[Python-ideas] Add nullifier argument to functools.reduce?

[Peter Otten]

David Mertz wrote:

>   def reduce_with_attractor(func, it, start=None, end_if=None):
>       it = iter(it)
>       start = start if start!=None else it.__next__()
>       return list(takewhile(lambda x: x!=end_if,
>                             accumulate(chain([start],it), func)))[-1]

Wouldn't it be better to break this into a function that limits a sequence 
and to combine that with the original reduce()?

>>> from functools import reduce
>>> from itertools import takewhile
>>> def stop_on(value, items):
...     return takewhile(lambda item: item != value, items)
... 
>>> list(stop_on(0, [1, 2, 3, 0, 4]))
[1, 2, 3]
>>> from operator import mul
>>> reduce(mul, stop_on(0, [1, 2, 3, 0, 4]))
6

My suggestion is to add a value-limited version of itertools.takewhile() 
rather than making reduce() more more powerful/complex.