[Python-ideas] for/except/else syntax

[Rob Cliffe]

I'm coming to this debate rather late but .....

I did find the for ... else syntax confusing when I learned Python, and I 
still do a little.

I would favour keeping for-loops exactly the same except allowing a more 
intelligible keyword instead of 'else', e.g.
'ifnobreak'.  Then moving towards deprecating and ultimately forbidding the 
use of 'else'.

Rob Cliffe

----- Original Message ----- 
From: "Arnaud Delobelle" <arnodel at googlemail.com>
To: "Yuvgoog Greenle" <ubershmekel at gmail.com>
Cc: <python-ideas at python.org>
Sent: Wednesday, October 07, 2009 10:15 AM
Subject: Re: [Python-ideas] for/except/else syntax


2009/10/7 Yuvgoog Greenle <ubershmekel at gmail.com>:
> I don't like your "except" and "else" only because it's not obvious
> when they execute.
>
> btw your example breaks the current "for..else" search idiom. C,
> suite_if_loop_is_not_broken, should be labelled
> suite_if_loop_is_not_broken_and_not_empty in your example code.
>
> As I understand it, pythoneers need a way to know what happened with a
> previous loop. Did it break? Did it execute at all? The answer to both
> questions is either True or False and developers need these answers
> only after the loop finished. If python wasn't in over it's head with
> reserved words I would suggest adding "loop":
>
> for i in SEQ:
> A
> if not loop.broke:
> C # suite_if_loop_is_not_broken
> elif loop.empty:
> B # suite_if_loop_body_is_not_executed
>
> For optimization I would suggest the "loop" variable only be
> instantiated if it's referenced. Also, there may be better names for
> "loop", "broke" and "empty".
>
> Two drawbacks:
> 1. python's in over it's head with reserved words.
> 2. developers might want to use the "loop" variable a mile down from
> the loop which would be ugly but their fault.
>
> I'm writing a summary of the previous for/else thread and hope that by
> the time I finish that (a week+), someone will invent a readable,
> low-cost syntax (optional but awesome if it can handle all 4:
> break/not break/empty/not empty).

Here's an idea with current syntax (python < 3):

class Loop(object):
    def __init__(self, iterable):
        self.iterable = iterable
    def __iter__(self):
        self.broken = True
        self.empty = True
        for i in self.iterable:
            self.empty = False
            yield i
        self.broken = False

# Example:

for string in 'spam', 'bot', '':
    loop = Loop(string)
    for i in loop:
        if i == 'a':
            break
    print repr(string), ':'
    if loop.broken: print 'broken',
    if loop.empty: print 'empty',
    print

# Output:
'spam' : broken
'bot' :
'' : empty

-- 
Arnaud
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