8000 gh-127750: Improve caching in singledispatchmethod by vodik · Pull Request #127751 · python/cpython · GitHub
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gh-127750: Improve caching in singledispatchmethod #127751

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52 changes: 34 additions & 18 deletions Lib/functools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1027,9 +1027,7 @@ def __init__(self, func):

self.dispatcher = singledispatch(func)
self.func = func

import weakref # see comment in singledispatch function
self._method_cache = weakref.WeakKeyDictionary()
self._method_cache = {}

def register(self, cls, method=None):
"""generic_method.register(cls, func) -> func
Expand All @@ -1039,30 +1037,48 @@ def register(self, cls, method=None):
return self.dispatcher.register(cls, func=method)

def __get__(self, obj, cls=None):
if self._method_cache is not None:
try:
_method = self._method_cache[obj]
except TypeError:
self._method_cache = None
except KeyError:
pass
else:
cache_key = id(obj)

try:
_obj_ref, _method = self._method_cache[id(obj)]
except KeyError:
pass
else:
if _obj_ref() is obj:
return _method

dispatch = self.dispatcher.dispatch
funcname = getattr(self.func, '__name__', 'singledispatchmethod method')
def _method(*args, **kwargs):
if not args:
raise TypeError(f'{funcname} requires at least '
'1 positional argument')
return dispatch(args[0].__class__).__get__(obj, cls)(*args, **kwargs)

def _remove(_):
self._method_cache.pop(cache_key, None)

import weakref # see comment in singledispatch function
try:
obj_ref = weakref.ref(obj, _remove)
except TypeError:
cache_key = None

def _method(*args, **kwargs):
if not args:
raise TypeError(f'{funcname} requires at least '
'1 positional argument')
return dispatch(args[0].__class__).__get__(obj, cls)(*args, **kwargs)
else:
def _method(*args, **kwargs):
if not args:
raise TypeError(f'{funcname} requires at least '
'1 positional argument')
# Make sure to use the weakref here to prevent storing
# a strong reference to obj in the cache
return dispatch(args[0].__class__).__get__(obj_ref(), cls)(*args, **kwargs)
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@vodik vodik Dec 9, 2024 •
edited
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Doesn't this introduce a data race though? If I assigned the method and then deleted the underlying obj? I'll have to try it.

Is a weakref even appropriate for this cache then? Or should I throw a ReferenceError?

-                return dispatch(args[0].__class__).__get__(obj_ref(), cls)(*args, **kwargs)
+                obj = obj_ref()
+                if obj is None:
+                    raise ReferenceError
+
+                return dispatch(args[0].__class__).__get__(obj, cls)(*args, **kwargs)

Or is this whole approach maybe not correct enough?


_method.__isabstractmethod__ = self.__isabstractmethod__
_method.register = self.register
update_wrapper(_method, self.func)

if self._method_cache is not None:
self._method_cache[obj] = _method
if cache_key:
self._method_cache[cache_key] = (obj_ref, _method)

return _method

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2024-12-08-21-03-55.gh-issue-127750.94rNtr.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve caching of :class:`functools.singledispatchmethod` to fix issues
where sometimes different object instances could collide with each other.
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