Python: zip-like function that pads to longest length?

Question

Is there a built-in function that works like zip() but that will pad the results so that the length of the resultant list is the length of the longest input rather than the shortest input?

>>> a=['a1']
>>> b=['b1','b2','b3']
>>> c=['c1','c2']

>>> zip(a,b,c)
[('a1', 'b1', 'c1')]

>>> What command goes here?
[('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]

Answer 1

itertools.izip_longest (note: it was renamed in py3k to zip_longest).

>>> list(itertools.izip_longest(a, b, c))
[('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]

Answer 2

You can either user itertools.izip_longest. But you need python 2.6 for that. Or you can use map with None. It is a little know feature of map but it doesn't work in py3k.

>>> map(None, a, b, c)
[('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]