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

Tim Peters tim.peters at gmail.com
Fri Apr 20 17:33:04 EDT 2018

[Guido, about g(items[idx] := idx := f()) ]

Does the PEP currently propose to allow that horrible example? I thought Tim Peters successfully pleaded to only allow a single "NAME := ".

I was "successful" only in that the two of us agreed that would be far less disruptive, and quite possibly an actual improvement ;-) But I only argued for limiting assignment expressions to the form

 identifier ":=" expression

I expected that, given that expressions "naturally nest", chained targets could still be specified:

a := b := c:= 5

but since they're all plain names there's no way to tell whether the bindings occur "left to right" or "right to left" short of staring at the generated code. I have no use case for chaining plain-name targets in assignment expressions, but didn't see a good reason to torture the implementation to forbid it. I expected chaining would just be an unused-in-practice possibility. Much like, e.g.,

a in b in c in d

is an unused-in-practice possibility.

And I'll take this opportunity to repeat the key point for me: I tried hard, but never found a single case based on staring at real code where allowing fancier (than "plain name") targets would be a real improvement. In every case I thought it might help, it turned out that it really didn't unless Python also grew an analog to C's "comma operator" (take only the last result from a sequence of expressions). I'll also note that I asked if anyone else had a real-life example, and got no responses.

There were lots of "real life" cases where plain-name targets allowed for code improvement, though.

You don't have to implement this restriction -- we know it's possible to implement, and if specifying this alone were to pull enough people from -1 to +0 there's a lot of hope!

Given my experience with trying to find use cases for fancier targets, and getting burned every time, I'm on the minus side of the current PEP, because - best I can tell - all the extra complexity would create an "attractive nuisance" :-(

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