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Update dict/OrderedDict differences with code equivalents. (GH-31563)

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Raymond Hettinger 2022-02-25 08:49:53 -06:00 committed by GitHub
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Doc/library/collections.rst
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@ -1092,18 +1092,35 @@ Some differences from :class:`dict` still remain:
Space efficiency, iteration speed, and the performance of update
operations were secondary.
* Algorithmically, :class:`OrderedDict` can handle frequent reordering
operations better than :class:`dict`. This makes it suitable for tracking
recent accesses (for example in an `LRU cache
<https://medium.com/@krishankantsinghal/my-first-blog-on-medium-583159139237>`_).
* The :class:`OrderedDict` algorithm can handle frequent reordering operations
better than :class:`dict`. As shown in the recipes below, this makes it
suitable for implementing various kinds of LRU caches.
* The equality operation for :class:`OrderedDict` checks for matching order.
A regular :class:`dict` can emulate the order sensitive equality test with
``p == q and all(k1 == k2 for k1, k2 in zip(p, q))``.
* The :meth:`popitem` method of :class:`OrderedDict` has a different
signature. It accepts an optional argument to specify which item is popped.
* :class:`OrderedDict` has a :meth:`move_to_end` method to
efficiently reposition an element to an endpoint.
A regular :class:`dict` can emulate OrderedDict's ``od.popitem(last=True)``
with ``d.popitem()`` which is guaranteed to pop the rightmost (last) item.
A regular :class:`dict` can emulate OrderedDict's ``od.popitem(last=False)``
with ``(k := next(iter(d)), d.pop(k))`` which will return and remove the
leftmost (first) item if it exists.
* :class:`OrderedDict` has a :meth:`move_to_end` method to efficiently
reposition an element to an endpoint.
A regular :class:`dict` can emulate OrderedDict's ``od.move_to_end(k,
last=True)`` with ``d[k] = d.pop(k)`` which will move the key and its
associated value to the rightmost (last) position.
A regular :class:`dict` does not have an efficient equivalent for
OrderedDict's ``od.move_to_end(k, last=False)`` which moves the key
and its associated value to the leftmost (first) position.
* Until Python 3.8, :class:`dict` lacked a :meth:`__reversed__` method.