[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Tim Peters tim.peters at gmail.com
Sat Apr 21 14:02:00 EDT 2018

[Matthew Woodcraft <matthew at woodcraft.me.uk>]

I would like to suggest one more motivating example for "Capturing condition values": multiple regex matches with 'elif'.

if match := re.search(pat1, text): print("Found one:", match.group(0)) elif match := re.search(pat2, text): print("Found two:", match.group(0)) elif match := re.search(pat3, text): print("Found three:", match.group(0)) Without assignment expressions, you have an annoying choice between a cascade of 'else's with an ever-increasing indent and evaluating all the matches up front (so doing unnecessary work).

That's a reasonable use, but would more likely be written like so today:

for tag, pat in (("one", pat1), ("two", pat2), ("three", pat3). ("four", pat4), ...): match = re.search(pat, text) if match: print("Found", tag + ":", match.group(0)) break

Which would still read a bit nicer if the first two loop body lines could be collapsed to

if match := re.search(pat, text):

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