How to Pad the Results of Python\'s zip() to the Length of the Longest Input?

How to Zip and Pad to the Longest Length in Python

In Python, the zip() function is used to combine elements from two or more iterable objects. However, zip() only considers the length of the shortest input. What if you want to pad the results to the length of the longest input?

itertools.zip_longest

In Python 3, the itertools.zip_longest() function provides this functionality. It takes multiple iterables and returns an iterator that yields tuples containing elements from the iterables, with None values padding the shorter iterables.

import itertools

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

result = list(itertools.zip_longest(a, b, c))
print(result)
# Output: [('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]

fillvalue Parameter

You can specify a fillvalue parameter to use instead of None for padding.

result = list(itertools.zip_longest(a, b, c, fillvalue='foo'))
print(result)
# Output: [('a1', 'b1', 'c1'), ('foo', 'b2', 'c2'), ('foo', 'b3', 'foo')]

Python 2 Alternatives

For Python 2, you have two options:

1. map(None, ...): This Python 2-specific feature of map can achieve the same result as zip_longest().

result = map(None, a, b, c)
print(list(result))
# Output: [('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]

2. itertools.izip_longest: This function was introduced in Python 2.6 and provides the same functionality as zip_longest().

from itertools import izip_longest

result = list(izip_longest(a, b, c))
print(result)
# Output: [('a1', 'b1', 'c1'), (None, 'b2', 'c2'), (None, 'b3', None)]

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