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PEP 572: Python assignment expressions has been accepted

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317 points est | 3 comments | 03 Jul 18 08:30 UTC | HN request time: 0.869s | 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.
replies(10): >>17448567 #>>17448570 #>>17448571 #>>17448572 #>>17448575 #>>17448579 #>>17448584 #>>17448591 #>>17448617 #>>17448638 #
est ◴[03 Jul 18 09:03 UTC] No.17448572[source]▶
It's a controversial PEP https://www.python.org/dev/peps/pep-0572/ which allows you to write Python like this:

    def foo():
        if n := randint(0, 3):
            return n ** 2
        return 1337


    [(x, y, x/y) for x in input_data if (y := f(x)) > 0]
replies(7): >>17448580 #>>17448633 #>>17448694 #>>17448731 #>>17448946 #>>17449000 #>>17449023 #
stfwn ◴[03 Jul 18 10:09 UTC] No.17448946[source]▶
This immediately looks useful for things like:

    if foo := bar[baz]:
        bar[baz] += 1
        return foo
    else:
        bar[baz] = 1
        return 0
Where foo is a dict keeping track of multiple things, and a non-existing key (baz) is never an error but rather the start of a new count. Faster and more readable than

    if baz in list(bar.keys()):
    ....
Similar to Swift’s ‘if let’, it seems.
replies(4): >>17448968 #>>17449008 #>>17449051 #>>17449074 #
1. eesmith ◴[03 Jul 18 10:48 UTC] No.17449074[source]▶
The place I see using it is in (quoting Python's "python.exe-gdb.py"):

        m = re.match(r'\s*(\d+)\s*', args)
        if m:
            start = int(m.group(0))
            end = start + 10

        m = re.match(r'\s*(\d+)\s*,\s*(\d+)\s*', args)
        if m:
            start, end = map(int, m.groups())
With the new syntax this becomes:

        if m := re.match(r'\s*(\d+)\s*', args):
            start = int(m.group(0))
            end = start + 10

        if m := re.match(r'\s*(\d+)\s*,\s*(\d+)\s*', args)
            start, end = map(int, m.groups())
This pattern occurs just often enough to be a nuisance. For another example drawn from the standard library, here's modified code from "platform.py"

    # Parse the first line
    if (m := _lsb_release_version.match(firstline)) is not None:
        # LSB format: "distro release x.x (codename)"
        return tuple(m.groups())

    # Pre-LSB format: "distro x.x (codename)"
    if (m := _release_version.match(firstline)) is not None:
        return tuple(m.groups())

    # Unknown format... take the first two words
    if l := firstline.strip().split():
        version = l[0]
        if len(l) > 1:
            id = l[1]
replies(1): >>17454726 #
2. est ◴[04 Jul 18 01:14 UTC] No.17454726[source]▶
It' a problem with re module really.

re.match should return a match object no matter what, and .group() should return strings, empty string if non were matched.

replies(1): >>17455316 #
3. eesmith ◴[04 Jul 18 04:44 UTC] No.17455316[source]▶
I don't see how that would improve things. Could you sketch a solution based around your ideas?