Re: while expression feature proposal

Discussion in 'Python' started by Tim Chase, Oct 24, 2012.

  1. Tim Chase

    Tim Chase Guest

    On 10/24/12 17:26, Cameron Simpson wrote:
    > On 24Oct2012 16:54, Tim Chase <> wrote:
    > | On 10/24/12 16:34, Ian Kelly wrote:
    > | > The idiomatic way to do this is:
    > | >
    > | > while True:
    > | > VAR = EXPR
    > | > if not VAR:
    > | > break
    > | > BLOCK
    > |
    > | It may be idiomatic, but that doesn't stop it from being pretty
    > | ugly.
    > Yes, but more flexible because it accomodates loops where the natural
    > place for the test is partway through the loop instead of right at the
    > top, which is quite common.

    Just like the "with" doesn't stop you from using a try/finally block
    to do something similar, you can still write mid-loop exits as code
    currently exists. The proposed syntax just makes coming from other
    languages easier to translate.

    > | I must say I really like the parity of Dan's
    > | while EXPR as VAR:
    > | BLOCK
    > | proposal with the "with" statement.
    > Well, it's nice. But usually EXPR will be a boolean.

    I think the most common use-cases are boolean'ish. The falsehood
    means stop, but the truthiness value is not merely True. The common
    use-case I hit is the

    f = file('foo.bin')
    while True:
    data =
    if not data: break
    process(data) #not just data=True

    which tidily becomes

    while as data:

    > m = re_FUNKYPATTERN.match(test_string)
    > if m:
    > do stuff with the results of the match, using "m"
    > If I could write this as:
    > if re_FUNKYPATTERN.match(test_string) as m:
    > do stuff with the results of the match, using "m"

    I'm -0 on this. I don't like the pattern, but I'm not sure the "if
    CONDITION as FOO" syntax yields nearly the functional & readable
    improvement of "while THING as FOO".

    > then some cascading parse decisions would feel a bit cleaner. Where I
    > current have this:
    > m = re_CONSTRUCT1.match(line)
    > if m:
    > ... handle construct 1 ...
    > else:
    > m = re_CONSTRUCT2.match(line)
    > if m:
    > ... handle construct 2 ...
    > else:
    > m = re_CONSTRUCT3.match(line)

    For those, I often end up putting the regexps in a list and
    iterating accordingly (which I have done multiple times):

    regexes = [
    for regex in regexes:
    m = regex.match(line)
    if m:

    Granted, it does expect that you want to handle the results
    uniformly, but this could (usually) be mitigated with a dispatch
    function as well

    regex_dispatches = [
    (re_one, handle1),
    (re_two, handle2),
    (re_three, handle3),
    for regex, fn in regex_dispatches:
    m = regex.match(line):
    if m:
    fn(m, extra_context)

    > But I'm still -0 on it, because it supplants the glaringly obvious:
    > m = ...
    > assignment with the far less in your face:
    > possibly-long-expr as m

    If it were replacing standard assignment, I'd be as off-the-charts
    -1 as possible. Yech. But when cued by the "while" and
    indentation, it's not quite so bad. It still feels yuckyâ„¢ in an
    "if", but I find the improvement in the "while" is certainly worthwhile.

    > With statements and except statements have concrete use cases for the
    > "as" part that aren't doable without it, but the while/ form
    > can always be written in the current convention.

    The syntax for the with *could* have been something like

    with foo = file("foo.txt"):
    for line in foo:

    but the "as" was chosen and I think it makes sense in the context.


    (To me, the "if CONDITION as VAR" feels much like the ternary

    x = foo if bar else baz

    which made it into official syntax; I find the "while CONDITION as
    VAR" much more readable/valuable in light of both)
    Tim Chase, Oct 24, 2012
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