Outerzip / zip longest function (with multiple fill values)

Question

Is there a Python function an "outer-zip", which is a extension of zip with different default values for each iterable?

a = [1, 2, 3]   # associate a default value 0
b = [4, 5, 6, 7] # associate b default value 1

zip(a,b)  # [(1, 4), (2, 5), (3, 6)]

outerzip((a, 0), (b, 1)) = [(1, 4), (2, 5), (3, 6), (0, 7)]
outerzip((b, 0), (a, 1)) = [(4, 1), (5, 2), (6, 3), (7, 1)]

I can almost replicate this outerzip function using map, but with None as the only default:

map(None, a, b) # [(1, 4), (2, 5), (3, 6), (None, 7)]

Note1: The built-in zip function takes an arbitrary number of iterables, and so should an outerzip function. (e.g. one should be able to calculate outerzip((a,0),(a,0),(b,1)) similarly to zip(a,a,b) and map(None, a, a, b).)

Note2: I say "outer-zip", in the style of this haskell question, but perhaps this is not correct terminology.

Answer 1

It's called izip_longest (zip_longest in python-3.x):

>>> from itertools import zip_longest
>>> a = [1,2,3]
>>> b = [4,5,6,7]
>>> list(zip_longest(a, b, fillvalue=0))
[(1, 4), (2, 5), (3, 6), (0, 7)]

Answer 2

You could modify zip_longest to support your use case for general iterables.

from itertools import chain, repeat

class OuterZipStopIteration(Exception):
    pass

def outer_zip(*args):
    count = len(args) - 1

    def sentinel(default):
        nonlocal count
        if not count:
            raise OuterZipStopIteration
        count -= 1
        yield default

    iters = [chain(p, sentinel(default), repeat(default)) for p, default in args]
    try:
        while iters:
            yield tuple(map(next, iters))
    except OuterZipStopIteration:
        pass


print(list(outer_zip( ("abcd", '!'), 
                      ("ef", '@'), 
                      (map(int, '345'), '$') )))

Answer 3

This function can be defined by extending each inputted list and zipping:

def outerzip(*args):
    # args = (a, default_a), (b, default_b), ...
    max_length = max( map( lambda s: len(s[0]), args))
    extended_args = [ s[0] + [s[1]]*(max_length-len(s[0])) for s in args ]
    return zip(*extended_args)

outerzip((a, 0), (b, 1)) # [(1, 4), (2, 5), (3, 6), (0, 7)]