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.
diff --git a/Lib/functools.py b/Lib/functools.py
index b3428a4..51048f5 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -868,3 +868,58 @@
@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