM Grant Edwards said... Damn. I should give up and go golfing. +1 QOTW Emile -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Oct 7, 6:10 pm, Rogério Brito rbr...@ime.usp.br wrote:
Hi there.
I am used to some languages like C, but I am just a complete newbie with
Python
and, while writing some small snippets, I had encountered some problems, with
which I would sincerely appreciate any help, since I appreciate this language
to
write my running pseudocode in and I am seriously thinking of using it to
teach some algorithms classes.
1 - The first issue that I am having is that I don't seem to be able to, say,
use something that would be common for people writing programs in C: defining
a
one-dimensional vector and only initializing it when needed.
For instance, in C, I would write something like:
int v[20];
for (i = 0; i 20; i++)
v[i] = 0;
Note that I only define the vector v (and its size) at the beginning but
initialize it latter during the code per-se.
My first try to write it in Python was something like this:
v = []
for i in range(20):
v[i] = 0
Unfortunately, this doesn't work, as I get an index out of bounds when trying
to
index the v list. Of course, the main difference between the two snippets is
that, in C, I declared v to have 20 positions, while in python I initialized
it
to be the empty list and, indeed, it has an empty set of indexes.
What is the Pythonic way of writing code like this? So far, I have found many
alternatives and I would like to write code that others in the Python
community
would find natural to read. Some of the things that crossed my mind:
v = [0 for i in range(20)]
v = [0] * 20
v = []
for i in range(20): v.append(0)
What should I prefer? Any other alternative?
If possible, I would like to simply declare the list and fill it latter in my
program, as lazily as possible (this happens notoriously when one is using a
technique of programming called dynamic programming where initializing all
positions of a table may take too much time in comparison to the filling of
the
array).
2 - If I declare a class with some member variables, is is strictly necessary
for me to qualify those members in a method in that class? For instance, if I
define:
class C:
f = 1
def g(self):
return f
I get an annoying message when I try to call the g method in an object of type
C, telling me that there's no global symbol called f. If I make g return
self.f
instead, things work as expected, but the code loses some readability.
Is there any way around this or is that simply a matter of life?
I have some other questions, but I will save them for latter.
Please, keep in mind that I am a newbie in Python. Despite that, I am enjoying
the little that I know.
Thank you very much in advance,
--
Rogério Brito : rbr...@{ime.usp.br,gmail.com} : GPG key
4096R/BCFChttp://rb.doesntexist.org: Packages for LaTeX :
algorithms.berlios.de
DebianQA:http://qa.debian.org/developer.php?login=rbrito%40ime.usp.br
How about:
v = [None] * 20
That way, you're not initializing with an artifical value like 0.
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Re: Many newbie questions regarding python
On Oct 8, 10:27 am, Steven D'Aprano st...@remove-this- cybersource.com.au wrote: v = [0 for i in range(20)] Absolutely not. Such a code snippet is very common, in fact I've done it myself, but it is a hammer solution -- to a small boy with a hammer, everything looks like a nail that needs hammering. Writing such a list comprehension is a list comp solution. v = [0] * 20 Yes, this is the solution. But the list comp version will work as expected for mutable types, whereas the 'solution' only works for immutable types. If anything, I feel like the list comp version is the correct solution because of its reliability, whereas the multiplication form feels like either a lucky naive approach or relies on the reader to know the type of the initialising value and its mutability. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Oct 7, 7:10 pm, Rogério Brito rbr...@ime.usp.br wrote:
Hi there.
I am used to some languages like C, but I am just a complete newbie with
Python
and, while writing some small snippets, I had encountered some problems, with
which I would sincerely appreciate any help, since I appreciate this language
to
write my running pseudocode in and I am seriously thinking of using it to
teach some algorithms classes.
1 - The first issue that I am having is that I don't seem to be able to, say,
use something that would be common for people writing programs in C: defining
a
one-dimensional vector and only initializing it when needed.
For instance, in C, I would write something like:
int v[20];
for (i = 0; i 20; i++)
v[i] = 0;
Note that I only define the vector v (and its size) at the beginning but
initialize it latter during the code per-se.
My first try to write it in Python was something like this:
v = []
for i in range(20):
v[i] = 0
Unfortunately, this doesn't work, as I get an index out of bounds when trying
to
index the v list. Of course, the main difference between the two snippets is
that, in C, I declared v to have 20 positions, while in python I initialized
it
to be the empty list and, indeed, it has an empty set of indexes.
What is the Pythonic way of writing code like this? So far, I have found many
alternatives and I would like to write code that others in the Python
community
would find natural to read. Some of the things that crossed my mind:
v = [0 for i in range(20)]
v = [0] * 20
v = []
for i in range(20): v.append(0)
What should I prefer? Any other alternative?
If possible, I would like to simply declare the list and fill it latter in my
program, as lazily as possible (this happens notoriously when one is using a
technique of programming called dynamic programming where initializing all
positions of a table may take too much time in comparison to the filling of
the
array).
Just to emphasize what Andreas said:
While
v = [0] * 20
is nice and good,
don't do this
v = [ [] ] * 20
or this
v = [ {} ] * 20
until you have played around with it on the interactive prompt to
understand how it works.
The difference of behavior of mutable vs immutable objects is one of
the main pitfalls for beginners in Python. Everything is very
consistent once you understand the assignment semantics, but it really
confuses people accustomed to other languages that work differently.
--
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Re: Many newbie questions regarding python
On Thu, 7 Oct 2010 18:34:58 -0700 (PDT) alex23 wuwe...@gmail.com wrote: On Oct 8, 10:27 am, Steven D'Aprano st...@remove-this- cybersource.com.au wrote: v = [0 for i in range(20)] Absolutely not. Such a code snippet is very common, in fact I've done it myself, but it is a hammer solution -- to a small boy with a hammer, everything looks like a nail that needs hammering. Writing such a list comprehension is a list comp solution. v = [0] * 20 Yes, this is the solution. But the list comp version will work as expected for mutable types, whereas the 'solution' only works for immutable types. If anything, I feel like the list comp version is the correct solution because of its reliability, whereas the multiplication form feels like either a lucky naive approach or relies on the reader to know the type of the initialising value and its mutability. The correct solution is the one that works the way you want it to work (that's my definition, anyway). There is nothing lucky about building a list via [value]*count. It repeats (but not so much duplicates!) value count times. It is well defined (and, I think, justified) behavior. It ceases to be as useful when mutable types are involved, but so what? Use something else, then. And I think it is not too much to ask of a reader of Python to know that integers are immutable. There are enough pitfalls that make this knowledge rather important to have. (I'm actually not convinced that this works as an argument, but there you go.) TL;DR: Don't say correct, say appropriate. /W -- To reach me via email, replace INVALID with the country code of my home country. But if you spam me, I'll be one sour Kraut. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Oct 7, 4:10 pm, Rogério Brito rbr...@ime.usp.br wrote:
[snip]
v = [0 for i in range(20)]
v = [0] * 20
v = []
for i in range(20): v.append(0)
What should I prefer? Any other alternative?
The Pythonic way is to not to preinitialize the list at all. Don't
put anything in the list until you have the data you need.
If possible, I would like to simply declare the list and fill it latter in my
program, as lazily as possible (this happens notoriously when one is using a
technique of programming called dynamic programming where initializing all
positions of a table may take too much time in comparison to the filling of
the
array).
So, if I understand you, you are thinking of your list as a table with
dynamically calculated entries, and want to calculate the entries upon
request.
Three possibilities:
1. Initialize the list using v = [None]*n (I recomment using None
instead of 0 for this, in most cases)
2. Use a dict instead. Dict items pop into existence if you assign
with a key that doesn't exist.
v = {}
Then you can do
v[1] = a
v[10] = n
v[999] = c
3. Consider numpy, which allows preallocation of lists:
v = np.zeros(100)
[snip]
For instance, if I
define:
class C:
f = 1
def g(self):
return f
I get an annoying message when I try to call the g method in an object of type
C, telling me that there's no global symbol called f. If I make g return
self.f
instead, things work as expected, but the code loses some readability.
Is there any way around this or is that simply a matter of life?
Matter of life. It's that way by design.
Carl Banks
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Many newbie questions regarding python
Hi there.
I am used to some languages like C, but I am just a complete newbie with Python
and, while writing some small snippets, I had encountered some problems, with
which I would sincerely appreciate any help, since I appreciate this language to
write my running pseudocode in and I am seriously thinking of using it to
teach some algorithms classes.
1 - The first issue that I am having is that I don't seem to be able to, say,
use something that would be common for people writing programs in C: defining a
one-dimensional vector and only initializing it when needed.
For instance, in C, I would write something like:
int v[20];
for (i = 0; i 20; i++)
v[i] = 0;
Note that I only define the vector v (and its size) at the beginning but
initialize it latter during the code per-se.
My first try to write it in Python was something like this:
v = []
for i in range(20):
v[i] = 0
Unfortunately, this doesn't work, as I get an index out of bounds when trying to
index the v list. Of course, the main difference between the two snippets is
that, in C, I declared v to have 20 positions, while in python I initialized it
to be the empty list and, indeed, it has an empty set of indexes.
What is the Pythonic way of writing code like this? So far, I have found many
alternatives and I would like to write code that others in the Python community
would find natural to read. Some of the things that crossed my mind:
v = [0 for i in range(20)]
v = [0] * 20
v = []
for i in range(20): v.append(0)
What should I prefer? Any other alternative?
If possible, I would like to simply declare the list and fill it latter in my
program, as lazily as possible (this happens notoriously when one is using a
technique of programming called dynamic programming where initializing all
positions of a table may take too much time in comparison to the filling of the
array).
2 - If I declare a class with some member variables, is is strictly necessary
for me to qualify those members in a method in that class? For instance, if I
define:
class C:
f = 1
def g(self):
return f
I get an annoying message when I try to call the g method in an object of type
C, telling me that there's no global symbol called f. If I make g return self.f
instead, things work as expected, but the code loses some readability.
Is there any way around this or is that simply a matter of life?
I have some other questions, but I will save them for latter.
Please, keep in mind that I am a newbie in Python. Despite that, I am enjoying
the little that I know.
Thank you very much in advance,
--
Rogério Brito : rbr...@{ime.usp.br,gmail.com} : GPG key 4096R/BCFC
http://rb.doesntexist.org : Packages for LaTeX : algorithms.berlios.de
DebianQA: http://qa.debian.org/developer.php?login=rbrito%40ime.usp.br
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Re: Many newbie questions regarding python
On 08/10/2010 00:10, Rogério Brito wrote: Hi there. I am used to some languages like C, but I am just a complete newbie with Python and, while writing some small snippets, I had encountered some problems, with which I would sincerely appreciate any help, since I appreciate this language to write my running pseudocode in and I am seriously thinking of using it to teach some algorithms classes. 1 - The first issue that I am having is that I don't seem to be able to, say, use something that would be common for people writing programs in C: defining a one-dimensional vector and only initializing it when needed. For instance, in C, I would write something like: int v[20]; for (i = 0; i 20; i++) v[i] = 0; Note that I only define the vector v (and its size) at the beginning but initialize it latter during the code per-se. My first try to write it in Python was something like this: v = [] for i in range(20): v[i] = 0 Unfortunately, this doesn't work, as I get an index out of bounds when trying to index the v list. Of course, the main difference between the two snippets is that, in C, I declared v to have 20 positions, while in python I initialized it to be the empty list and, indeed, it has an empty set of indexes. What is the Pythonic way of writing code like this? So far, I have found many alternatives and I would like to write code that others in the Python community would find natural to read. Some of the things that crossed my mind: v = [0 for i in range(20)] v = [0] * 20 v = [] for i in range(20): v.append(0) What should I prefer? Any other alternative? If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). Python doesn't have declarations. The code: v = [] simply creates an empty list and binds the name 'v' to it. If you want to create a list containing 20 zeros then: v = [0] * 20 is the Pythonic way. Which one you do depends on the particular problem you're working on; do whichever makes the most sense. 2 - If I declare a class with some member variables, is is strictly necessary for me to qualify those members in a method in that class? For instance, if I define: class C: f = 1 def g(self): return f I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. If I make g return self.f instead, things work as expected, but the code loses some readability. Is there any way around this or is that simply a matter of life? The name 'f' in that case is an attribute of the class itself. If you want it to be an attribute of an instance of the class then do something like this: class C: def __init__(self): self.f = 1 def g(self): return self.f You should write Python code in the Python idiom and not try to look for the Python equivalent of a C idiom. In other words, don't try to write a C program in Python! You might want to read The Zen of Python. Just type: help(this) at the Python prompt. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Thu, 07 Oct 2010 20:10:14 -0300 Rogério Brito rbr...@ime.usp.br wrote: I am used to some languages like C, but I am just a complete newbie with Python and, while writing some small snippets, I had encountered some problems, with which I would sincerely appreciate any help, since I appreciate this language to write my running pseudocode in and I am seriously thinking of using it to teach some algorithms classes. Let me congratulate you on your choice. 1 - The first issue that I am having is that I don't seem to be able to, say, use something that would be common for people writing programs in C: defining a one-dimensional vector and only initializing it when needed. [snip] I could be cheeky here and ask Do you really need this? REALLY?. And I will: Really? ;) Note that I only define the vector v (and its size) at the beginning but initialize it latter during the code per-se. [snip] What is the Pythonic way of writing code like this? So far, I have found many alternatives and I would like to write code that others in the Python community would find natural to read. Some of the things that crossed my mind: v = [0 for i in range(20)] Pretty good. v = [0] * 20 This will break on you if you replace the zeros with mutable objects. Try it, you'll be unpleasantly surprised. I guess it's OK for numbers, though. Or tuples, if need be. v = [] for i in range(20): v.append(0) That's the older (pre list comp) way, I guess. No need for it anymore, unless the initialization is more complicated than this. If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). Nah, don't worry too much about performance. You can do that when your program is too slow. I know that this isn't too satisfying, but seriously: Try to write programs for humans, not for the computer. (I'm talking about Python here; in C it's certainly par for the course to try to think like a computer does.) 2 - If I declare a class with some member variables, is is strictly necessary for me to qualify those members in a method in that class? For instance, if I define: class C: f = 1 def g(self): return f I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. If I make g return self.f instead, things work as expected, but the code loses some readability. No it doesn't. (... pedagogical pause ...) Python kind of forces you to be *very* explicit about which f you mean. And this is a good thing. If you mean the global f, then say f. If you mean the instance attribute of the current instance then say self.f and if you mean the class attribute then say C.f (or, if you fear you're going to be renaming C a lot, self.__class__.f). Note here that self.f refers to the f as accessible by the specific *instance* of the class, *not* the class attribute C.f. If you don't change f on the instance, then they'll coincide, otherwise they won't. Example (warning: not Python 3 compatible!): [wilde...@localhost ~]$ ipython Python 2.5.2 (r252:60911, Sep 30 2008, 15:41:38) Type copyright, credits or license for more information. IPython 0.8.4 -- An enhanced Interactive Python. ? - Introduction and overview of IPython's features. %quickref - Quick reference. help - Python's own help system. object? - Details about 'object'. ?object also works, ?? prints more. In [1]: class C: ...: f = 1 ...: def g(self): ...: return self.f ...: ...: In [2]: c = C(); In [3]: print c.g() 1 In [4]: C.f = 2 In [5]: print c.g() 2 In [6]: c.f = 3 In [7]: print c.g() 3 In [8]: print C.f 2 Is there any way around this or is that simply a matter of life? Yes. I have some other questions, but I will save them for latter. Keep 'em coming. But be prepared for hearing get used to it many a time. Or, from the more harsh folks, don't try to write C in Python. :) Please, keep in mind that I am a newbie in Python. Despite that, I am enjoying the little that I know. Really? I never enjoy knowing just a little. ;) -- To reach me via email, replace INVALID with the country code of my home country. But if you spam me, I'll be one sour kraut. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Fri, 08 Oct 2010 00:46:41 +0100 MRAB pyt...@mrabarnett.plus.com wrote: In other words, don't try to write a C program in Python! Man, I'm good. :D /W -- To reach me via email, replace INVALID with the country code of my home country. But if you spam me, I'll be one sour kraut. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On 2010-10-07, Rogério Brito rbr...@ime.usp.br wrote: 1 - The first issue that I am having is that I don't seem to be able to, say, use something that would be common for people writing programs in C: defining a one-dimensional vector and only initializing it when needed. For instance, in C, I would write something like: int v[20]; for (i = 0; i 20; i++) v[i] = 0; Note that I only define the vector v (and its size) at the beginning but initialize it latter during the code per-se. You are reserving enough space to be defined as an array of 20 integers. Note that after you declare it, you *can* access those memory locations even before you initialize them. They therefore still contain something even if you have not yet defined what that something is. Note that uninitialized variables are a common C error. Python is dynamic. You never need to allocate memory for something, you simply assign something and the system takes care of the allocation for you. Before something is assigned, it cannot be addressed. Nothing takes up memory before it is assigned. Unfortunately, this doesn't work, as I get an index out of bounds when trying to index the v list. Of course, the main difference between the two snippets is that, in C, I declared v to have 20 positions, while in python I initialized it to be the empty list and, indeed, it has an empty set of indexes. What is the Pythonic way of writing code like this? So far, I have found many alternatives and I would like to write code that others in the Python community would find natural to read. Some of the things that crossed my mind: v = [0 for i in range(20)] v = [0] * 20 v = [] for i in range(20): v.append(0) What should I prefer? Any other alternative? It really would help to know what you are trying to with v. If we knew that, we might be able to make better suggestions. Usually in Python, there is no need to initialize elements in a list as you can simply add them as you need them. If possible, I would like to simply declare the list and fill it latter in my program, as lazily as possible (this happens notoriously when one is using a technique of programming called dynamic programming where initializing all positions of a table may take too much time in comparison to the filling of the array). What I *think* you *might* be looking for is a kind of sparse list. You can achieve this by using a dictionary with numeric keys rather then a list. Not that when you do this, you recreate the C problem of trying to access something that has not been allocated. You either need to make sure that a key exists before trying to access it or catch and handleKeyError. 2 - If I declare a class with some member variables, is is strictly necessary for me to qualify those members in a method in that class? For instance, if I define: class C: f = 1 def g(self): return f Note that you have assigned f to the class object and not to the instance objects. This is a common pitfall for new Python programmers. I suggest assigning f in the constructor unless you are really sure that you want to assign it to the class object which will be shared among the instance objects. I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. If I make g return self.f instead, things work as expected, but the code loses some readability. Is there any way around this or is that simply a matter of life? Each member functions has its own scope. The object itself is not part of this scope. Therefore, you must use the self object reference to access other members of the object containing the function. I don't really see the readability problem; but, you could work around it by creating a reference to the member at the start of the function: def g(self): f = self.f For this to work, f must be mutable. If f is not mutable then you can wrap f inside of a class in such a way that it is mutable. Do this at your own risk. In doing this, you are starting to play with fire. -- http://mail.python.org/mailman/listinfo/python-list
Re: Many newbie questions regarding python
On Thu, 07 Oct 2010 20:10:14 -0300, Rogério Brito wrote: What is the Pythonic way of writing code like this? So far, I have found many alternatives and I would like to write code that others in the Python community would find natural to read. Some of the things that crossed my mind: v = [0 for i in range(20)] Absolutely not. Such a code snippet is very common, in fact I've done it myself, but it is a hammer solution -- to a small boy with a hammer, everything looks like a nail that needs hammering. Writing such a list comprehension is a list comp solution. v = [0] * 20 Yes, this is the solution. But given your earlier explanation that you want to lazily initialise v, I don't think it applies, because it initialises it all at once. (Of course, such initialisation is cheap, but it does happen all at once.) If you are writing this: v = [] # declare v as an empty list do_something() do_something_else() v = [0]*20 # now initialise v before using it do_something_with(v) then such declarations are not necessary and are discouraged in Python. Having an unused, empty variable v floating around doing nothing is pointless in Python. Just write: do_something() do_something_else() v = [0]*20 # create v before using it do_something_with(v) v = [] for i in range(20): v.append(0) I would never write that literally. It's as bad as the list comp, only it takes up more space. It too also fails to be lazy initialisation. However, using append is the correct way when you have a situation where you need to dynamically grow the list, e.g.: v = [] for i in range(20): v.append(0) do_something_with(v) Two more alternatives: v = list(range(20)) # in Python 2.x you can leave out the call to list() or v = [] v.extend(range(20)) both initialise v to [0, 1, 2, 3, ... , 19] instead of [0, 0, ..., 0]. 2 - If I declare a class with some member variables, is is strictly necessary for me to qualify those members in a method in that class? For instance, if I define: In Python, we generally refer to attributes rather than members. class C: f = 1 def g(self): return f By the way, you have created a class attribute f which is shared by all instances. You probably want: class C: def __init__(self): self.f = 1 def g(self): return self.f I get an annoying message when I try to call the g method in an object of type C, telling me that there's no global symbol called f. No you don't. You get a message that there is no global NAME called f. You might think I'm being pedantic, but I'm not. If you're thinking in terms of C language, you probably think that there is a symbol table created by the compiler so that Python can look at a reference f and say oh, that's a member variable at compile time, and only the correct value needs to be looked up at runtime. But that's not Python's execution model. *Everything* in Python is looked up dynamically at runtime in namespaces. (The only exceptions are statements.) So when you write f, Python *cannot* know at compile time whether it is a local variable, a non-local, a global, an attribute (member) or something else. It must look the name up in one or more namespaces at runtime. (Of course, you might already know this, in which case, yes, I'm just being pedantic *grins* ) If I make g return self.f instead, things work as expected, but the code loses some readability. On the contrary, it increases readability, because it explicitly tells the reader that you are accessing an attribute f rather than a local variable. Remember that Python uses nested namespaces. Inside the method C.g above the namespaces that are searched are: local variables any non-local (nested) functions or closures (none in this example) global variables built-ins in that order. Each namespace is independent, and Python will never try to guess that f is an attribute of the instance or class unless you explicitly tell it so. For example, an attribute instance.len will never block access to the built-in function len(). (Actually, there is one exception... when a class statement is executed for the first time, creating the class, local variables of the class block are identified as attributes. This is a case of practicality beats purity, since otherwise Python would need extra syntax for creating methods and class attributes.) Attributes have their own search order: instance attributes class attributes attributes of any base classes When you refer to instance.attribute, the namespaces are searched in that order for the name attribute. The locals and globals are not searched. Hope this helps, -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Newbie questions for Python usage
Hello, I am fairly new to Python and after exploring the language some I havesome questions: I understand HOW to use the lambda operator, but WHY would you want to use it? Can anyone please give an example of WHY you would need it as opposed to just declaring a function either in the local scope, or outside? I would like to be able to declare as a member attribute a file object, however because there is no declaration of variable types like there is in C++, there doesnt seem to be a way to do this without first making a fobj = open() call. Is this true? Now for an os import question for Windows. I wish to automate the builds of VS.NET 2005 and I can do so by executing the os.system(...) command however I cannot see how to execute the vcvars32.cmd first to set environment variables and then execute the actual command-line for the build itself. Thanks for the help all! Caolan-- http://mail.python.org/mailman/listinfo/python-list
Re: Newbie questions for Python usage
Caolan wrote: 1. I understand HOW to use the lambda operator, but WHY would you want to use it? Can anyone please give an example of WHY you would need it as opposed to just declaring a function either in the local scope, or outside? you don't *need* it, because callback = lambda arg: expression is, for most practical purposes, the same thing as def callback(arg): return expression (the only difference is that the __name__ attribute for the function objects will differ; all lambdas are named lambda, while objects created by def have the original name.) however, in some cases, it's may be convenient to use the lambda form, for stylistic reasons. 2. I would like to be able to declare as a member attribute a file object, however because there is no declaration of variable types like there is in C++, there doesn’t seem to be a way to do this without first making a fobj = open(…) call. Is this true? not sure what you mean, really -- attributes are not typed, and there's no way to declare them. just assign to the attribute and be done with it. if you want to use a placeholder value, use None: class foo: def __init__(self): self.file = None # not opened yet def open(self, name): self.file = open(name) or class foo: file = None # shared placeholder def __init__(self): pass def open(self, name): self.file = open(name) Now for an os import question for Windows. I wish to automate the builds of VS.NET 2005 and I can do so by executing the os.system(...) command however I cannot see how to execute the vcvars32.cmd first to set environment variables and then execute the actual command-line for the build itself. there's no easy way to do that: environment variables set by a sub- process isn't available to the main process. the easiest way to do this might be to simply generate a short temporary BAT-file for each command, and do os.system() on that file: f = open(temp.bat, w) f.write(@call vcvars32.bat\n) f.write(cl ...\n) f.close() os.system(f.name) os.remove(f.name) /F -- http://mail.python.org/mailman/listinfo/python-list
RE: Newbie questions for Python usage
Title: Re: Newbie questions for Python usage
Thanks
Fred.
just assign to the attribute and be done
with it. if you want to use a placeholder value, use
None:
I thought I had tried that already but got an error.
I'll try it again, and as for the 2nd one, I was hoping to avoid the usage of
.cmd or .bat files altogether. Thanks!
-Caolan O'Domhnaill
From:
[EMAIL PROTECTED] on behalf of Fredrik
LundhSent: Tue 8/22/2006 4:08 PMTo:
python-list@python.orgSubject: Re: Newbie questions for Python
usage
Caolan wrote: 1. I understand HOW
to use the lambda operator, but WHY would
you want to use it? Can anyone
please give an example of WHY you
would need it as opposed to just
declaring a function either in the
local scope, or outside?you don't *need* it,
because callback = lambda arg:
_expression_is, for most practical purposes, the same thing
as def
callback(arg): return
_expression_(the only difference is that the __name__ attribute for the
functionobjects will differ; all lambdas are named "lambda", while
objectscreated by "def" have the original name.)however, in some
cases, it's may be convenient to use the lambda form,for stylistic
reasons. 2. I would like to be able to declare as
a member attribute a file object,
however because there is no declaration of variable
types like there is in C++, there
doesnt seem to be a way to do this
without first making a fobj = open() call. Is this true?not sure what
you mean, really -- attributes are not typed, and there'sno way to "declare"
them. just assign to the attribute and be done withit. if you
want to use a placeholder value, use None: class
foo: def
__init__(self):
self.file = None # not opened
yet def open(self,
name):
self.file = open(name)or class
foo: file = None # shared
placeholder def
__init__(self):
pass def open(self,
name):
self.file = open(name) Now for an os import question for Windows. I
wish to automate the builds of VS.NET 2005 and I can do so by executing
the os.system(...) command however I cannot see how to execute the
vcvars32.cmd first to set environment variables and then execute the
actual command-line for the build itself.there's no easy way to
do that: environment variables set by a sub-process isn't available to the
main process.the easiest way to do this might be to simply generate a
short temporaryBAT-file for each command, and do os.system() on that
file: f = open("temp.bat",
"w") f.write("@call
vcvars32.bat\n") f.write("cl
...\n") f.close()
os.system(f.name)
os.remove(f.name)/F--http://mail.python.org/mailman/listinfo/python-list
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