[Python-Dev] PEP 572: Assignment Expressions

[Tim Peters]

[Matthew Woodcraft]
>>> 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))

[Steven D'Aprano <steve at pearwood.info>]
>> I don't think that a bunch of generic "do_something_with" functions is
>> precisely "realistic".
>>
>> If I saw something like that, I'd try very hard to find a way to
>> refactor it into code like this:
>>
>> for handler in handlers:
>>     if handler.match(text):
>>         handler.process()
>>         break
>> else:
>>     # handle no-match case here
>>
>> where the knowledge of what to search for, where to search for it, how
>> to search for it, and what to do when found, was encapsulated in the
>> handler objects. Your tastes may vary.
>>
>> But your point is well-taken that the version with binding assignment
>> (thanks Tim!) is nicer to read than the current procedural version:
>>
>> match = re.search(pat1, text)
>> if match:
>>     do_something_with(match.group(0))
>> else:
>>     match = re.search(pat2, text)
>>     if match:
>>         do_something_else_with(match.group(0), match.group(1))
>>     else:
>>         match = = re.search(pat3, text)
>>         do_some_other_things_with(match.group(0))
>>         and_also_with(match.group(1), match.group(2))
>>
>> I just don't think it counts as a motivating use-case distinct from the
>> single match case.

[Guido]
> The version of this code found in reality is not as regular as the example
> quoted, and the rebuttal "but I would rewrite it with a loop" shoots a straw
> man. To me the if-elif-elif portion of the example is very much a separate
> motivation, since being able to put the assignment in the elif clause avoids
> runaway indentation. I've regretted not being able to use elif in this kind
> of situation many times, whereas in the single match case I don't find it a
> burden to assign the variable in a separate statement preceding the
> if-clause. (I guess this is a case of "flat is better than nested" -- thanks
> Tim! :-)

Au contraire - thank you for forcing me to channel you succinctly lo
those many years ago ;-)

And for pointing out this real use case, which I'm not sure has been
stressed before.  The PEP could clearly use more motivating examples,
and this is a fine class of them.  Few things are more maddening than
runaway cascading indentation :-(

And noting again that a simple "binding expression" (my neologism for
`identifier ":=" expression`, to break the reflexive horror at
imagining the full complexity of assignment statements being allowed
everywhere expressions are allowed) is sufficient to address it.