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Sat Apr 21 06:12:35 EDT 2018 · https://mail.python.org/pipermail/python-dev/2018-April/152838.html

On Sat, Apr 21, 2018 at 6:38 PM, Anthony Flury via Python-Dev
<python-dev at python.org> wrote:
> On 21/04/18 08:46, Chris Angelico wrote:
>>
>> doubled_items = [x for x in (items := get_items()) if x * 2 in items]
>>
>> This will leak 'items' into the surrounding scope (but not 'x').
>
> At the risk of stating the obvious - wasn't there work in Python 3 to
> prevent leakage from comprehensions ?
>>
>> [x for x in x if x] # This works
>> [x for y in x if x := y] # UnboundLocalError
>
>
> The standard library example given earlier notwithstanding, I can see no
> benefit in using the same name as the iterator and the loop target name. To
> be honest I have trouble parsing that first version, and keeping track of
> which x is which (especially which x is being used in the conditional
> clause) : surely this would be better : [x_item for x_item in x if x_item]
>
> Your 2nd example makes no sense to me as to the intention of the code - the
> re-use of the name x is confusing at best.
>

I agree. The change in behaviour caused by PEP 572 is basically only
going to be visible if you reuse a name, or in a very few other cases
like yield expressions:

def gen():
    yield [x for x in (yield 1)]

g = gen()
next(g)
g.send(range(5))

Once again, the outermost iterable is bizarre in this way.

ChrisA

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[Python-Dev] PEP 572: Assignment Expressions