[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings
Paul Moore
p.f.moore at gmail.com
Sat Mar 24 07:49:33 EDT 2018
On 24 March 2018 at 09:18, Chris Angelico <rosuav at gmail.com> wrote:
> On Sat, Mar 24, 2018 at 8:07 PM, Christoph Groth
> <christoph at 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.
Wow, that's subtle (in a bad way!). I'd much rather that assignments
don't leak at all - that seems to me to be the only correct design,
although I understand that implementation practicalities mean it's
hard to do.
There's a lot of context snipped here. Is this about the variant that
just does assignment without the new scope? If it is, then is there a
similar issue with the actual proposal, or is that immune to this
problem (I suspect that it's not immune, although the details may
differ).
Paul
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