zip()

zip()

What it is

zip(*iterables) pairs up elements from two or more iterables position by position, yielding tuples, and stops at the shortest input — it never pads or errors for mismatched lengths by default.

Before this feature

names = ["Alice", "Bob"]
ages = [30, 25, 40]  # note: 3 items, one more than names

pairs = []
for i in range(min(len(names), len(ages))):
    pairs.append((names[i], ages[i]))

After this feature

names = ["Alice", "Bob"]
ages = [30, 25, 40]

pairs = list(zip(names, ages))
print(pairs)  # [('Alice', 30), ('Bob', 25)] -- the extra 40 is silently dropped

for name, age in zip(names, ages):
    print(f"{name} is {age}")

# Unzipping — the inverse operation
pairs = [('Alice', 30), ('Bob', 25)]
names, ages = zip(*pairs)
print(names, ages)  # ('Alice', 'Bob') (30, 25)

Why this is better

• No manual index bookkeeping: pairing two lists is a one-liner instead of an indexed loop.
• Works with any number of iterables: zip(a, b, c) pairs three (or more) sequences at once, not just two.
• Lazy: zip returns an iterator, computing pairs on demand rather than building the whole list upfront — matters for large or infinite iterables.

Key notes / edge cases

• zip silently truncates to the shortest iterable — a length mismatch is not an error, which can hide bugs. Use itertools.zip_longest(*iterables, fillvalue=None) if you need to pad instead of truncate.
• zip(*pairs) — unpacking a list of tuples back into separate sequences — is a common and slightly non-obvious trick (zip is its own inverse when combined with *).
• In Python 2, zip() returned a list; in Python 3 it returns an iterator, so wrap it in list(...) if you need to index into the result or iterate it more than once.

Quick practice

1. What does list(zip([1,2,3], ['a','b'])) return?

   Answer

   `[(1, 'a'), (2, 'b')]` — it stops at the shorter iterable (length 2), silently dropping the `3`.

2. How do you zip together three lists instead of two?

   Answer

   `zip(list1, list2, list3)` — `zip` accepts any number of iterables.

3. What would you use instead of zip if you wanted mismatched-length inputs padded rather than truncated?

   Answer

   `itertools.zip_longest(*iterables, fillvalue=...)`.