Loops

Python · Reference cheat sheet

📋 Overview

Python iterates with for over any iterable and while for condition-driven loops. Prefer for + comprehensions over index arithmetic. Control flow: break, continue, else on loops (runs if no break), and enumerate / zip for paired iteration.

🔧 Core concepts

range(stop), range(start, stop[, step]) for integer sequences.

💡 Examples

Enumerate, zip, unpacking:

names = ["Ada", "Bob"]
scores = [10, 7]

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

for name, score in zip(names, scores, strict=True):  # 3.10+
    print(f"{name}: {score}")

Loop else (search):

needle = 4
for n in [1, 2, 3]:
    if n == needle:
        print("found")
        break
else:
    print("not found")

Comprehensions and generators:

squares = [n * n for n in range(5) if n % 2 == 0]
gen = (n * n for n in range(1_000_000))  # lazy
total = sum(gen)

While with sentinel:

from collections.abc import Iterator

def read_chunks() -> Iterator[str]:
    yield "a"
    yield "b"

it = iter(read_chunks())
while True:
    try:
        chunk = next(it)
    except StopIteration:
        break
    print(chunk)

⚠️ Pitfalls

• Do not mutate a list you are iterating; iterate a copy or build a new list.
• for i in range(len(xs)) is usually worse than for x in xs / enumerate.
• Infinite while True needs a clear exit; prefer iterators when possible.
• zip truncates to the shortest iterable unless strict=True.
• Creating huge lists with comprehensions can exhaust memory—use generators.

🔗 Related

• Dictionaries
• Types
• Strings
• *args and **kwargs
• Async / await
• Examples: merge lists