History question about for .. else

David Bolen db3l at fitlinxx.com
Wed Nov 8 03:36:38 CET 2000


Dave Cole <djc at object-craft.com.au> writes:

> I just used the else clause on a for loop for the first time and my
> business partner does not like it.  We launched into a discussion
> about the construct.
> 
>         for value in list:
>             if condition:
>                 # do something
>                 break
>         else:
>             # do something else
> 
> My partner is of the opinion that the syntax is ugly, and he would
> avoid it if at all possible.  I argued that it is a zero-cost
> detection of "falling of the end of the loop" as it prevents you
> having to use an additional condition variable and subsequent test.
> 
> I would be interested in knowing the history behind the introduction
> of the else clause on the for loop.

It sort of seems difficult to understand why:

    found = 0

    for value in list:                        for value in list:
        if condition:                             if condition:
            # do something      could be              # do something
            found = 1           less ugly             break
            break               than ->

    if not found:                             else:
        # do something                            # do something


I can't speak directly to the history, but I know that my own code in
the past has often used the flag pattern (although in C code I in a
for loop I sometimes avoid the flag variable in favor of just
inverting the for termination expression), so I was actually
pleasantly surprised to see it so directly supported (and to me
elegantly) by Python.

--
-- David
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