please help explain this result

[Paul Kölle]

Am 17.10.2010 13:48, schrieb Steven D'Aprano:
> On Sun, 17 Oct 2010 03:58:21 -0700, Yingjie Lan wrote:
>
>> Hi,
>>
>> I played with an example related to namespaces/scoping. The result is a
>> little confusing:
>
> [snip example of UnboundLocalError]
>
> Python's scoping rules are such that if you assign to a variable inside a
> function, it is treated as a local. In your function, you do this:
>
> def f():
>      a = a + 1
>
> Since a is treated as a local, when you enter the function the local a is
> unbound -- it does not have a value. So the right hand side fails, since
> local a does not exist, and you get an UnboundLocalError. You are trying
> to get the value of local "a" when it doesn't have a value.
Oh really? Can you explain the following?

 >>> a = {}
 >>> def foo():
...     a['a'] = 'lowercase a'
...     print a.keys()
...
 >>> foo()
['a']
 >>> a
{'a': 'lowercase a'}
 >>> def bar():
...     a['b'] = a['a'].replace('a', 'b')
...
 >>> bar()
 >>> a
{'a': 'lowercase a', 'b': 'lowercbse b'}
 >>>

cheers
  Paul