---
title: "Python itertools Cheatsheet"
slug: "python-itertools"
category: "cheatsheets"
tags: ["cheatsheets", "python", "itertools", "functional", "iterators", "standard-library"]
status: "stable"
last_updated: 2026-10-01
summary: "Python itertools functions for chaining, slicing, combinatorics, grouping, and running reductions, with the groupby sorting rule and infinite-iterator gotchas."
related: ["[[coding/python]]", "[[cheatsheets/python-collections]]", "[[cheatsheets/python-string-formatting]]", "[[cheatsheets/python-regex]]", "[[coding/python-performance]]", "[[cheatsheets/index|Cheatsheets]]"]
---

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## Overview

Every `itertools` function returns a lazy iterator: nothing runs until you iterate, and an iterator can be consumed only once. Wrap in `list()` when you need a sequence. Data-structure helpers are on [[cheatsheets/python-collections]].

## Chain, repeat, and slice

Use these to avoid building intermediate lists.

| Function | Example | Yields |
| --- | --- | --- |
| `chain(*its)` | `chain([1,2],[3])` | 1, 2, 3 |
| `chain.from_iterable(its)` | `chain.from_iterable([[1,2],[3]])` | 1, 2, 3 (lazy over the outer iterable) |
| `zip_longest(*its, fillvalue=None)` | `zip_longest([1,2],[3])` | (1,3), (2,None) |
| `count(start, step)`, `cycle(it)`, `repeat(x, n)` | `count(10, 5)` | 10, 15, 20, ... (infinite unless `repeat` has `n`) |
| `islice(it, start, stop, step)` | `islice(range(100), 5, 15, 2)` | 5, 7, 9, 11, 13 |
| `takewhile(pred, it)`, `dropwhile(pred, it)` | `takewhile(lambda x: x<5, [1,3,6,2])` | 1, 3 |
| `filterfalse(pred, it)`, `compress(data, sel)` | `compress("ABCD", [1,0,1,0])` | A, C |
| `batched(it, n)` | `batched("abcde", 2)` | ('a','b'), ('c','d'), ('e',) (3.12+; `strict=` in 3.13) |
| `pairwise(it)` | `pairwise("abc")` | ('a','b'), ('b','c') (3.10+) |
| `starmap(fn, it)`, `tee(it, n)` | `starmap(pow, [(2,3)])` | 8; `tee` makes independent copies. |

## Combinatorics

Check the result count before materializing a combinatoric iterator.

| Function | Example | Count |
| --- | --- | --- |
| `product(*its, repeat=1)` | `product([0,1], repeat=2)` | n^r; same as nested loops. |
| `permutations(it, r)` | `permutations([1,2,3], 2)` | n! / (n-r)! |
| `combinations(it, r)` | `combinations([1,2,3], 2)` | n! / (r! (n-r)!) |
| `combinations_with_replacement(it, r)` | `combinations_with_replacement([1,2], 2)` | Items may repeat. |

## groupby and accumulate

`groupby` starts a new group whenever the key changes, so sort by the same key first.

```python
from itertools import groupby, accumulate
import operator

rows = sorted(rows, key=lambda r: r["dept"])
by_dept = {k: list(g) for k, g in groupby(rows, key=lambda r: r["dept"])}

list(accumulate([1, 2, 3, 4]))                  # [1, 3, 6, 10]
list(accumulate([3, 1, 4, 1, 5], max))          # [3, 3, 4, 4, 5]
list(accumulate([1, 2, 3], initial=10))         # [10, 11, 13, 16]
list(accumulate([1, 2, 3, 4], operator.mul))    # [1, 2, 6, 24]
```

Use `accumulate` for the running series and `functools.reduce` for only the final value.

## Gotchas

Avoid these; each yields empty, repeated, or infinite output.

| Gotcha | Fix |
| --- | --- |
| `groupby` on unsorted input yields repeated groups for one key. | Sort by the same key first. |
| Each group iterator is invalid once the outer iterator advances. | Consume it with `list(g)` before moving on. |
| `cycle`, `count`, and `repeat(x)` never stop. | Bound them with `islice` or `takewhile`. |
| `product(*[big] * 3)` builds n^3 tuples. | Check the count before materializing. |
| `islice` rejects negative indexes. | For the last n items use `deque(it, maxlen=n)`. |
| An exhausted iterator is empty on a second pass. | Use `tee`, or rebuild the iterator. |

## Related

- [[coding/python]]
- [[cheatsheets/python-collections]]
- [[cheatsheets/python-string-formatting]]
- [[cheatsheets/python-regex]]
- [[coding/python-performance]]