Re: function parameter scope python 2.5.2
J Kenneth King wrote: I was working on a program of some complexity recently and quickly caught the issue in my tests. I knew what was going on and fixed it expediently, but the behaviour confused me and I couldn't find any technical documentation on it so I figured I just didn't know what it was referred to in Python. Hence the post. :) Language Reference / Expressions / Primaries / Calls + Language Reference / Compound statements / Function definitions Hmm. Read by themselves, these are not as clear as they could be that what parameters get bound to are the argument objects. One really needs to have read the section on assignment statements first. -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
Peter Pearson <[EMAIL PROTECTED]> writes: > On Fri, 21 Nov 2008 10:12:08 -0500, J Kenneth King wrote: >> Steven D'Aprano <[EMAIL PROTECTED]> writes: >>> >>> I am curious why you thought that. What made you think Python should/did >>> make a copy of weird_obj.words when you pass it to a function? > [snip] >> Of course if there is any further reading on the subject, I'd appreciate >> some links. > > As one relatively new Python fan to another, I recommend following > this newsgroup. Many important aspects of Python that several books > failed to drive through my skull are very clearly (and repeatedly) > explained here. Hang around for a week, paying attention to posts > with subjects like "Error in Python subscripts" (made-up example), > and curse me if you don't find it greatly rewarding. I do lurk more often than I post and sometimes I help out people new to Python or new to programming in general. I know how helpful usenet can be and usually this group in particular is quite special. It's good advice to read before you post; quite often the question has been proposed and answered long before it came to your little head (not you in particular; just general "you"). In this case, I was simply lacking the terminology to find what I was looking for on the subject. In such cases turning to the community seems like a fairly reasonable way to find clarification. I've only been programming in Python specifically for two years or so now, so I hope I can be forgiven. Cheers. -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
On Fri, 21 Nov 2008 10:12:08 -0500, J Kenneth King wrote: > Steven D'Aprano <[EMAIL PROTECTED]> writes: >> >> I am curious why you thought that. What made you think Python should/did >> make a copy of weird_obj.words when you pass it to a function? [snip] > Of course if there is any further reading on the subject, I'd appreciate > some links. As one relatively new Python fan to another, I recommend following this newsgroup. Many important aspects of Python that several books failed to drive through my skull are very clearly (and repeatedly) explained here. Hang around for a week, paying attention to posts with subjects like "Error in Python subscripts" (made-up example), and curse me if you don't find it greatly rewarding. -- To email me, substitute nowhere->spamcop, invalid->net. -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
Steven D'Aprano <[EMAIL PROTECTED]> writes: > On Thu, 20 Nov 2008 18:31:12 -0500, J Kenneth King wrote: > >> Of course I expected that recursive_func() would receive a copy of >> weird_obj.words but it appears to happily modify the object. > > I am curious why you thought that. What made you think Python should/did > make a copy of weird_obj.words when you pass it to a function? > > This is a serious question, I'm not trying to trap you into something :) Don't worry, I don't feel "trapped" in usenet. ;) It was more of an intuitive expectation than a suggestion that Python got something wrong. I was working on a program of some complexity recently and quickly caught the issue in my tests. I knew what was going on and fixed it expediently, but the behaviour confused me and I couldn't find any technical documentation on it so I figured I just didn't know what it was referred to in Python. Hence the post. :) I suppose I have some functional sensibilities and assumed that an object wouldn't let a non-member modify its properties even if they were mutable. Of course if there is any further reading on the subject, I'd appreciate some links. Cheers. -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
alex23 <[EMAIL PROTECTED]> writes: > On Nov 21, 9:40 am, J Kenneth King <[EMAIL PROTECTED]> wrote: >> Of course, providing a shallow (or deep as necessary) copy makes it >> work, I'm curious as to why the value passed as a parameter to a >> function outside the class is passed a reference rather than a copy. > > You're passing neither a reference nor a copy, you're passing the > object (in this case a list) directly: > > http://effbot.org/zone/call-by-object.htm Ah, thanks -- that's precisely what I was looking for. I knew it couldn't be a mistake; I just couldn't find the documentation on the behaviour since I didn't know what it was called in the python world. Cheers. -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
J Kenneth King <[EMAIL PROTECTED]> writes: > I recently encountered some interesting behaviour that looks like a bug > to me, but I can't find the appropriate reference to any specifications > to clarify whether it is a bug. > > Here's the example code to demonstrate the issue: > > class SomeObject(object): > > def __init__(self): > self.words = ['one', 'two', 'three', 'four', 'five'] > > def main(self): > recursive_func(self.words) > print self.words > > def recursive_func(words): > if len(words) > 0: > word = words.pop() > print "Popped: %s" % word > recursive_func(words) > else: > print "Done" > > if __name__ == '__main__': > weird_obj = SomeObject() > weird_obj.main() > > > The output is: > > Popped: five > Popped: four > Popped: three > Popped: two > Popped: one > Done > [] > > Of course I expected that recursive_func() would receive a copy of > weird_obj.words but it appears to happily modify the object. > > Of course a work around is to explicitly create a copy of the object > property befor passing it to recursive_func, but if it's used more than > once inside various parts of the class that could get messy. > > Any thoughts? Am I crazy and this is supposed to be the way python works? That's because Python isn't call-by-value. Or it is according to some, it's just that the values it passes are references. Which, according to others, is unnecessarily convoluted: it's call-by-object, or shall we call it call-by-sharing? At least everybody agrees it's not call-by-reference or call-by-name. There. I hope this helps! -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
On Thu, 20 Nov 2008 18:31:12 -0500, J Kenneth King wrote: > Of course I expected that recursive_func() would receive a copy of > weird_obj.words but it appears to happily modify the object. I am curious why you thought that. What made you think Python should/did make a copy of weird_obj.words when you pass it to a function? This is a serious question, I'm not trying to trap you into something :) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
On Nov 21, 6:31 am, J Kenneth King <[EMAIL PROTECTED]> wrote: > I recently encountered some interesting behaviour that looks like a bug > to me, but I can't find the appropriate reference to any specifications > to clarify whether it is a bug. > > Here's the example code to demonstrate the issue: > > class SomeObject(object): > > def __init__(self): > self.words = ['one', 'two', 'three', 'four', 'five'] > > def main(self): > recursive_func(self.words) > print self.words > > def recursive_func(words): > if len(words) > 0: > word = words.pop() > print "Popped: %s" % word > recursive_func(words) > else: > print "Done" > > if __name__ == '__main__': > weird_obj = SomeObject() > weird_obj.main() > > The output is: > > Popped: five > Popped: four > Popped: three > Popped: two > Popped: one > Done > [] > > Of course I expected that recursive_func() would receive a copy of > weird_obj.words but it appears to happily modify the object. > > Of course a work around is to explicitly create a copy of the object > property befor passing it to recursive_func, but if it's used more than > once inside various parts of the class that could get messy. > > Any thoughts? Am I crazy and this is supposed to be the way python works? You are passing a mutable object. So it can be changed. If you want a copy, use slice: >>> L = [1, 2, 3, 4, 5] >>> copy = L[:] >>> L.pop() 5 >>> L [1, 2, 3, 4] >>> copy [1, 2, 3, 4, 5] ...in your code... def main(self): recursive_func(self.words[:]) print self.words ...or... >>> def recursive_func(words): >>> words = words[:] >>> if len(words) > 0: >>> word = words.pop() >>> print "Popped: %s" % word >>> recursive_func(words) >>> else: >>> print "Done" >>> >>> words = ["one", "two", "three"] >>> recursive_func(words) Popped: three Popped: two Popped: one Done >>> words ['one', 'two', 'three'] Though I haven't been doing this long enough to know if that last example has any drawbacks. If we knew more about what you are trying to do, perhaps an alternative would be even better. - Rafe -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
On Nov 20, 6:40 pm, J Kenneth King <[EMAIL PROTECTED]> wrote: > J Kenneth King <[EMAIL PROTECTED]> writes: > > > > > I recently encountered some interesting behaviour that looks like a bug > > to me, but I can't find the appropriate reference to any specifications > > to clarify whether it is a bug. > > > Here's the example code to demonstrate the issue: > > > class SomeObject(object): > > > def __init__(self): > > self.words = ['one', 'two', 'three', 'four', 'five'] > > > def main(self): > > recursive_func(self.words) > > print self.words > > > def recursive_func(words): > > if len(words) > 0: > > word = words.pop() > > print "Popped: %s" % word > > recursive_func(words) > > else: > > print "Done" > > > if __name__ == '__main__': > > weird_obj = SomeObject() > > weird_obj.main() > > > The output is: > > > Popped: five > > Popped: four > > Popped: three > > Popped: two > > Popped: one > > Done > > [] > > > Of course I expected that recursive_func() would receive a copy of > > weird_obj.words but it appears to happily modify the object. > > > Of course a work around is to explicitly create a copy of the object > > property befor passing it to recursive_func, but if it's used more than > > once inside various parts of the class that could get messy. > > > Any thoughts? Am I crazy and this is supposed to be the way python works? > > Of course, providing a shallow (or deep as necessary) copy makes it > work, I'm curious as to why the value passed as a parameter to a > function outside the class is passed a reference rather than a copy. Why should it be a copy by default ? In Python all copies have to be explicit. George -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
On Nov 21, 9:40 am, J Kenneth King <[EMAIL PROTECTED]> wrote: > Of course, providing a shallow (or deep as necessary) copy makes it > work, I'm curious as to why the value passed as a parameter to a > function outside the class is passed a reference rather than a copy. You're passing neither a reference nor a copy, you're passing the object (in this case a list) directly: http://effbot.org/zone/call-by-object.htm -- http://mail.python.org/mailman/listinfo/python-list
Re: function parameter scope python 2.5.2
J Kenneth King <[EMAIL PROTECTED]> writes: > I recently encountered some interesting behaviour that looks like a bug > to me, but I can't find the appropriate reference to any specifications > to clarify whether it is a bug. > > Here's the example code to demonstrate the issue: > > class SomeObject(object): > > def __init__(self): > self.words = ['one', 'two', 'three', 'four', 'five'] > > def main(self): > recursive_func(self.words) > print self.words > > def recursive_func(words): > if len(words) > 0: > word = words.pop() > print "Popped: %s" % word > recursive_func(words) > else: > print "Done" > > if __name__ == '__main__': > weird_obj = SomeObject() > weird_obj.main() > > > The output is: > > Popped: five > Popped: four > Popped: three > Popped: two > Popped: one > Done > [] > > Of course I expected that recursive_func() would receive a copy of > weird_obj.words but it appears to happily modify the object. > > Of course a work around is to explicitly create a copy of the object > property befor passing it to recursive_func, but if it's used more than > once inside various parts of the class that could get messy. > > Any thoughts? Am I crazy and this is supposed to be the way python works? Of course, providing a shallow (or deep as necessary) copy makes it work, I'm curious as to why the value passed as a parameter to a function outside the class is passed a reference rather than a copy. -- http://mail.python.org/mailman/listinfo/python-list
