[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Tim Peters tim.peters at gmail.com
Mon Apr 23 01:44:44 EDT 2018

[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 ...: elif (m := re.match('(.):(.)', line)) and m.group(1) == m.group(2): 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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