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gh-106292: restore checking __dict__ in cached_property.__get__ #106380

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Merged
merged 5 commits into from
Jul 5, 2023
Merged

gh-106292: restore checking __dict__ in cached_property.__get__ #106380

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32eec48
gh-106292: restore checking __dict__ in cached_property.__get__
carljm Jul 3, 2023
43ed758
add news entry
carljm Jul 3, 2023
7a7fd43
Merge branch 'main' into cachedpropdoublecheck
carljm Jul 4, 2023
465dbd3
update comment to not make promises we can't guarantee
carljm Jul 4, 2023
19b5320
Apply suggestions from code review
carljm Jul 5, 2023
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23 changes: 13 additions & 10 deletions Lib/functools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -957,9 +957,10 @@ def __isabstractmethod__(self):


################################################################################
### cached_property() - computed once per instance, cached as attribute
### cached_property() - property result cached as instance attribute
################################################################################

_NOT_FOUND = object()

class cached_property:
def __init__(self, func):
Expand Down Expand Up @@ -990,15 +991,17 @@ def __get__(self, instance, owner=None):
f"instance to cache {self.attrname!r} property."
)
raise TypeError(msg) from None
val = self.func(instance)
try:
cache[self.attrname] = val
except TypeError:
msg = (
f"The '__dict__' attribute on {type(instance).__name__!r} instance "
f"does not support item assignment for caching {self.attrname!r} property."
)
raise TypeError(msg) from None
val = cache.get(self.attrname, _NOT_FOUND)
if val is _NOT_FOUND:
val = self.func(instance)
try:
cache[self.attrname] = val
except TypeError:
msg = (
f"The '__dict__' attribute on {type(instance).__name__!r} instance "
f"does not support item assignment for caching {self.attrname!r} property."
)
raise TypeError(msg) from None
return val

__class_getitem__ = classmethod(GenericAlias)
19 changes: 19 additions & 0 deletions Lib/test/test_functools.py
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Expand Up @@ -3037,6 +3037,25 @@ def test_access_from_class(self):
def test_doc(self):
self.assertEqual(CachedCostItem.cost.__doc__, "The cost of the item.")

def test_subclass_with___set__(self):
"""Caching still works for a subclass defining __set__."""
class readonly_cached_property(py_functools.cached_property):
def __set__(self, obj, value):
raise AttributeError("read only property")

class Test:
def __init__(self, prop):
self._prop = prop

@readonly_cached_property
def prop(self):
return self._prop

t = Test(1)
self.assertEqual(t.prop, 1)
t._prop = 999
self.assertEqual(t.prop, 1)


if __name__ == '__main__':
unittest.main()
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2023-07-03-15-09-44.gh-issue-106292.3npldV.rst
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@@ -0,0 +1,4 @@
Check for an instance-dict cached value in the :meth:`__get__` method of
:func:`functools.cached_property`. This better matches the pre-3.12 behavior
and improves compatibility for users subclassing
:func:`functools.cached_property` and adding a :meth:`__set__` method.