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

Chris Angelico rosuav at gmail.com
Wed Jun 27 21:57:45 EDT 2018

On Thu, Jun 28, 2018 at 11:29 AM, Ivan Pozdeev via Python-Dev
<python-dev at python.org> wrote:
> On 28.06.2018 2:44, Greg Ewing wrote:
>> Ivan Pozdeev via Python-Dev wrote:
>>> for me, the primary use case for an assignment expression is to be able
>>> to "catch" a value into a variable in places where I can't put an assignment
>>> statement in, like the infamous `if re.match() is not None'.
>> This seems to be one of only about two uses for assignment
>> expressions that gets regularly brought up. The other is
>> the loop-and-a-half, which is already adequately addressed
>> by iterators.
>> So maybe instead of introducing an out-of-control sledgehammer
>> in the form of ":=", we could think about addressing this
>> particular case.
>> Like maybe adding an "as" clause to if-statements:
>>    if pattern.match(s) as m:
>>       do_something_with(m)
> I've skimmed for the origins of "as" (which I remember seeing maybe even
> before Py3 was a thing) and found this excellent analysis of modern
> languages which is too a part of the PEP 572 discussion:
> https://mail.python.org/pipermail/python-ideas/2018-May/050920.html
> It basically concludes that most recently-created languages do not have
> assignment expressions; they rather allow assignment statement(s?) before
> the tested expression in block statements (only if/while is mentioned. `for'
> is not applicable because its exit condition in Python is always the
> iterable's exhaustion, there's nothing in it that could be used as a
> variable).

Now read this response.



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