On Sat, Mar 24, 2018 at 8:07 PM, Christoph Groth
<christ...@grothesque.org> wrote:
> Chris Angelico wrote:
>
>> Thank you; both of these have now been incorporated into the document.
>
> Thanks! Just a small comment. You wrote in the PEP draft:
>
>> # Name bindings inside list comprehensions usually won't leak
>> ...
>> # But occasionally they will!
>
> I don't understand what you mean here. If the (y as x) syntax is to
> have, as you say, "the exact same semantics as regular assignment", then
> assignments inside list comprehensions would always "leak". But this is
> a good thing, because this is consistent with how Python behaves.
Except that a list comprehension is implemented using an inner
function. Very approximately:
x = [n * m for n in range(4) for m in range(5)]
def <listcomp>(iter):
ret = []
for n in iter:
for m in range(5):
ret.append(n * m)
return ret
x = <listcomp>(iter(range(4))
So the first (outermost) iterable is actually evaluated in the
caller's scope, but everything else is inside a subscope. Thus an
assignment inside that first iterable WILL leak into the surrounding
scope; but anywhere else, it won't.
ChrisA
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