[Python-ideas] Conditional Assignment in If Statement

[Chris Angelico]

On Tue, Oct 18, 2016 at 9:11 AM, Michael duPont <michael at mdupont.com> wrote:
> What does everyone think about:
>
> if foo = get_foo():
>     bar(foo)
>
> as a means to replace:
>
> foo = get_foo()
> if not foo:
>     bar(foo)
> del foo
>
> Might there be some better syntax or a different keyword? I constantly run into this sort of use case.

I'm pretty sure that syntax is never going to fly, for a variety of
reasons (to see most of them, just read up a C style guide). But this
syntax has been proposed now and then, analogously with the 'with'
statement:

if get_foo() as foo:
    bar(foo)

Be careful of your definitions, though. You've said these as equivalent:

if foo = get_foo():
    bar(foo)

foo = get_foo()
if foo is not None:
    bar(foo)

foo = get_foo()
if not foo:
    bar(foo)
del foo

There are three quite different conditions here. Your last two are
roughly opposites of each other; but also, most people would expect
"if foo = get_foo()" to be the same condition as "if get_foo()", which
is not the same as "if get_foo() is not None". The semantics most
likely to be accepted would be for "if get_foo() as foo:" to use the
standard boolification rules of Python (and then make 'foo' available
in both 'if' and 'else' blocks). Would you support that? If so, check
out some of the previous threads on the subject - this is far from the
first time it's been discussed, and most likely won't be the last.

ChrisA