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[3.10] bpo-44782: Improve OrderedDict recipe for LRU cache variants (GH-27536) #27566

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Merged
miss-islington merged 1 commit into from
Aug 2, 2021
Merged

[3.10] bpo-44782: Improve OrderedDict recipe for LRU cache variants (GH-27536) #27566

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48 changes: 32 additions & 16 deletions Doc/library/collections.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1171,27 +1171,43 @@ original insertion position is changed and moved to the end::
self.move_to_end(key)

An :class:`OrderedDict` would also be useful for implementing
variants of :func:`functools.lru_cache`::
variants of :func:`functools.lru_cache`:

class LRU(OrderedDict):
'Limit size, evicting the least recently looked-up key when full'
.. testcode::

def __init__(self, maxsize=128, /, *args, **kwds):
self.maxsize = maxsize
super().__init__(*args, **kwds)
class LRU:

def __getitem__(self, key):
value = super().__getitem__(key)
self.move_to_end(key)
def __init__(self, func, maxsize=128):
self.func = func
self.maxsize = maxsize
self.cache = OrderedDict()

def __call__(self, *args):
if args in self.cache:
value = self.cache[args]
self.cache.move_to_end(args)
return value
value = self.func(*args)
if len(self.cache) >= self.maxsize:
self.cache.popitem(False)
self.cache[args] = value
return value

def __setitem__(self, key, value):
if key in self:
self.move_to_end(key)
super().__setitem__(key, value)
if len(self) > self.maxsize:
oldest = next(iter(self))
del self[oldest]
.. doctest::
:hide:

>>> def square(x):
... return x ** 2
...
>>> s = LRU(square, maxsize=5)
>>> actual = [(s(x), s(x)) for x in range(20)]
>>> expected = [(x**2, x**2) for x in range(20)]
>>> actual == expected
True
>>> actual = list(s.cache.items())
>>> expected = [((x,), x**2) for x in range(15, 20)]
>>> actual == expected
True


:class:`UserDict` objects
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