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Date: Sun, 29 May 2011 03:01:52 -0700
Subject: Re: scope of function parameters
From: Chris Rebert <crebert@ucsd.edu>
To: Henry Olders <henry.olders@mcgill.ca>
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6523

On Sun, May 29, 2011 at 1:30 AM, Henry Olders <henry.olders@mcgill.ca> wrot=
e:
> I just spent a considerable amount of time and effort debugging a program=
. The made-up code snippet below illustrates the problem I encountered:
>
> def main():
> =C2=A0 =C2=A0 =C2=A0 =C2=A0a =3D ['a list','with','three elements']
> =C2=A0 =C2=A0 =C2=A0 =C2=A0print a
> =C2=A0 =C2=A0 =C2=A0 =C2=A0print fnc1(a)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0print a
>
> def fnc1(b):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0return fnc2(b)
>
> def fnc2(c):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0c[1] =3D 'having'
> =C2=A0 =C2=A0 =C2=A0 =C2=A0return c
>
> This is the output:
> ['a list', 'with', 'three elements']
> ['a list', 'having', 'three elements']
> ['a list', 'having', 'three elements']
>
> I had expected the third print statement to give the same output as the f=
irst, but variable a had been changed by changing variable c in fnc2.

To be more accurate, the list object referred to by `a` was modified
through c, due to the fact that a, b, and c all refer to the very same
object in this case.

> It seems that in Python, a variable inside a function is global unless it=
's assigned. This rule has apparently been adopted in order to reduce clutt=
er by not having to have global declarations all over the place.
>
> I would have thought that a function parameter would automatically be con=
sidered local to the function.
<snip>
> Are there others who feel as I do that a function parameter should always=
 be local to the function? Or am I missing something here?

Function parameters *are* local variables. Function parameters are
indeed local in that *rebinding* them has no effect outside of the
function:

def foo(a):
    a =3D 42

def bar():
    b =3D 1
    foo(b)
    print b

bar() #=3D> outputs 1

As you've observed, *mutating* the object a variable refers to is
another matter entirely. Python does not use call-by-value and does
not copy objects unless explicitly requested to, as you've
encountered. But it does not use call-by-reference either, as my
example demonstrates. Like several other popular contemporary
languages, Python uses call-by-object for parameter passing; a good
explanation of this model can be found at
http://effbot.org/zone/call-by-object.htm  It's well worth reading.

Cheers,
Chris