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gh-141335: Deque as a Growable Ring Buffer#141337

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matthiasgoergens wants to merge5 commits intopython:main
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@matthiasgoergensmatthiasgoergens commentedNov 10, 2025•
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Inspired by Rust's VecDeque, we have rewritten Python'scollections.deque to use a growable ring buffer implementation. This change improves memory efficiency and performance for various operations, including indexing and slicing.

Specifically, indexing is now O(1), and slicing is supported at all in O(k) time, where k is the number of elements in the slice.

Unix. They are also useful for tracking transactions and other pools of data
where only the most recent activity is of interest.
where only the most recent activity is of interest. Passing a *maxlen*
greater than:data:`sys.maxsize` raises:exc:`ValueError`.
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We could also leave the behaviour the same as before (raiseOverflowError), or fix it to be in line with what eg slicing for lists does (which handles huge numbers gracefully.)

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Pull Request Overview

This PR reimplements thecollections.deque data structure from a doubly-linked list of blocks to a growable ring buffer (circular array), authored by Matthias Goergens. The primary goals are to improve performance for indexing operations and add slicing support.

Key Changes

  • Replaced the block-based doubly-linked list implementation with a power-of-2 sized ring buffer using a single dynamically allocated array
  • Added O(1) indexing support (previously O(n) in the middle) and O(k) slicing support for deques
  • Updated documentation to reflect the new capabilities and performance characteristics

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 14 comments.

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FileDescription
Modules/_collectionsmodule.cComplete rewrite of deque internals from block-based to ring buffer implementation, including all operations (append, pop, rotate, etc.)
Modules/clinic/_collectionsmodule.c.hUpdated generated clinic code for deque_rotate to accept PyObject* instead of Py_ssize_t
Lib/test/test_deque.pyAdded extensive hypothesis-based property tests and new test cases for slicing functionality
Lib/collections/init.pyAdded import for reverse iterator type needed for pickle support
Doc/whatsnew/3.14.rstDocumented new slicing support and maxlen validation change
Doc/library/collections.rstUpdated documentation to reflect O(1) indexing, slicing support, and maxlen validation behavior

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/* collections module implementation of a deque() datatype
Written and maintained by Raymond D. Hettinger <python@rcn.com>
Originally written by Raymond D. Hettinger <python@rcn.com>
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@rhettinger I'm happy to handle this however you feel like, including leaving the whole comment as is.

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Hmm, looks like I broke some of the tests. Will fix.

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@rhettingerrhettingerAwaiting requested review from rhettingerrhettinger is a code owner

@AA-TurnerAA-TurnerAwaiting requested review from AA-Turner

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