function parameter scope python 2.5.2

[J Kenneth King]

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(obje ct):

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?

[J Kenneth King]

Peter Pearson <pp******@nowhe re.invalidwrite s:

On Fri, 21 Nov 2008 10:12:08 -0500, J Kenneth King wrote:

>Steven D'Aprano <st***@REMOVE-THIS-cybersource.com .auwrites:

>>>
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.

[Terry Reedy]

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.