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PEP 572: Python assignment expressions has been accepted

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317 points est | 5 comments | 03 Jul 18 08:30 UTC | HN request time: 0.001s | source
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oooooof ◴[03 Jul 18 09:00 UTC] No.17448560[source]▶
What is it? The link points to a discussion more deep than I’m willing to read.
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OskarS ◴[03 Jul 18 09:12 UTC] No.17448617[source]▶
Basically it's about adding := as an "assignment expression operator", that does assignment and returns the value as an expression. That is, take this regex example:

    match1 = re1.match(text)

    if match1 is not None:
        do_stuff()
    else:
        match2 = re2.match(text)

        if match2 is not None:
            do_other_stuff()
Which is a bit clunky. you only want to evaluate match2 in case match1 fails, but that means a new level of nesting. Instead, with this proposal, you could do this:

    if (match1 := re1.match(text)) is not None:
        do_stuff();
    elif (match2 := re2.match(text)) is not None:
        do_other_stuff()
Evaluate and assign in the if-statement itself. This is not dissimilar to the equals operator in C. In C, you would frequently find loops like `while ((c = read()) != EOF) { ... }`. This would presumably allow a similar pattern in python as well.

More information can be found in PEP-572: https://www.python.org/dev/peps/pep-0572/

replies(1): >>17448750 #
1. oblio ◴[03 Jul 18 09:35 UTC] No.17448750[source]▶
Hehe. More chances for C-style bugs like:

if (a = b) /* Oooops, meant a == b! */

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2. toxik ◴[03 Jul 18 09:58 UTC] No.17448889[source]▶
Difference is bigger, C is `if (a = b)` vs `if (a == b)`. Python is `if (a := b)` vs `if a == b`
3. afraca ◴[03 Jul 18 09:58 UTC] No.17448893[source]▶
Except it's more likely you're accidentally inserting a character twice than inserting another extra character (':')
4. MaxBarraclough ◴[03 Jul 18 10:00 UTC] No.17448905[source]▶
Presumably that's why they've gone with the far more sensible ":=" syntax.

The use of "=" for assignment has long been a pet peeve of mine. It was a mistake when C did it, and it's been a mistake for so many subsequent languages to copy it.

"=" shouldn't be an operator at all, it makes a lot more sense to use ":=" and "==".

Pascal's use of ":=" for assignment and "=" for equality, strikes me as almost as clear.

Still, at least C makes consistent use of '=' for assignment, unlike that god-forsaken trainwreck of a language, VB.Net, which uses it for both assignment and for equality depending on context.

5. bluecalm ◴[03 Jul 18 20:39 UTC] No.17453446[source]▶
It's not a problem in C anymore as modern compilers warn about that so you had to put additional parenthesis to make it clearer.

I like C way of assignment being an expression. I think having separate statement and then assignment expresdion is a mess. It's still useful though as Python was missing where keyword like feature from Haskell which is necessary to avoid duplicating computation in list comprehension.