[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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