[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20 (original) (raw)

Tim Peters tim.peters at gmail.com
Wed Apr 25 19:55:56 EDT 2018

[Tim]

To my eyes, this is genuinely harder to follow, despite its relative brevity:

while total != (total := total + term):

[Antoine]

Does it even work? Perhaps if the goal is to stop when total is NaN, but otherwise?

[Chris]

Yes, it does, because the first "total" is looked up before the rebinding happens. It's 100% unambiguous to the compiler... but still pretty unclear to a human. And I think the multiple use of 'total' is to blame for that. So I agree with Tim that this particular example is better in longhand.

[Antoine]

"Better" is an understatement :-( Now that I understood it (thanks for the explanation),

Ah, sorry - I had no idea it was the "left to right evaluation" part you weren't seeing. Next time explain why you think something is broken?

the shorthand version appears completely bonkers.

I wouldn't go that far, but I already said I wouldn't write it that way.

However, without looking at real code, people are just flat-out guessing about how bad - or good - things can get, no matter how confident they sound.

So at least give me credit for presenting the worst brief binding-expression example you've seen too ;-)

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