mirror of https://github.com/python/cpython
bpo-21145: Add cached_property decorator in functools (#6982)
Robust caching of calculated properties is harder than it looks at first glance, so add a solid, well-tested implementation to the standard library.
This commit is contained in:
Carl Meyer
Nick Coghlan
parent
216b745eaf
commit
d658deac60
|
|
@ -20,6 +20,39 @@ function for the purposes of this module.
|
|||
|
||||
The :mod:`functools` module defines the following functions:
|
||||
|
||||
.. decorator:: cached_property(func)
|
||||
|
||||
Transform a method of a class into a property whose value is computed once
|
||||
and then cached as a normal attribute for the life of the instance. Similar
|
||||
to :func:`property`, with the addition of caching. Useful for expensive
|
||||
computed properties of instances that are otherwise effectively immutable.
|
||||
|
||||
Example::
|
||||
|
||||
class DataSet:
|
||||
def __init__(self, sequence_of_numbers):
|
||||
self._data = sequence_of_numbers
|
||||
|
||||
@cached_property
|
||||
def stdev(self):
|
||||
return statistics.stdev(self._data)
|
||||
|
||||
@cached_property
|
||||
def variance(self):
|
||||
return statistics.variance(self._data)
|
||||
|
||||
.. versionadded:: 3.8
|
||||
|
||||
.. note::
|
||||
|
||||
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).
|
||||
|
||||
|
||||
.. function:: cmp_to_key(func)
|
||||
|
||||
Transform an old-style comparison function to a :term:`key function`. Used
|
||||
|
|
|
|||
|
|
@ -868,3 +868,58 @@ class singledispatchmethod:
|
|||
@property
|
||||
def __isabstractmethod__(self):
|
||||
return getattr(self.func, '__isabstractmethod__', False)
|
||||
|
||||
|
||||
################################################################################
|
||||
### cached_property() - computed once per instance, cached as attribute
|
||||
################################################################################
|
||||
|
||||
_NOT_FOUND = object()
|
||||
|
||||
|
||||
class cached_property:
|
||||
def __init__(self, func):
|
||||
self.func = func
|
||||
self.attrname = None
|
||||
self.__doc__ = func.__doc__
|
||||
self.lock = RLock()
|
||||
|
||||
def __set_name__(self, owner, name):
|
||||
if self.attrname is None:
|
||||
self.attrname = name
|
||||
elif name != self.attrname:
|
||||
raise TypeError(
|
||||
"Cannot assign the same cached_property to two different names "
|
||||
f"({self.attrname!r} and {name!r})."
|
||||
)
|
||||
|
||||
def __get__(self, instance, owner):
|
||||
if instance is None:
|
||||
return self
|
||||
if self.attrname is None:
|
||||
raise TypeError(
|
||||
"Cannot use cached_property instance without calling __set_name__ on it.")
|
||||
try:
|
||||
cache = instance.__dict__
|
||||
except AttributeError: # not all objects have __dict__ (e.g. class defines slots)
|
||||
msg = (
|
||||
f"No '__dict__' attribute on {type(instance).__name__!r} "
|
||||
f"instance to cache {self.attrname!r} property."
|
||||
)
|
||||
raise TypeError(msg) from None
|
||||
val = cache.get(self.attrname, _NOT_FOUND)
|
||||
if val is _NOT_FOUND:
|
||||
with self.lock:
|
||||
# check if another thread filled cache while we awaited lock
|
||||
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
|
||||
|
|
|
|||
|
|
@ -2313,5 +2313,171 @@ class TestSingleDispatch(unittest.TestCase):
|
|||
with self.assertRaisesRegex(TypeError, msg):
|
||||
f()
|
||||
|
||||
|
||||
class CachedCostItem:
|
||||
_cost = 1
|
||||
|
||||
def __init__(self):
|
||||
self.lock = py_functools.RLock()
|
||||
|
||||
@py_functools.cached_property
|
||||
def cost(self):
|
||||
"""The cost of the item."""
|
||||
with self.lock:
|
||||
self._cost += 1
|
||||
return self._cost
|
||||
|
||||
|
||||
class OptionallyCachedCostItem:
|
||||
_cost = 1
|
||||
|
||||
def get_cost(self):
|
||||
"""The cost of the item."""
|
||||
self._cost += 1
|
||||
return self._cost
|
||||
|
||||
cached_cost = py_functools.cached_property(get_cost)
|
||||
|
||||
|
||||
class CachedCostItemWait:
|
||||
|
||||
def __init__(self, event):
|
||||
self._cost = 1
|
||||
self.lock = py_functools.RLock()
|
||||
self.event = event
|
||||
|
||||
@py_functools.cached_property
|
||||
def cost(self):
|
||||
self.event.wait(1)
|
||||
with self.lock:
|
||||
self._cost += 1
|
||||
return self._cost
|
||||
|
||||
|
||||
class CachedCostItemWithSlots:
|
||||
__slots__ = ('_cost')
|
||||
|
||||
def __init__(self):
|
||||
self._cost = 1
|
||||
|
||||
@py_functools.cached_property
|
||||
def cost(self):
|
||||
raise RuntimeError('never called, slots not supported')
|
||||
|
||||
|
||||
class TestCachedProperty(unittest.TestCase):
|
||||
def test_cached(self):
|
||||
item = CachedCostItem()
|
||||
self.assertEqual(item.cost, 2)
|
||||
self.assertEqual(item.cost, 2) # not 3
|
||||
|
||||
def test_cached_attribute_name_differs_from_func_name(self):
|
||||
item = OptionallyCachedCostItem()
|
||||
self.assertEqual(item.get_cost(), 2)
|
||||
self.assertEqual(item.cached_cost, 3)
|
||||
self.assertEqual(item.get_cost(), 4)
|
||||
self.assertEqual(item.cached_cost, 3)
|
||||
|
||||
def test_threaded(self):
|
||||
go = threading.Event()
|
||||
item = CachedCostItemWait(go)
|
||||
|
||||
num_threads = 3
|
||||
|
||||
orig_si = sys.getswitchinterval()
|
||||
sys.setswitchinterval(1e-6)
|
||||
try:
|
||||
threads = [
|
||||
threading.Thread(target=lambda: item.cost)
|
||||
for k in range(num_threads)
|
||||
]
|
||||
with support.start_threads(threads):
|
||||
go.set()
|
||||
finally:
|
||||
sys.setswitchinterval(orig_si)
|
||||
|
||||
self.assertEqual(item.cost, 2)
|
||||
|
||||
def test_object_with_slots(self):
|
||||
item = CachedCostItemWithSlots()
|
||||
with self.assertRaisesRegex(
|
||||
TypeError,
|
||||
"No '__dict__' attribute on 'CachedCostItemWithSlots' instance to cache 'cost' property.",
|
||||
):
|
||||
item.cost
|
||||
|
||||
def test_immutable_dict(self):
|
||||
class MyMeta(type):
|
||||
@py_functools.cached_property
|
||||
def prop(self):
|
||||
return True
|
||||
|
||||
class MyClass(metaclass=MyMeta):
|
||||
pass
|
||||
|
||||
with self.assertRaisesRegex(
|
||||
TypeError,
|
||||
"The '__dict__' attribute on 'MyMeta' instance does not support item assignment for caching 'prop' property.",
|
||||
):
|
||||
MyClass.prop
|
||||
|
||||
def test_reuse_different_names(self):
|
||||
"""Disallow this case because decorated function a would not be cached."""
|
||||
with self.assertRaises(RuntimeError) as ctx:
|
||||
class ReusedCachedProperty:
|
||||
@py_functools.cached_property
|
||||
def a(self):
|
||||
pass
|
||||
|
||||
b = a
|
||||
|
||||
self.assertEqual(
|
||||
str(ctx.exception.__context__),
|
||||
str(TypeError("Cannot assign the same cached_property to two different names ('a' and 'b')."))
|
||||
)
|
||||
|
||||
def test_reuse_same_name(self):
|
||||
"""Reusing a cached_property on different classes under the same name is OK."""
|
||||
counter = 0
|
||||
|
||||
@py_functools.cached_property
|
||||
def _cp(_self):
|
||||
nonlocal counter
|
||||
counter += 1
|
||||
return counter
|
||||
|
||||
class A:
|
||||
cp = _cp
|
||||
|
||||
class B:
|
||||
cp = _cp
|
||||
|
||||
a = A()
|
||||
b = B()
|
||||
|
||||
self.assertEqual(a.cp, 1)
|
||||
self.assertEqual(b.cp, 2)
|
||||
self.assertEqual(a.cp, 1)
|
||||
|
||||
def test_set_name_not_called(self):
|
||||
cp = py_functools.cached_property(lambda s: None)
|
||||
class Foo:
|
||||
pass
|
||||
|
||||
Foo.cp = cp
|
||||
|
||||
with self.assertRaisesRegex(
|
||||
TypeError,
|
||||
"Cannot use cached_property instance without calling __set_name__ on it.",
|
||||
):
|
||||
Foo().cp
|
||||
|
||||
def test_access_from_class(self):
|
||||
self.assertIsInstance(CachedCostItem.cost, py_functools.cached_property)
|
||||
|
||||
def test_doc(self):
|
||||
self.assertEqual(CachedCostItem.cost.__doc__, "The cost of the item.")
|
||||
|
||||
|
||||
if __name__ == '__main__':
|
||||
unittest.main()
|
||||
|
|
|
|||
|
|
@ -0,0 +1,2 @@
|
|||
Add ``functools.cached_property`` decorator, for computed properties cached
|
||||
for the life of the instance.
|
||||
Loading…
Reference in New Issue