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bpo-42781: Document the mechanics of cached_property from a user viewpoint #24031

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bpo-42781: Document the mechanics of cached_property from a user poin…
rhettinger Dec 31, 2020
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Make the example data immutable
rhettinger Dec 31, 2020
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18 changes: 14 additions & 4 deletions Doc/library/functools.rst
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Expand Up @@ -62,16 +62,26 @@ The :mod:`functools` module defines the following functions:
Example::

class DataSet:

def __init__(self, sequence_of_numbers):
self._data = sequence_of_numbers
self._data = tuple(sequence_of_numbers)

@cached_property
def stdev(self):
return statistics.stdev(self._data)

@cached_property
def variance(self):
return statistics.variance(self._data)
The mechanics of :func:`cached_property` are somewhat different from
:func:`property`. A regular property blocks attribute writes unless a
setter is defined. In contrast, a *cached_property* allows writes.

The *cached_property* decorator only runs on lookups and only when an
attribute of the same name doesn't exist. When it does run, the
*cached_property* writes to the attribute with the same name. Subsequent
attribute reads and writes take precedence over the *cached_property*
method and it works like a normal attribute.

The cached value can be cleared by deleting the attribute. This
allows the *cached_property* method to run again.

Note, this decorator interferes with the operation of :pep:`412`
key-sharing dictionaries. This means that instance dictionaries
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