Re: New to Python - block grouping (spaces)

References	(2 earlier) <552F772B.8030201@rece.vub.ac.be> <CAPTjJmoeN1EYGmNW4ssfSBMhhAjwtbfeQnEbFxOKxNARL7VizA@mail.gmail.com> <mailman.334.1429178988.12925.python-list@python.org> <552f9264$0$11092$c3e8da3@news.astraweb.com> <552F97EA.5080401@rece.vub.ac.be>
Date	2015-04-16 23:41 +1000
Subject	Re: New to Python - block grouping (spaces)
From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.343.1429191686.12925.python-list@python.org> (permalink)

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On Thu, Apr 16, 2015 at 9:07 PM, Antoon Pardon
<antoon.pardon@rece.vub.ac.be> wrote:
> On 04/16/2015 12:43 PM, Steven D'Aprano wrote:
>> On Thursday 16 April 2015 20:09, Antoon Pardon wrote:
>>
>>> I beg to differ. The most common occurence is a loop with a break
>>> condition in the middle I would prefer such a loop to be written as
>>> follows:
>>>
>>> repeat:
>>>     some
>>>     code
>>> break_when condition:
>>>     more
>>>     code

The case of a loop structure with its condition in the middle is one
that few languages support, so the physical structure has to be
something like:

goto middle
while not condition:
    more code
    label middle
    some code

or

while True:
    some code
    if condition: break
    more code

or maybe

some code
while not condition:
    more code
    some code

But I'm not sure how you could represent this more appropriately,
regardless of your indentation. Unindenting an "if" in the middle of a
loop doesn't instantly scream "this is the loop header". Using a goto
to jump half way into a loop is a really REALLY bad idea in most
programs (and it's illegal in lots of languages anyway). Repeating the
setup code is fine if it's a single line, but not else.

Similar to this is the capturing condition. Say you want to process
lines until you get to one that consists solely of a full stop. In C,
you can do this (kinda); in Pike, where strings are first-class
objects (like Python, unlike C), you can definitely do it,
syntactically:

while ((line=get_next_line()) != ".")
    process_line(line);

Perfectly legal. Not perfectly readable. In Python, your options are a
helper generator function:

def next_line():
    while True:
        line = get_next_line()
        if line==".": break
        yield line
for line in next_line():
    process_line(line)

or a loop with a critical line of code duplicated above and at the
bottom of the loop (risks 'continue' failure):

line = get_next_line()
while line!=".":
    process_line(line)
    line = get_next_line()

or the "mid-loop break" model, which is what makes this similar to the above:

while True:
    line = get_next_line()
    if line==".": break
    process_line(line)

There's no nice way to spell "grab this and retain it". But again, I'm
not sure how a change of indentation could improve this.

>> That structure makes no sense to me. Why is the "break_when" *outside* of
>> the loop? Why does the "break_when condition" introduce a new block?
>
> How do you mean outside the loop? Do you consider the "else" outside the
> if statement?

The "break_when" is part of the loop structure, not the loop body.
With an entry-checked condition, the loop structure is flush left, and
the loop body is indented:

x, y = 0, 1
while x < y:
    x = (x + y) / 2
    y = f(x)

Some languages have a similar construct for an exit-checked condition, eg REXX:

do until x > y
    /* as above */
end

or BASIC-derived languages:

do
    rem as above
loop while x < y

In all three cases, the condition is on a line that's not indented. So
logically, the mid-checked loop could also go flush left:

do
    some code
while condition
    more code
end

Whether this actually improves readability or not is a separate
question. The "break_when" isn't so much *outside* the loop as simply
*not inside* the loop.

ChrisA

Thread

New to Python - block grouping (spaces) Blake McBride <blake1024@gmail.com> - 2015-04-15 21:07 -0700
  Re: New to Python - block grouping (spaces) Paul Rubin <no.email@nospam.invalid> - 2015-04-15 21:48 -0700
  Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-16 14:51 +1000
  Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-16 15:49 +1000
    Re: New to Python - block grouping (spaces) Paul Rubin <no.email@nospam.invalid> - 2015-04-15 23:11 -0700
      Re: New to Python - block grouping (spaces) William Ray Wing <wrw@mac.com> - 2015-04-16 09:00 -0400
    Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-16 11:51 +0100
      Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-17 03:10 +1000
        Re: New to Python - block grouping (spaces) Tim Chase <python.list@tim.thechases.com> - 2015-04-16 12:49 -0500
        Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-16 22:04 +0100
        Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-17 11:37 +0200
    Re: New to Python - block grouping (spaces) Serhiy Storchaka <storchaka@gmail.com> - 2015-04-16 22:14 +0300
  Re: New to Python - block grouping (spaces) alister <alister.nospam.ware@ntlworld.com> - 2015-04-16 07:46 +0000
    Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 10:47 +0200
    Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-16 19:34 +1000
    Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 12:09 +0200
      Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-16 20:43 +1000
        Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 13:07 +0200
          Re: New to Python - block grouping (spaces)yhoni alister <alister.nospam.ware@ntlworld.com> - 2015-04-16 13:18 +0000
            Re: New to Python - block grouping (spaces)yhoni BartC <bc@freeuk.com> - 2015-04-16 14:44 +0100
              Re: New to Python - block grouping (spaces)yhoni Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-16 18:46 +0100
              Re: New to Python - block grouping (spaces)yhoni alister <alister.nospam.ware@ntlworld.com> - 2015-04-16 18:03 +0000
            Re: New to Python - block grouping (spaces)yhoni Chris Angelico <rosuav@gmail.com> - 2015-04-16 23:50 +1000
            Re: New to Python - block grouping (spaces)yhoni Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 16:09 +0200
              Re: New to Python - block grouping (spaces)yhoni alister <alister.nospam.ware@ntlworld.com> - 2015-04-16 18:04 +0000
        Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-16 23:41 +1000
        Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 15:57 +0200
  Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-16 13:17 +0100
  Re: New to Python - block grouping (spaces) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-04-16 14:48 +0200
  Re: New to Python - block grouping (spaces) Simmo <square.steve@gmail.com> - 2015-04-16 14:37 +0100
  Re: New to Python - block grouping (spaces) Blake McBride <blake1024@gmail.com> - 2015-04-16 07:52 -0700
    Re: New to Python - block grouping (spaces) Blake McBride <blake1024@gmail.com> - 2015-04-16 08:01 -0700
      Re: New to Python - block grouping (spaces) alister <alister.nospam.ware@ntlworld.com> - 2015-04-16 18:08 +0000
        Re: New to Python - block grouping (spaces) memilanuk <memilanuk@gmail.com> - 2015-04-16 11:28 -0700
    Re: New to Python - block grouping (spaces) Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2015-04-16 15:05 +0000
      Re: New to Python - block grouping (spaces) edmondo.giovannozzi@gmail.com - 2015-04-20 03:00 -0700
    Re: New to Python - block grouping (spaces) memilanuk <memilanuk@gmail.com> - 2015-04-16 08:05 -0700
    Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-16 16:05 +0100
      Re: New to Python - block grouping (spaces) CHIN Dihedral <dihedral88888@gmail.com> - 2015-04-19 11:46 -0700
    Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-17 01:03 +1000
    Re: New to Python - block grouping (spaces) Grant Edwards <invalid@invalid.invalid> - 2015-04-16 15:21 +0000
    Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-17 03:41 +1000
      Re: New to Python - block grouping (spaces) Ron Adam <ron3200@gmail.com> - 2015-04-16 14:54 -0400
    EditorConfig for cross-editor consistent code style (was: New to Python - block grouping (spaces)) Ben Finney <ben+python@benfinney.id.au> - 2015-04-17 06:10 +1000
    Re: New to Python - block grouping (spaces) Michael Torrie <torriem@gmail.com> - 2015-04-17 09:44 -0600
      Re: New to Python - block grouping (spaces) Grant Edwards <invalid@invalid.invalid> - 2015-04-17 16:28 +0000
        Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-17 18:05 +0100
          Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-17 10:13 -0700
          Re: New to Python - block grouping (spaces) sohcahtoa82@gmail.com - 2015-04-17 11:13 -0700
            Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-17 23:28 +0300
          Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-18 07:10 +1000
          Re: New to Python - block grouping (spaces) Dan Sommers <dan@tombstonezero.net> - 2015-04-18 01:18 +0000
            Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-17 19:22 -0700
              Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-19 12:44 +0100
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-19 23:23 +1000
                Re: New to Python - block grouping (spaces) wxjmfauth@gmail.com - 2015-04-19 07:22 -0700
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 07:01 -0700
          Re: New to Python - block grouping (spaces) Michael Torrie <torriem@gmail.com> - 2015-04-17 20:14 -0600
            Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-18 09:53 +0300
          Re: New to Python - block grouping (spaces) Ben Finney <ben+python@benfinney.id.au> - 2015-04-18 12:22 +1000
            Re: New to Python - block grouping (spaces) Larry Hudson <orgnut@yahoo.com> - 2015-04-17 22:28 -0700
            Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-18 10:00 +0300
              Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-18 00:13 -0700
                Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-18 10:42 +0300
              Re: New to Python - block grouping (spaces) Michael Torrie <torriem@gmail.com> - 2015-04-19 11:15 -0600
                Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-19 23:41 +0300
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 19:00 -0700
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 12:54 +1000
                Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 13:05 +1000
                Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-20 08:09 +0300
            Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-19 12:38 +0100
              Re: New to Python - block grouping (spaces) Ben Finney <ben+python@benfinney.id.au> - 2015-04-19 22:59 +1000
                Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-19 22:42 +0100
                Re: New to Python - block grouping (spaces) Ron Adam <ron3200@gmail.com> - 2015-04-19 19:28 -0400
                Re: New to Python - block grouping (spaces) Ben Finney <ben+python@benfinney.id.au> - 2015-04-20 09:59 +1000
                Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-20 01:30 +0100
              Re: New to Python - block grouping (spaces) Dave Angel <davea@davea.name> - 2015-04-19 09:18 -0400
              Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-19 23:22 +1000
              Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-19 14:30 +0100
              Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 01:15 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 09:03 -0700
                Re: New to Python - block grouping (spaces) Mel Wilson <mwilson@the-wire.com> - 2015-04-19 17:38 +0000
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 03:53 +1000
                Re: New to Python - block grouping (spaces) Mel Wilson <mwilson@the-wire.com> - 2015-04-19 18:25 +0000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 19:08 -0700
                Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 12:24 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 19:43 -0700
                Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 13:03 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 20:28 -0700
                Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 13:44 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-20 19:18 -0700
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 20:30 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-20 04:51 -0700
                Re: New to Python - block grouping (spaces) albert@spenarnc.xs4all.nl (Albert van der Horst) - 2015-04-25 17:42 +0000
                Re: New to Python - block grouping (spaces) BartC <bc@freeuk.com> - 2015-04-20 13:05 +0100
                Re: New to Python - block grouping (spaces) wxjmfauth@gmail.com - 2015-04-24 01:50 -0700
                Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-20 03:38 +0100
                Re: New to Python - block grouping (spaces) llanitedave <llanitedave@birdandflower.com> - 2015-04-21 08:29 -0700
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-21 10:49 -0700
                Re: New to Python - block grouping (spaces) llanitedave <llanitedave@birdandflower.com> - 2015-04-21 14:35 -0700
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-21 20:11 -0700
                Re: New to Python - block grouping (spaces) llanitedave <llanitedave@birdandflower.com> - 2015-04-21 21:05 -0700
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-22 04:37 -0700
                Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-22 13:05 +0100
                Re: New to Python - block grouping (spaces) Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-04-20 20:38 +1200
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 20:15 +1000
                Re: New to Python - block grouping (spaces) Dan Sommers <dan@tombstonezero.net> - 2015-04-19 18:07 +0000
                Re: New to Python - block grouping (spaces) Chris Angelico <rosuav@gmail.com> - 2015-04-20 06:03 +1000
                Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-19 18:46 -0700
                Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 12:42 +1000
              Re: New to Python - block grouping (spaces) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-20 03:46 +1000
                Re: New to Python - block grouping (spaces) Paul Rubin <no.email@nospam.invalid> - 2015-04-19 13:36 -0700
          Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-24 22:06 -0700
            Re: New to Python - block grouping (spaces) Marko Rauhamaa <marko@pacujo.net> - 2015-04-25 10:27 +0300
              Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-25 09:52 -0700
    Re: New to Python - block grouping (spaces) Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-04-24 08:31 +0100
    Re: New to Python - block grouping (spaces) Michael Torrie <torriem@gmail.com> - 2015-04-24 08:03 -0600
  Re: New to Python - block grouping (spaces) Rustom Mody <rustompmody@gmail.com> - 2015-04-16 10:59 -0700
    Re: New to Python - block grouping (spaces) Rob Gaddi <rgaddi@technologyhighland.invalid> - 2015-04-16 18:45 +0000

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