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bpo-42127: Document effect of cached_property on key-sharing dictionaries (GH-22930) (GH-22955)

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Doc/library/functools.rst
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@ -73,17 +73,32 @@ The :mod:`functools` module defines the following functions:
def variance(self):
return statistics.variance(self._data)
.. versionadded:: 3.8
Note, this decorator interferes with the operation of :pep:`412`
key-sharing dictionaries. This means that instance dictionaries
can take more space than usual.
.. note::
This decorator requires that the ``__dict__`` attribute on each instance
Also, this decorator requires that the ``__dict__`` attribute on each instance
be a mutable mapping. This means it will not work with some types, such as
metaclasses (since the ``__dict__`` attributes on type instances are
read-only proxies for the class namespace), and those that specify
``__slots__`` without including ``__dict__`` as one of the defined slots
(as such classes don't provide a ``__dict__`` attribute at all).
If a mutable mapping is not available or if space-efficient key sharing
is desired, an effect similar to :func:`cached_property` can be achieved
by a stacking :func:`property` on top of :func:`cache`::
class DataSet:
def __init__(self, sequence_of_numbers):
self._data = sequence_of_numbers
@property
@cache
def stdev(self):
return statistics.stdev(self._data)
.. versionadded:: 3.8
.. function:: cmp_to_key(func)