Chris Rebert wrote:

On Wed, Jul 15, 2009 at 3:50 PM, Jan Kaliszewski wrote:

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Hello,

The issue has been coming back repeatedly:

How we could elegantly and pythonic'ly avoid repeating ourselves if we need the control flow structure that e.g. in PASCAL has the form:

repeat ... until CONDITION

In Python we must use while-loop in not very DRY (Don't Repeat Yourself) manner:

SOME ACTIONS HERE ... while CONDITION: THE SAME ACTIONS AGAIN ...

You can fix that by just writing it as:

while True: SOME ACTIONS HERE if not CONDITION: break

If we were going to add 'repeat' then it could be: repeat: SOME ACTIONS HERE while CONDITION If 'while' starts a loop it ends with a colon and the body of the loop follows either on the same line or indented on the next lines. If 'while' ends a 'repeat' loop it doesn't end with a colon and there isn't anything following on the same line or indented on the next lines.

On Wed, Jul 15, 2009 at 5:16 PM, Jan Kaliszewski wrote:

16-07-2009 o 00:54 Chris Rebert wrote:

...

You can fix that by just writing it as:

while True:    SOME    ACTIONS    HERE    if not CONDITION: break

Yeah, but it's not the same :) because eyes must look for the actual loop condition somewhere-within-the-loop (after all, "while True" is common idiom with large field of usage, not only in such situations...).

Another solution I invented for myself is to use object which once evaluates to True, then always to False, e.g.:

 from itertools import chain, repeat class FirstTrue(object):       def __init__(self):           self._iter = chain([True], repeat(False))       def __nonzero__(self):  # __bool__ for Py3k           return next(self._iter)

Usage:

first = FirstTrue() while first or CONDITION:     SOME     ACTIONS     HERE

or, if we want to *evaluate* CONDITION also at first time:

first = FirstTrue() while CONDITION or first:     SOME     ACTIONS     HERE

I usually do: FIRST = 1 while CONDITION or FIRST: FIRST = 0 SOME ACTIONS HERE I use 1/0 instead of True/False because older Pythons loaded numeric constants faster than the names True/False (is that still the case?) - Josiah

Wouldn't be nice to have such factory as built-in or in itertools?

Best regards, *j

-- Jan Kaliszewski _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

Steven D'Aprano wrote:

On Thu, 16 Jul 2009 10:32:01 am Josiah Carlson wrote:

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I use 1/0 instead of True/False because older Pythons loaded numeric constants faster than the names True/False (is that still the case?)

...

From Python 2.6.1:

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...

...

import dis dis.dis(compile("while 1: break", "", "exec")) 1 0 SETUP_LOOP 4 (to 7) >> 3 BREAK_LOOP 4 JUMP_ABSOLUTE 3 >> 7 LOAD_CONST 0 (None) 10 RETURN_VALUE dis.dis(compile("while True: break", "", "exec")) 1 0 SETUP_LOOP 13 (to 16) >> 3 LOAD_NAME 0 (True) 6 JUMP_IF_FALSE 5 (to 14) 9 POP_TOP 10 BREAK_LOOP 11 JUMP_ABSOLUTE 3 >> 14 POP_TOP 15 POP_BLOCK >> 16 LOAD_CONST 0 (None) 19 RETURN_VALUE

True is a constant in Python 3.0.

In 3.1

...

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dis.dis(compile("while True: break", "", "exec")) 1 0 SETUP_LOOP 4 (to 7) >> 3 BREAK_LOOP 4 JUMP_ABSOLUTE 3 >> 7 LOAD_CONST 0 (None) 10 RETURN_VALUE

Same as while 1 tjr

On Wed, Jul 15, 2009 at 2:16 PM, Jan Kaliszewski wrote:

16-07-2009 o 00:54 Chris Rebert wrote:

...

You can fix that by just writing it as:

while True:    SOME    ACTIONS    HERE    if not CONDITION: break

Yeah, but it's not the same :) because eyes must look for the actual loop condition somewhere-within-the-loop (after all, "while True" is common idiom with large field of usage, not only in such situations...).

I think we can just piggyback off of Python's existing optional support for braces, BEGIN, END, etc.: while True: #DO WHILE COND { something # } if cond: break :-D My favorite proposal out of the last discussion of do-while was to make "while:" mean "while True:". Combined with Python's optional support for braces, it sounds like a winner to me! :-D Optionally-yrs, -- Carl Johnson

Steven D'Aprano wrote:

On Thu, 16 Jul 2009 08:50:06 am Jan Kaliszewski wrote:

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Hello,

The issue has been coming back repeatedly:

And not very long ago at that.

So often that there's even a PEP* that documents the pros and cons of different ideas and points out that a viable syntactic challenger to the "while True with inner break" approach has yet to be found. The current idiom does have the downside of potentially hiding the termination condition, but it otherwise handles generalised looping very nicely. (Putting the break on a separate line also helps a great deal with the visibility of the termination condition) Cheers, Nick. * http://www.python.org/dev/peps/pep-0315/ -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia ---------------------------------------------------------------