[Python-ideas] A comprehension scope issue in PEP 572

Jacco van Dorp j.van.dorp at deonet.nl
Fri May 11 03:33:06 EDT 2018

> Since this is all about scope, while I'm not 100% sure of what Guido
> meant, I assumed he was saying "p can only have one scope in the
> synthetic function:  local or non-local, not both, and local is what I
> propose".  For example, let's flesh out his example a bit more:
>     p = 42
>     [p := p for p in range(10) if p == 3]
>     print(p) # 42?  3?  9?
> If `p` is local to the listcomp, it must print 42.  If `p` is
> not-local, it must print 9.  If it's some weird mixture of both, 3
> makes most sense (the only time `p := p` is executed is when the `for`
> target `p` is 3).

With my limited experience, I'd consider 3 to make most sense, but 9
when thinking about it in the expanded form.

If it's not 3 tho, then the following would make most sense:

SyntaxError("Cannot re-bind for target name in a list comprehension")
# Or something more clear.

And the rest of that mail that convinces me even more that an error
would be the correct solution here.

Before I got on this mailinglist, i never even knew comprehensions
introduced a new scope. I'm really that new. Two years ago I'd look up
stackoverflow to check the difference between overriding and extending
a method and to verify whether I made my super() calls the right way.

If something goes to weird, I think just throwing exceptions is a
sensible solution that keeps the language simple, rather than making
that much of a headache of something so trivially avoided.


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