Python defined or operator

Question

Is there some conditional operator in Python which would evaluate the first operand and return its value if it's not None, or if it's None, evaluate and return the second operand? I know that a or b can almost do this, except that it does not strictly distinguish None and False. Looking for something similar to // operator in Perl.

The goal is to write both a and b only once, therefore alternative a if a is not None else b doesn't work either - both a and b can be expensive expressions.

PEP 505 proposes such an operator (`a ?? b`), but it's suspended in limbo. — chepner, Jun 16 '23 at 19:19
Your title says "defined", your text says "not `None`. Which one is it? — Kelly Bundy, Jun 16 '23 at 19:19

Hack5 · Accepted Answer · 2023-06-16T19:23:05.920

5

How about using the walrus operator?

tmp if (tmp := a()) is not None else b()

It will be faster than writing your own function, as function calls in Python incur a large overhead.

edited Jun 16 '23 at 19:23

answered Jun 16 '23 at 19:18

Hack5

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Elvis is `?:`, `:=` is walrus. – Czaporka Jun 16 '23 at 19:22
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Oops :/ it's been a long day – Hack5 Jun 16 '23 at 19:23

compilation-error · Answer 2 · 2023-06-16T19:23:36.000

1

This works: a_ if (a_ := a) is not None else b - would this not account for a being only evaluated once?

Also see: What is the correct syntax for Walrus operator with ternary operator?

edited Jun 16 '23 at 19:23

answered Jun 16 '23 at 19:19

compilation-error

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Pretty sure that's invalid, since order of operations will make that `(a_ := (a is not None))`. I think that should be `(a_ := a) is not None` – Samathingamajig Jun 16 '23 at 19:21
Thanks @Samathingamajig. Edited answer – compilation-error Jun 16 '23 at 19:23

Python defined or operator

2 Answers2