[Python-Dev] Call for prudence about PEP-572 (original) (raw)

Terry Reedy tjreedy at udel.edu
Sat Jul 7 15:58:48 EDT 2018

On 7/7/2018 12:53 PM, Tim Peters wrote:

[Guido]

... As to why you might want to use := in a function call, I could imagine writing if validate(name := re.search(pattern, line).group(1)): return name

If name has to be non-blank to pass validate, one can avoid the assignment within the function call be adding a redundant pre-test.

if name := re.search(pattern, line).group(1) and validate(name): return name

Giampaolo would presumably prefer this, but I don't think such preference should be enforced on everyone.

If name == '' is valid, then the alternative is the current one, using a separate assignment statement.

name = re.search(pattern, line).group(1) if validate(name): return name

When I was staring at my code, I never mentioned the very first plausible use I bumped into (in code I was actively working on at the time):

while not probableprime(p := randrange(lo, hi)):  pass # and now p is likely a random prime in range

As long as lo excludes 0:

while p := randrange(lo, hi) and not probable_prime(p): continue

I can see how someone might prefer this stylistically, but it is buggy. If this is contained in a function (very likely) and lo could be <= 0, because it is either passed in or calculated, 0 could be passed on a likely prime!

I never mentioned it because I expected it would annoy people on 3(!) counts:

- assigning in a function call

This is a style preference that people can and will disagree on. In any case, I think correctness trumps beauty, just as it trumps speed.

- reducing the loop body to pass

I fixed that ;-). 'continue' better expresses the 'try again' part of English versions, such as "While the trial value is not acceptable, try again."

- using the binding long after the loop ended

The same is true for the current 4-line loop and a half.

while True: p = randrange(lo, hi) if probable_prime(p): break # p used somewhere else

Indeed, for those reasons it wasn't "an obvious" win to me - or an obvious loss.  So I just moved on.

However, after staring at hundreds of other cases, it does strike me as "a small win" today - my brain cells have rewired to recognize more ":=" patterns at a glance. Whether that's a good thing or not I don't know, but it is a real thing ;-)

I must admit that I too am already more comfortable with := now than I was originally.

-- Terry Jan Reedy

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