python · rhettinger · Aug 4, 2021 · Aug 4, 2021 · Aug 4, 2021 · Aug 4, 2021
@@ -18,7 +18,7 @@
 from collections import namedtuple
 # import types, weakref  # Deferred to single_dispatch()
 from reprlib import recursive_repr
-from _thread import RLock
+from threading import RLock, Condition, get_ident
 from types import GenericAlias
 
 
@@ -930,13 +930,15 @@ def __isabstractmethod__(self):
 
 _NOT_FOUND = object()
 
-
 class cached_property:
+
     def __init__(self, func):
         self.func = func
         self.attrname = None
         self.__doc__ = func.__doc__
-        self.lock = RLock()
+        self.updater_lock = RLock()
+        self.cv = Condition(RLock())
+        self.updater = {}
 
     def __set_name__(self, owner, name):
         if self.attrname is None:
@@ -961,21 +963,68 @@ def __get__(self, instance, owner=None):
                 f"instance to cache {self.attrname!r} property."
             )
             raise TypeError(msg) from None
+
+        # Quickly and atomically determine which thread is reponsible
+        # for doing the update so that other threads can wait for the
+        # update to complete. If the current thread is already running,
+        # allow the reentrant thread to proceed rather than waiting.
+        key = id(self)
+        this_thread = get_ident()
+        with self.updater_lock:
+            reentrant = self.updater.get(key) == this_thread
+            wait = self.updater.setdefault(key, this_thread) != this_thread
+
+        # Only if this particular instance is currently being updated,
+        # block and wait for the updating thread to finish. Other
+        # instances won't have to wait.
+        while wait:
+            with self.cv:
+                while this_thread != self.updater.get(key, this_thread):
+                    self.cv.wait()
+                with self.updater_lock:
+                    reentrant = self.updater.get(key) == this_thread
+                    wait = self.updater.setdefault(key, this_thread) != this_thread
+
+        # If a value has already been stored, use it.
         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
+        if val is not _NOT_FOUND:
+            if not reentrant:
+                with self.updater_lock:
+                    self.updater.pop(key, None)
+                with self.cv:
+                    self.cv.notify_all()
+            return val
+
+        # Call the underlying function to compute the value.
+        try:
+            val = self.func(instance)
+        except Exception:
+            with self.updater_lock:
+                self.updater.pop(key, None)
+            with self.cv:
+                self.cv.notify_all()
+            raise
+
+        # Attempt to store the value
+        try:
+            cache[self.attrname] = val
+        except TypeError:
+            with self.updater_lock:
+                self.updater.pop(key, None)
+            with self.cv:
+                self.cv.notify_all()
+            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
+
+        # Value has been computed and cached.  Now return it.
+        if not reentrant:
+            with self.updater_lock:
+                self.updater.pop(key, None)
+            with self.cv:
+                self.cv.notify_all()
         return val
 
     __class_getitem__ = classmethod(GenericAlias)
diff --git a/Lib/threading.py b/Lib/threading.py
@@ -3,7 +3,6 @@
 import os as _os
 import sys as _sys
 import _thread
-import functools
 
 from time import monotonic as _time
 from _weakrefset import WeakSet
@@ -1477,6 +1476,8 @@ def _register_atexit(func, *arg, **kwargs):
     if _SHUTTING_DOWN:
         raise RuntimeError("can't register atexit after shutdown")
 
+    # Lazy import to prevent an import cycle
+    import functools
     call = functools.partial(func, *arg, **kwargs)
     _threading_atexits.append(call)
 

diff --git a/Misc/NEWS.d/next/Library/2021-08-04-18-30-25.bpo-43468.PlU--J.rst b/Misc/NEWS.d/next/Library/2021-08-04-18-30-25.bpo-43468.PlU--J.rst
@@ -0,0 +1,3 @@
+Fixed locking for functools.cached_property() to have instance specific
+locking.  Formerly, it blocked all instances of the same class, making it
+unsuitable for parallel I/O bound computations (which were a key use case).