diff options
| -rw-r--r-- | Lib/functools.py | 23 | ||||
| -rw-r--r-- | Lib/test/test_functools.py | 19 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Library/2023-07-03-15-09-44.gh-issue-106292.3npldV.rst | 4 |
3 files changed, 36 insertions, 10 deletions
diff --git a/Lib/functools.py b/Lib/functools.py index 4d5e270..8518450 100644 --- a/Lib/functools.py +++ b/Lib/functools.py @@ -957,9 +957,10 @@ class singledispatchmethod: ################################################################################ -### 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): @@ -990,15 +991,17 @@ class cached_property: 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) diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py index d668fa4..c4eca0f 100644 --- a/Lib/test/test_functools.py +++ b/Lib/test/test_functools.py @@ -3037,6 +3037,25 @@ class TestCachedProperty(unittest.TestCase): 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() diff --git a/Misc/NEWS.d/next/Library/2023-07-03-15-09-44.gh-issue-106292.3npldV.rst b/Misc/NEWS.d/next/Library/2023-07-03-15-09-44.gh-issue-106292.3npldV.rst new file mode 100644 index 0000000..2335093 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2023-07-03-15-09-44.gh-issue-106292.3npldV.rst @@ -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. |