Zip, Enumerate, Map, Filter

Python · Reference cheat sheet

📋 Overview

enumerate, zip, map, and filter are built-in iteration helpers. Prefer enumerate/zip and comprehensions in idiomatic Python; map/filter shine with existing callables.

🔧 Core concepts

All return iterators (except sorted → list). Consume once unless recreated.

💡 Examples

enumerate and zip:

names = ["ada", "bob", "cy"]
scores = [9, 7, 8]

for i, name in enumerate(names, start=1):
    print(i, name)

for name, score in zip(names, scores, strict=True):
    print(f"{name}: {score}")

unzip:

pairs = [("a", 1), ("b", 2)]
letters, nums = zip(*pairs)
print(letters, nums)

map / filter vs comps:

nums = [1, 2, 3, 4]
print(list(map(str, nums)))
print(list(filter(lambda n: n % 2 == 0, nums)))

# Prefer:
print([str(n) for n in nums])
print([n for n in nums if n % 2 == 0])

dict from parallel lists:

keys = ["host", "port"]
vals = ["localhost", "8000"]
cfg = dict(zip(keys, vals, strict=True))

⚠️ Pitfalls

• zip stops at the shortest iterable unless strict=True (then errors on mismatch).
• map/filter are lazy — wrap list() for reuse/debug.
• enumerate on a dict iterates keys — use .items() when you need pairs.
• Star-unzip zip(*xs) fails on empty xs.
• Heavy lambda with map/filter is usually clearer as a comprehension.

🔗 Related

• Loops
• Comprehensions
• Lambda
• Iterators
• Unpacking
• Examples: list to dict