Weird local variables behaviors

Sebastjan Trepca trepca at gmail.com
Sat Jun 21 09:05:15 CEST 2008


I see, intuitively one would think it would try to get it from global
context as it's not yet bound in the local.

Thanks for the explanation.

Sebastjan


On Sat, Jun 21, 2008 at 5:48 AM, Dan Bishop <danb_83 at yahoo.com> wrote:
> On Jun 20, 7:32 pm, Matt Nordhoff <mnordh... at mattnordhoff.com> wrote:
>> Sebastjan Trepca wrote:
>> > Hey,
>>
>> > can someone please explain this behavior:
>>
>> > The code:
>>
>> > def test1(value=1):
>> >     def inner():
>> >         print value
>> >     inner()
>>
>> > def test2(value=2):
>> >     def inner():
>> >         value = value
>> >     inner()
>>
>> > test1()
>> > test2()
>>
>> > [trepca at sauron ~/dev/tests]$ python locals.py
>> > 1
>> > Traceback (most recent call last):
>> >   File "locals.py", line 13, in <module>
>> >     test2()
>> >   File "locals.py", line 10, in test2
>> >     inner()
>> >   File "locals.py", line 9, in inner
>> >     value = value
>> > UnboundLocalError: local variable 'value' referenced before assignment
>>
>> > Why can't he find the variable in the second case?
>>
>> > Thanks, Sebastjan
>>
>> Python doesn't like when you read a variable that exists in an outer
>> scope, then try to assign to it in this scope.
>>
>> (When you do "a = b", "b" is processed first. In this case, Python
>> doesn't find a "value" variable in this scope, so it checks the outer
>> scope, and does find it. But then when it gets to the "a = " part...
>> well, I don't know, but it doesn't like it.)
>
> In a language like C++, the scope of a variable is determined by the
> declaration.
>
> int x; // A
> class Example
> {
>   int x; // B
>   void f()
>   {
>      int x; // C
>      x = 42; // Which x?
>   }
> };
>
> The "x" referred to in the statement "x = 42;" refers to local
> variable of Example::f.  If line C were removed, then it would refer
> to the member variable of class Example.  And if line B were removed,
> then it would refer to the global variable.
>
> In Python, however, there are no declarations.  Therefore, it requires
> another approach.  What it chose was:
>
> (1) Explicit "self" for object attributes.
> (2) A function's local variables are defined as those appearing on the
> left side of an assignment.  Whether the name happens to refer to a
> global is NOT considered.
> --
> http://mail.python.org/mailman/listinfo/python-list
>



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