[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part) (original) (raw)

Tim Peters tim.peters at gmail.com
Thu Jun 28 02:31:04 EDT 2018

[Chris Barker]

... So what about:

l = [x:=i for i in range(3)] vs g = (x:=i for i in range(3)) Is there any way to keep these consistent if the "x" is in the regular local scope?

I'm not clear on what the question is. The list comprehension would bind l to [0, 1, 2] and leave the local x bound to 2. The second example binds g to a generator object, which just sits there unexecuted. That has nothing to do with the PEP, though.

If you go on to do, e.g.,

l = list(g)

then, same as the listcomp, l will be bound to [0, 1, 2] and the local x will be left bound to 2.

The only real difference is in when the x:=i for i in range(3) part gets executed. There's no new twist here due to the PEP. Put a body B in a listcomp and any side effects due to executing B happen right away, but put B in a genexp and they don't happen until you force the genexp to yield results.

For example, do you think these two are "consistent" today?

l = [print(i) for i in range(3)] g = (print(i) for i in range(3))

? If so, nothing essential changes by replacing "print(i)" with "x := i" - in either case the side effects happen when the body is executed.

But if you don't think they're already consistent, then nothing gets less consistent either ;-) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20180628/89b72375/attachment.html>

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