Iterators

Python · Reference cheat sheet

📋 Overview

An iterable can produce an iterator via iter(obj). An iterator yields values with next(it) until StopIteration. For-loops, comprehensions, and unpacking all consume iterators.

🔧 Core concepts

Protocol	Methods
Iterable	`__iter__` → iterator
Iterator	`__iter__` (returns self) + `__next__`
`iter(x)`	Get iterator
`next(it, default)`	Next value or default
Sequence	`__len__` + `__getitem__` also iterable

Many built-ins are iterable: list, dict, range, files, generators. Custom classes can implement the protocols or use generators.

💡 Examples

Manual iteration:

it = iter([10, 20, 30])
print(next(it))
print(next(it))
print(next(it, None))
print(next(it, None))  # None — exhausted

Custom iterator:

from collections.abc import Iterator

class CountDown:
    def __init__(self, start: int) -> None:
        self.current = start

    def __iter__(self) -> Iterator[int]:
        return self

    def __next__(self) -> int:
        if self.current <= 0:
            raise StopIteration
        self.current -= 1
        return self.current + 1

print(list(CountDown(3)))  # [3, 2, 1]

Iterable (fresh iterator each time):

class Repeat:
    def __init__(self, value: str, times: int) -> None:
        self.value = value
        self.times = times

    def __iter__(self):
        for _ in range(self.times):
            yield self.value

r = Repeat("x", 3)
print(list(r), list(r))  # both work

tee / consume carefully:

from itertools import tee

a, b = tee(range(3), 2)
print(list(a), list(b))

⚠️ Pitfalls

Iterators are exhausted after one pass; iterables like list can re-iterate.
Calling iter on an iterator returns itself — no rewind.
Mutating a collection while iterating is unsafe.
StopIteration inside generators becomes runtime errors in some contexts (PEP 479).
Infinite iterators need explicit stopping (islice, break).

Iterators

Iterators

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

On this page