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map_longest and map_strict (Python recipe) by Steven D'Aprano
ActiveState Code (http://code.activestate.com/recipes/577687/)

In Python 3, the map builtin silently drops any excess items in its input:

>>> a = [1, 2, 3]
>>> b = [2, 3, 4]
>>> c = [3, 4, 5, 6, 7]
>>> list(map(lambda x,y,z: x*y+z, a, b, c))
[5, 10, 17]

In Python 2, map pads the shorter items with None, while itertools.imap drops the excess items. Inspired by this, and by itertools.zip_longest, I have map_longest that takes a value to fill missing items with, and map_strict that raises an exception if a value is missing.

>>> list(map_longest(lambda x,y,z: x*y+z, a, b, c, fillvalue=0))
[5, 10, 17, 6, 7]
>>> list(map_strict(lambda x,y,z: x*y+z, a, b, c))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in map_strict
ValueError: too few items in iterable
Python, 14 lines
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from itertools import zip_longest

def map_longest(func, *iterables, fillvalue=None):
    for t in zip_longest(*iterables, fillvalue=fillvalue):
        yield func(*t)

def map_strict(func, *iterables, exception=None):
    sentinel = object()
    for t in zip_longest(*iterables, fillvalue=sentinel):
        if sentinel in t:
            if exception is None:
                exception = ValueError('too few items in iterable')
            raise exception
        yield func(*t)
Tags: iteration, map