[Python-Dev] PEP 572: Assignment Expressions

[Matthew Woodcraft]

On 2018-04-21 19:02, Tim Peters wrote:
> [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

Well, that's a reason to make the example a bit more realistic, then.

Say:

if match := re.search(pat1, text):
    do_something_with(match.group(0))
elif match := re.search(pat2, text):
    do_something_else_with(match.group(0), match.group(1))
elif match := re.search(pat3, text):
    do_some_other_things_with(match.group(0))
    and_also_with(match.group(1), match.group(2))

-M-