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

Steven D'Aprano steve at pearwood.info
Sat Apr 28 04:33:47 EDT 2018

On Fri, Apr 27, 2018 at 04:24:35PM -0400, Wes Turner wrote:

if ((1) or (x := 3)): if ((y := func(x)) if x else (x := 3))

Wes, there is absolutely nothing new here. This sort of error is already possible in Python.

Do you see a lot of code like:

if (1 or sequence.append(3) or sequence[-1]):

in real life? If you do, then I'm really, really sorry that you are forced to deal with such rubbish code, but honestly, the vast bulk of Python programmers do not write like that, and they won't write this either:

if (1 or (x := 3)):

[...]

Assignment expressions, though they are noticeable :=, may not actually define the variable in cases where that part of the line doesn't run but reads as covered.

The same applies to any operation at all.

/sarcasm I guess adding print() to the language was a mistake, because we can write rubbish code like this:

if 1 or print(x):

and then be confused by the fact that x doesn't get printed. /end sarcasm

In another post, you insisted that we need to warn in the PEP and the docs not to do this sort of thing. Should we also go through and add these warnings to list.append, dict.update, set.add, etc?

I trust that the answer to that is obviously no.

And neither do we have to assume that people who use binding-expressions will lose their minds and start writing the sort of awful code that they don't write with anything else.

-- Steve

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