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).
It, however, doesn't say anything about constructs that are not block
statements but are equivalent to them, like the ternary operator. (In
comprehensions, filter conditions are the bits equivalent to if/while
statements.)
--
Regards,
Ivan
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