Deque as a Growable Ring Buffer

Inspired by Rust's VecDeque, we have rewritten Python's
collections.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.

Author: matthiasgoergens

Doc/library/collections.rst

@@ -479,7 +479,8 @@ or subtracting from an empty counter.
     corresponding number of items are discarded from the opposite end.  Bounded
     length deques provide functionality similar to the ``tail`` filter in
     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`.
 
 
     Deque objects support the following methods:
@@ -591,9 +592,11 @@ or subtracting from an empty counter.
 
 In addition to the above, deques support iteration, pickling, ``len(d)``,
 ``reversed(d)``, ``copy.copy(d)``, ``copy.deepcopy(d)``, membership testing with
-the :keyword:`in` operator, and subscript references such as ``d[0]`` to access
-the first element.  Indexed access is *O*\ (1) at both ends but slows to *O*\ (*n*) in
-the middle.  For fast random access, use lists instead.
+the :keyword:`in` operator, subscript references such as ``d[0]`` to access
+the first element, and slicing notation like ``d[i:j:k]`` which returns a new
+deque of the same type (including subclasses) while preserving ``maxlen``.
+Indexed access is *O*\ (1).  Slicing is *O*\ (k) where *k* is the number of
+elements in the slice.
 
 Starting in version 3.5, deques support ``__add__()``, ``__mul__()``,
 and ``__imul__()``.

Doc/whatsnew/3.14.rst

@@ -2193,6 +2193,15 @@ bdb
   to the :class:`~bdb.Bdb` class's new *backend* parameter.
   (Contributed by Tian Gao in :gh:`124533`.)
 
+collections
+-----------
+
+* Slicing a :class:`collections.deque <collections.deque>` is now supported.  Our deque also support
+  constant time indexing.
+
+* Passing a *maxlen* larger than :data:`sys.maxsize` to :class:`collections.deque`
+  now raises :exc:`ValueError` instead of :exc:`OverflowError`.
+
 
 difflib
 -------

Lib/collections/__init__.py

@@ -54,6 +54,12 @@
 except ImportError:
     pass
 
+try:
+    # Ditto for reverse iterators (used by pickle reducers)
+    from _collections import _deque_reverse_iterator  # noqa: F401
+except ImportError:
+    pass
+
 try:
     from _collections import defaultdict
 except ImportError: