There is a XXX in v3.5.0 shows that need to dealing with deque maxlen setting case in deque_repeat.

Although there have common case for deque size 1 with maxlen, other size of deque with maxlen still using for-loop to extend the deque, without any detection.

Adding this fast break will reduce the execution speed when repeat deque with maxlen.

---
$ cat tests.py
from collections import deque
for _ in range(10:
    d = deque(maxlen=100_000)
    d.insert(0, 0)
    d.insert(0, 10)
    d * 10_000_000
$ time ./python_with_patch tests.py
$ time ./python tests.py

Wouldn't be better to update the number of repeats before the loop rather than checking at every iteration?

if (deque->maxlen >= 0 && n * size > deque->maxlen)
    n = (deque->maxlen + size - 1) / size;

Serhiy: yes, your advice is better than checking inside the loop.

I have updated this to the commit, thanks!

The code looks fine.  Please change the comment to something like:

/* Reduce the number of repetitions when maxlen would be exceeded */

Raymond: comment has changed, pushed on to GitHub.

There is an opportunity of further optimisation. For example deque(range(100000), maxlen=100001) *= 2 copies 100000 elements while it is enough to copy just 1 element. But I don't know whether it is worth to optimise this.

New changeset 357bad7101df196d7f4ea3dc6ed9a6436afb022c by Raymond Hettinger (Louie Lu) in branch 'master':
bpo-29634: Reduce deque repeat execution when maxlen exist and size is not 1 (#255) (#255)
https://github.com/python/cpython/commit/357bad7101df196d7f4ea3dc6ed9a6436afb022c