[Python-Dev] Call for prudence about PEP-572
tjreedy at udel.edu
Sat Jul 7 15:58:48 EDT 2018
On 7/7/2018 12:53 PM, Tim Peters wrote:
> 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):
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)
> 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 probable_prime(p := randrange(lo, hi)):
> # 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):
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
> I never mentioned it because I expected it would annoy people on 3(!)
> - 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
> - using the binding long after the loop ended
The same is true for the current 4-line loop and a half.
p = randrange(lo, hi)
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
Terry Jan Reedy
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