[Python-ideas] if condition: break idiom

[Leif Walsh]

On Sat, Sep 20, 2008 at 11:41 PM, Carl Johnson <carl at carlsensei.com> wrote:
> def while(x):
>    if x > 10:
>        raise Break
>    else:
>        return x
>
> [while(x) for x in range(20)]  #produces [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>
> Right now, you can do something this with list(generator expression) by
> raising StopIteration, but it is considered a hack, and it doesn't work with
> list comprehensions.

I imagine what you are thinking of is:

"""
def While(x):
  if x > 10:
    raise StopIteration
  else:
    return x

list(While(x) for x in range(20))
"""

This doesn't seem too horrific to me, and certainly doesn't merit
adding Break and Continue as Exception-like objects.  I feel this
would just make it unbearably hard for beginners moving in from <every
other language that has looping block constructs and uses break and
continue as statements>.

An alternate construct could be something like:

"""
def While(x):
  if x > 10:
    return None
  else:
    return x

[w for w in (While(x) for x in range(20)) if w is not None]
"""

Of course, this looks silly in this example, but in a context where
While were to do something nontrivial, I think it would be suitable.

-- 
Cheers,
Leif