zip_longest¶

When combining iterables of unequal length, Python's built-in zip() silently truncates to the shortest input — which can cause data loss. zip_longest() from the itertools module solves this by continuing until every iterable is exhausted, filling in a default value for the shorter ones.

Zipping Different Length Iterables¶

The following example pairs a 3-element list with a 5-element list. The fillvalue parameter specifies what to substitute for the missing entries in the shorter iterable.

from itertools import zip_longest

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c', 'd', 'e']

result = list(zip_longest(list1, list2, fillvalue='*'))
print(result)

[(1, 'a'), (2, 'b'), (3, 'c'), ('*', 'd'), ('*', 'e')]

Custom Fill Value¶

The default fill value is None, but you can pass any object via the fillvalue parameter. This is especially useful when the fill value must be meaningful in downstream processing — for example, using 'N/A' for missing names.

from itertools import zip_longest

names = ['Alice', 'Bob']
scores = [95, 87, 92, 88]

result = list(zip_longest(names, scores, fillvalue='N/A'))
for name, score in result:
    print(f"{name}: {score}")

Alice: 95
Bob: 87
N/A: 92
N/A: 88