zip()
Iterates over multiple iterables in parallel, pairing items.
for a, b in zip(iter1, iter2):
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What it does
Overview
Iterates over multiple iterables in parallel, pairing items.
zip() is a built-in Python function that aggregates elements from multiple iterables (such as lists, tuples, or strings) into an iterator of tuples. The i-th tuple contains the i-th element from each of the input arguments. It utilizes lazy evaluation, meaning it generates elements on-the-fly and consumes very little memory even when processing massive datasets. A critical technical detail is the 'shortest wins' behavior: zip() stops as soon as the shortest input iterable is exhausted, which can lead to silent data loss if sequences are of unequal length. Since Python 3.10, the 'strict' parameter can be set to True to raise a ValueError if lengths do not match. Performance-wise, zip() runs in O(N) time where N is the length of the shortest iterable and O(1) auxiliary space beyond the resulting tuples.
Quick reference
Syntax
for a, b in zip(iter1, iter2):
...Inputs
Parameters
See it in practice
Examples
Parallel Iteration and Combination
products = ['Laptop', 'Mouse', 'Keyboard']
prices = [1200, 25, 75]
for item, price in zip(products, prices):
print(f'Product: {item}, Price: ${price}')Product: Laptop, Price: $1200 Product: Mouse, Price: $25 Product: Keyboard, Price: $75
Combines two related lists into pairs for simultaneous iteration in a single loop.
Matrix Transposition
matrix = [[1, 2, 3], [4, 5, 6]]
transposed = list(zip(*matrix))
print(transposed)[(1, 4), (2, 5), (3, 6)]
Uses the unpacking operator (*) with zip to swap rows and columns of a 2D data structure.
Building Dictionaries Dynamically
headers = ['id', 'username', 'email']
user_data = [1001, 'dev_user', 'dev@example.com']
user_dict = dict(zip(headers, user_data))
print(user_dict){'id': 1001, 'username': 'dev_user', 'email': 'dev@example.com'}
Pairs keys and values together and passes the iterator directly to the dict() constructor.
Debug faster
Common Errors
LogicError
Cause: The input iterables have different lengths, causing zip() to silently drop trailing data from the longer iterables.
Fix: Use itertools.zip_longest() if you need to preserve all data, or set strict=True (Python 3.10+) to catch length mismatches.
list(zip([1, 2, 3], ['a', 'b']))StopIteration / Exhaustion
Cause: zip() returns an iterator, which can only be traversed once. Attempting to iterate over the same zip object a second time will yield nothing.
Fix: Convert the zip object to a list or tuple if you need to reuse the aggregated data multiple times.
z = zip([1], [2])
list(z)
print(list(z))Runtime support
Compatibility
Source: MDN Web Docs
Common questions
Frequently Asked Questions
Iterates over multiple iterables in parallel, pairing items.
iterables: Two or more iterables to combine.
LogicError: Use itertools.ongest() if you need to preserve all data, or set strict=True (Python 3.10+) to catch length mismatches. StopIteration / Exhaustion: Convert the zip object to a list or tuple if you need to reuse the aggregated data multiple times.