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Mon Apr 23 01:44:44 EDT 2018 · https://mail.python.org/pipermail/python-dev/2018-April/152886.html

[Guido]
> In reality there often are other conditions being applied to the match for
> which `if expr as name` is inadequate. The simplest would be something like
>
>   if ...:
>       <something>
>   elif (m := re.match('(.*):(.*)', line)) and m.group(1) == m.group(2):
>     <whatever>
>
> And the match() call may not even be the first thing to check -- e.g. we
> could have
>
>     elif line is not None and (m := re.match('(.*):(.*)', line)) and m.group(1) == m.group(2):

I find myself warming more to binding expressions the more I keep them
in mind while writing new code.  And I think it may be helpful to
continue showing real examples where they would help.

Today's example:  I happened to code this a few hours ago:

diff = x - x_base
if diff:
    g = gcd(diff, n)
    if g > 1:
        return g

It's not really hard to follow, but two levels of nesting "feels
excessive", as does using the names "diff" and "g" three times each.
It's _really_ an "and" test:  if the diff isn't 0 and gcd(diff, n) >
1, return the gcd.  That's how I _thought_ of it from the start.

Which this alternative expresses directly:

if (diff := x - x_base) and (g := gcd(diff, n)) > 1:
    return g

That's so Pythonic I could cry ;-)

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Showing content from https://mail.python.org/pipermail/python-dev/2018-April/152886.html below:

[Python-Dev] PEP 572: Assignment Expressions