[Python-ideas] For-loop variable scope: simultaneous possession and ingestion of cake

[Bruce Leban]

On Sun, Oct 5, 2008 at 10:16 PM, Carl Johnson <carl at carlsensei.com> wrote:

> On 2008/10/05, at 6:47 pm, Bruce Leban wrote:
>
>    i = 0
>>   def f():
>>        i += 1
>>        return lambda: i
>>
>>
> I'm not sure I see what you're getting at. In Python 2.6 and 3.0rc1 this
> raises "UnboundLocalError: local variable 'i' referenced before assignment."
> If you want to do what it looks like you want to do, you have to use
> "nonlocal i" or "global i".
>

Yup. I wrote that a bit too quickly.

>
>
>>   i = 0
>>   def f():
>>        i += 1
>>        return lambda: immanentize i
>>
>> This will return lambda: 1, then lambda: 2, etc. right? No. It returns
>> lambda: 0, lambda: 0, etc.
>>
>
> To me, it is transparently clear that this will return lambda: 0 every
> time. That's what immanentization does. If you want lambda: 1, etc., use
> "nonlocal i".
>
>
Consider this:

i = 0
def f():
    global i
    i += 1
    return lambda: immanentize 1

when does immanentize get evaluated? when f is defined or when the lambda is
evaluated? From what you wrote, it sounds like you think it's evaluated when
f is defined. OK, so how do I get the equivalent of:

def f():
    global i
    i += 1
    lambda i=i: i

using immanentize?
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