PEP 572: Python assignment expressions has been accepted

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korijn ◴[03 Jul 18 11:10 UTC] No.17449188[source]▶

It feels like we are repeating a mistake the PHP community made years ago. PHP supports assignment in expressions, and the community seems to avoid its use. There are plenty of examples of stackoverflow questions and answers where the problem is caused by coders overlooking an assignment in an expression. You just don't expect assignment to occur in an expression, and its visually similar enough to gloss over it.

replies(4): >>17449245 #>>17449289 #>>17449406 #>>17450205 #

1. sametmax ◴[03 Jul 18 11:57 UTC] No.17449406[source]▶

>>17449188 #

In PHP, the "if ($a = 1)" and "if ($a == 1)" look a lot like each others.

In the Python version, the new "(a := 1)" will stand out compare to the canonical "a == 1" as the ':' and '()' are unexpected here, and scream that it's not regular if test.

replies(3): >>17451677 #>>17451814 #>>17453916 #

2. dvlsg ◴[03 Jul 18 16:48 UTC] No.17451677[source]▶

>>17449406 (TP) #

Same problem in Javascript. There's actually a yoda lint rule that encourages you to write your code like (1 === a) instead, so you don't accidentally make that mistake.

replies(1): >>17453341 #

3. korijn ◴[03 Jul 18 17:00 UTC] No.17451814[source]▶

>>17449406 (TP) #

I'm curious to see how it will pan out in practise, but I'm not getting my hopes up.

I made a flake8 plugin to forbid assignment expressions:

https://github.com/Korijn/flake8-assignexp

Will be released once python 3.8 is out and I can test it.

replies(1): >>17453563 #

4. makapuf ◴[03 Jul 18 20:21 UTC] No.17453341[source]▶

>>17451677 #

I hate Yoda assignment in C. Does no good when assigning to variables. I think it's better to use the weird (on purpose) ((x=13)) and forbid x=13

5. geoelectric ◴[03 Jul 18 20:57 UTC] No.17453563[source]▶

>>17451814 #

That seems unnecessarily reactionary, but ok. I agree with the post you're responding to that having a completely separate token for inline assignment fixes the majority of issues that have historically come up with it (which mostly come down to forgetting an = in the ==). Might be worth feeling out the trajectory of the feature before proactively banning it from codebases you control.

replies(1): >>17455994 #

6. thomasahle ◴[03 Jul 18 22:00 UTC] No.17453916[source]▶

>>17449406 (TP) #

The '()'s are not always needed though, as in the PEP examples `(y := f(x), x/y)` and `(x := 0, x := 1)`.

The alternative `f(x) as y` syntax looked nice to my eyes, and doesn't introduce new symbols. However I'm sure they are right, that it would have bad corner cases.

7. korijn ◴[04 Jul 18 08:34 UTC] No.17455994{3}[source]▶

>>17453563 #

You're right. Like I said, I'm curious to see how it will pan out, but I'm not getting my hopes up. I'm just not a fan of this new syntax. :) IMHO null-safe navigation operators like ?. and ?[] are much more important to have.

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