[Python-Dev] set/dict comprehensions don't leak in Py2.7 - intentional?

[Michael Foord]

On 05/07/2010 14:12, Tim Golden wrote:
> On 05/07/2010 14:06, Stefan Behnel wrote:
>> Hi,
>>
>> I only noticed now that set/dict comprehensions were backported to 2.7
>> without letting the loop Variables leak into the surrounding scope. So
>> they now behave different from list comprehensions. Is this intentional
>> or just a backporting oversight?
>>
>> I'm asking because we had a discussion about the 'right' semantics
>> several times on the Cython list, and the current behaviour is that all
>> comprehensions leak when compiling Py2 code and the do not leak when
>> compiling Py3 code. I would hate to fix that up to match 2.7 and then
>> see that the behaviour in Py2.7 is to be considered an accident.
>
> Depends on your definition of "comprehension", but generator expressions
> have always not leaked, so these are already different in Py2.x:
>
> [i for i in range (10)]
>
> list (i for i in range (10))
>
> I had understood (without being able to put my finger on a relevant 
> thread)
> that all comprehensions were going not leak their loop variables in the
> future.

Similarly my understanding was that list comprehensions leaking was an 
unfortunate implementation detail and that to depend on it was therefore 
"a bug". I don't see it as an issue that set and dict comprehensions 
don't leak in 2.7 (and in fact it is a good thing as it makes it harder 
to write code that works in 2.7 but fails in 3.x).

Michael

>
> TJG
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