[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings

[Chris Angelico]

On Sun, Mar 25, 2018 at 3:08 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
> On 03/24/2018 09:00 PM, Chris Angelico wrote:
>>
>> On Sun, Mar 25, 2018 at 2:59 PM, Ethan Furman wrote:
>>>
>>> On 03/24/2018 08:51 PM, Chris Angelico wrote:
>
>
>>>> Let's suppose we have assignment expressions. I'm going to use "(expr
>>>> as name)" syntax for this example.
>>>>
>>>> a = [(1 as b) for c in (d as e) if (2 as f)]
>>>>
>>>> Which of these six names is local to the comprehension and which can
>>>> leak? Due to the requirements of class scope, 'd' must be looked up in
>>>> the outer scope. That means that its corresponding 'as e' must also
>>>> land in the outer scope. [...]
>>>
>>>
>>>
>>> Why?
>>>
>>> d = 9
>>> def func():
>>>    e = d
>>>
>>> d is looked up outside of func, but e is still local to func.
>>
>>
>> No, it's looked up inside func, and the value is found outside of
>> func. Consider:
>>
>> d = 9
>> def func():
>>      e = d
>>      d = 1
>
>
> That is invalid Python.

No, it's perfectly valid Python... it just happens that it'll raise
UnboundLocalError when it tries to look up 'd' :)

>> What's happening with a comprehension is more like:
>>
>> d = 9
>> def func(_iter):
>>      ... body of comprehension
>> func((d as e))
>>
>> which is looking it up _outside_ the function, then passing it as a
>> parameter.
>
>
> Looks like a buggy implementation detail.  Any assignments that happen
> inside a listcomp should be effective only inside the listcomp.

Leaky abstraction, perhaps. I don't think it's buggy as such.

ChrisA