for: else: - any practical uses for the else clause?

[Paul Rubin]

Peter Otten <__peter__ at web.de> writes:
> I like
> 
> def blocks_til_mark(stack):
>     for block in stack:
>         if block.is_marked():
>             return
>         yield block
>     raise SomeError
> height = sum(block.height for block in blocks_til_mark(stack))

Oh my, I realize now that I mis-read the original, and my examples
were all wrong.  Shows how the explicit loop isn't necessarily so
clear ;-).  Note that unlike the original, the loop above fails to set
height = 0 in the case where the exception gets raise.

I'd maybe just scan the stack twice.  The rescan is needed only if
there can be blocks with height <= 0.  Otherwise, you can just raise
SomeError if height == 0:

    height = sum(b.height for b in 
         itertools.takewhile(lambda: not block.is_marked(), stack))

    if height == 0 and True not in (b.is_marked() for b in stack):
         raise SomeError