behavior difference for mutable and immutable variable in function definition

[Carsten Haese]

On Fri, 2007-05-04 at 14:30 -0700, jianbing.chen at gmail.com wrote:
> Hi,
> 
> Can anyone explain the following:
> 
> Python 2.5 (r25:51908, Apr  9 2007, 11:27:23)
> [GCC 4.1.1 20060525 (Red Hat 4.1.1-1)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> def foo():
> ...     x = 2
> ...
> >>> foo()
> >>> def bar():
> ...     x[2] = 2
> ...
> >>>
> >>> bar()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 2, in bar
> NameError: global name 'x' is not defined

"x = 2" binds the name 'x' in foo's local namespace to the object '2'.
For this, it doesn't matter whether the name 'x' was previously bound to
anything.

"x[2] = 2" is a shorthand notation for the method call
"x.__setitem__(2,2)". This requires the name 'x' to be bound to some
object that has a __setitem__ method.

-Carsten