behavior difference for mutable and immutable variable in function definition

[jianbing.chen@gmail.com]

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

Thanks,
Jianbing

[James Stroud]

Re: behavior difference for mutable and immutable variable in functiondefinit ion

jianbing.chen@g mail.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
>
Thanks,
Jianbing
>

1. Each function call creates its own namespace, so "x" in foo() is
"isolated" from the global namespace or from calls of bar().
2. Think of assignment as assigning a name to a value rather than
"putting a value" into the name. When you assign, you completely change
the identity of name, rather than changing the contents of the name.

For example:


pyx = object()
pyid(x)
1074201696
pyx = object()
pyid(x)
1074201704

Notice how the identity (id) of x changes.

James

[Carsten Haese]

Re: behavior difference for mutable and immutable variable infunction definition

On Fri, 2007-05-04 at 14:30 -0700, jianbing.chen@g mail.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

[7stud]

Re: behavior difference for mutable and immutable variable in function definition

On May 4, 3:30 pm, jianbing.c...@g mail.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.>>d ef 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
>
Thanks,
Jianbing

The first function is completely irrelevant unless you expect this to
work:

x = 2
x[2] = 2

Traceback (most recent call last):
File "test1.py", line 2, in ?
x[2] = 2
TypeError: object does not support item assignment

So that leaves you with:

>def bar():

>
... x[2] = 2
...
>

>bar()

Would you expect this to work:

x[2] = 2
print x

[Roger Miller]

Re: behavior difference for mutable and immutable variable in function definition

On May 4, 12:39 pm, 7stud <bbxx789_0...@y ahoo.comwrote:

On May 4, 3:30 pm, jianbing.c...@g mail.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.>>d ef 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

>

Thanks,
Jianbing

>
The first function is completely irrelevant unless you expect this to
work:
>
x = 2
x[2] = 2
>
Traceback (most recent call last):
File "test1.py", line 2, in ?
x[2] = 2
TypeError: object does not support item assignment
>
So that leaves you with:
>

>>def bar():

>

... x[2] = 2
...

>

>>bar()

>
Would you expect this to work:
>
x[2] = 2
print x

I will sympathize with the OP to the extent that the message "global
name 'x' is not defined" is a bit misleading. All that the interpreter
really knows is that 'x' is not defined, locally or globally, and it
should probably not presume to guess the coder's intention.

[jianbing.chen@gmail.com]

Re: behavior difference for mutable and immutable variable in function definition

On May 4, 5:14 pm, Carsten Haese <cars...@uniqsy s.comwrote:

On Fri, 2007-05-04 at 14:30 -0700, jianbing.c...@g mail.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

This makes sense.

Thank you.