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

Tim Peters tim.peters at gmail.com
Mon Apr 23 18:04:55 EDT 2018

[Ethan Furman <ethan at stoneleaf.us>]

So I really like being able to make the assignment in the expression, but I have a really hard time parsing it with the name first.

... On the other hand, if it were using the "as" keyword: if (x - xbase as diff) and (gcd(diff, n) as g) > 1: return g I would parse as: if x - xbase as diff and gcd(diff, n) as g > 1: return g For me at least, the last is much more readable. Thinking about it some more, the problem (or maybe just my problem) is that I see an "if" or "while" and the I look for the thing that is True or False, and using the ":=" syntax the first thing I see is a placeholder for a result that doesn't exist yet, making me constantly scan backwards and forwards to put all the pieces in the correct place. With "as", it just flows forwards.

I can read it fine either way, and don't much care. A possible advantage of an "as" operator is that its precedence could be set to bind just a tad stronger than comparisons (which include "is" and "is not" in Python), and then, e.g.,

if f() as result is not None:
    do something with result

could work as intended. So long as people can't get "assignment statements" out of their heads,

if result := f() is not None:

groups instead as

if result := (f() is not None):

which would almost never be intended. Maybe spelling it "as" instead could break that.

However, against "as" is that its current use in "with" statements does something quite different:

with f() as name:

does not bind the result of f() to name, but the result of f().__enter__(). Whether that "should be" fatal, I don't know, but it's at least annoying ;-)

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