Zipping lists of unequal size

I have two lists

a = [1,2,3]
b = [9,10]

I want to combine (zip) these two lists into one list c such that

c = [(1,9), (2,10), (3, )]

Is there any function in standard library in Python to do this?

Solution 1:

Normally, you use itertools.zip_longest for this:

>>> import itertools
>>> a = [1, 2, 3]
>>> b = [9, 10]
>>> for i in itertools.zip_longest(a, b): print(i)
... 
(1, 9)
(2, 10)
(3, None)

But zip_longest pads the shorter iterable with Nones (or whatever value you pass as the fillvalue= parameter). If that's not what you want then you can use a comprehension to filter out the Nones:

>>> for i in (tuple(p for p in pair if p is not None) 
...           for pair in itertools.zip_longest(a, b)):
...     print(i)
... 
(1, 9)
(2, 10)
(3,)

but note that if either of the iterables has None values, this will filter them out too. If you don't want that, define your own object for fillvalue= and filter that instead of None:

sentinel = object()

def zip_longest_no_fill(a, b):
    for i in itertools.zip_longest(a, b, fillvalue=sentinel):
        yield tuple(x for x in i if x is not sentinel)

list(zip_longest_no_fill(a, b))  # [(1, 9), (2, 10), (3,)]

Solution 2:

Another way is map:

a = [1, 2, 3]
b = [9, 10]
c = map(None, a, b)

Although that will too contain (3, None) instead of (3,). To do that, here's a fun line:

c = (tuple(y for y in x if y is not None) for x in map(None, a, b))