"""Weak reference support for Python.
This module is an implementation of PEP 205:
https://peps.python.org/pep-0205/
"""
# Naming convention: Variables named "wr" are weak reference objects;
# they are called this instead of "ref" to avoid name collisions with
# the module-global ref() function imported from _weakref.
from _weakref import (
getweakrefcount,
getweakrefs,
ref,
proxy,
CallableProxyType,
ProxyType,
ReferenceType,
_remove_dead_weakref)
from _weakrefset import WeakSet
import _collections_abc # Import after _weakref to avoid circular import.
import sys
import itertools
ProxyTypes = (ProxyType, CallableProxyType)
__all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
"WeakKeyDictionary", "ReferenceType", "ProxyType",
"CallableProxyType", "ProxyTypes", "WeakValueDictionary",
"WeakSet", "WeakMethod", "finalize"]
_collections_abc.MutableSet.register(WeakSet)
class WeakMethod(ref):
"""
A custom `weakref.ref` subclass which simulates a weak reference to
a bound method, working around the lifetime problem of bound methods.
"""
__slots__ = "_func_ref", "_meth_type", "_alive", "__weakref__"
def __new__(cls, meth, callback=None):
try:
obj = meth.__self__
func = meth.__func__
except AttributeError:
raise TypeError("argument should be a bound method, not {}"
.format(type(meth))) from None
def _cb(arg):
# The self-weakref trick is needed to avoid creating a reference
# cycle.
self = self_wr()
if self._alive:
self._alive = False
if callback is not None:
callback(self)
self = ref.__new__(cls, obj, _cb)
self._func_ref = ref(func, _cb)
self._meth_type = type(meth)
self._alive = True
self_wr = ref(self)
return self
def __call__(self):
obj = super().__call__()
func = self._func_ref()
if obj is None or func is None:
return None
return self._meth_type(func, obj)
def __eq__(self, other):
if isinstance(other, WeakMethod):
if not self._alive or not other._alive:
return self is other
return ref.__eq__(self, other) and self._func_ref == other._func_ref
return NotImplemented
def __ne__(self, other):
if isinstance(other, WeakMethod):
if not self._alive or not other._alive:
return self is not other
return ref.__ne__(self, other) or self._func_ref != other._func_ref
return NotImplemented
__hash__ = ref.__hash__
class WeakValueDictionary(_collections_abc.MutableMapping):
"""Mapping class that references values weakly.
Entries in the dictionary will be discarded when no strong
reference to the value exists anymore
"""
# We inherit the constructor without worrying about the input
# dictionary; since it uses our .update() method, we get the right
# checks (if the other dictionary is a WeakValueDictionary,
# objects are unwrapped on the way out, and we always wrap on the
# way in).
def __init__(self, other=(), /, **kw):
def remove(wr, selfref=ref(self), _atomic_removal=_remove_dead_weakref):
self = selfref()
if self is not None:
# Atomic removal is necessary since this function
# can be called asynchronously by the GC
_atomic_removal(self.data, wr.key)
self._remove = remove
self.data = {}
self.update(other, **kw)
def __getitem__(self, key):
o = self.data[key]()
if o is None:
raise KeyError(key)
else:
return o
def __delitem__(self, key):
del self.data[key]
def __len__(self):
return len(self.data)
def __contains__(self, key):
try:
o = self.data[key]()
except KeyError:
return False
return o is not None
def __repr__(self):
return "<%s at %#x>" % (self.__class__.__name__, id(self))
def __setitem__(self, key, value):
self.data[key] = KeyedRef(value, self._remove, key)
def copy(self):
new = WeakValueDictionary()
for key, wr in self.data.copy().items():
o = wr()
if o is not None:
new[key] = o
return new
__copy__ = copy
def __deepcopy__(self, memo):
from copy import deepcopy
new = self.__class__()
for key, wr in self.data.copy().items():
o = wr()
if o is not None:
new[deepcopy(key, memo)] = o
return new
def get(self, key, default=None):
try:
wr = self.data[key]
except KeyError:
return default
else:
o = wr()
if o is None:
# This should only happen
return default
else:
return o
def items(self):
for k, wr in self.data.copy().items():
v = wr()
if v is not None:
yield k, v
def keys(self):
for k, wr in self.data.copy().items():
if wr() is not None:
yield k
__iter__ = keys
def itervaluerefs(self):
"""Return an iterator that yields the weak references to the values.
The references are not guaranteed to be 'live' at the time
they are used, so the result of calling the references needs
to be checked before being used. This can be used to avoid
creating references that will cause the garbage collector to
keep the values around longer than needed.
"""
yield from self.data.copy().values()
def values(self):
for wr in self.data.copy().values():
obj = wr()
if obj is not None:
yield obj
def popitem(self):
while True:
key, wr = self.data.popitem()
o = wr()
if o is not None:
return key, o
def pop(self, key, *args):
try:
o = self.data.pop(key)()
except KeyError:
o = None
if o is None:
if args:
return args[0]
else:
raise KeyError(key)
else:
return o
def setdefault(self, key, default=None):
try:
o = self.data[key]()
except KeyError:
o = None
if o is None:
self.data[key] = KeyedRef(default, self._remove, key)
return default
else:
return o
def update(self, other=None, /, **kwargs):
d = self.data
if other is not None:
if not hasattr(other, "items"):
other = dict(other)
for key, o in other.items():
d[key] = KeyedRef(o, self._remove, key)
for key, o in kwargs.items():
d[key] = KeyedRef(o, self._remove, key)
def valuerefs(self):
"""Return a list of weak references to the values.
The references are not guaranteed to be 'live' at the time
they are used, so the result of calling the references needs
to be checked before being used. This can be used to avoid
creating references that will cause the garbage collector to
keep the values around longer than needed.
"""
return list(self.data.copy().values())
def __ior__(self, other):
self.update(other)
return self
def __or__(self, other):
if isinstance(other, _collections_abc.Mapping):
c = self.copy()
c.update(other)
return c
return NotImplemented
def __ror__(self, other):
if isinstance(other, _collections_abc.Mapping):
c = self.__class__()
c.update(other)
c.update(self)
return c
return NotImplemented
class KeyedRef(ref):
"""Specialized reference that includes a key corresponding to the value.
This is used in the WeakValueDictionary to avoid having to create
a function object for each key stored in the mapping. A shared
callback object can use the 'key' attribute of a KeyedRef instead
of getting a reference to the key from an enclosing scope.
"""
__slots__ = "key",
def __new__(type, ob, callback, key):
self = ref.__new__(type, ob, callback)
self.key = key
return self
def __init__(self, ob, callback, key):
super().__init__(ob, callback)
class WeakKeyDictionary(_collections_abc.MutableMapping):
""" Mapping class that references keys weakly.
Entries in the dictionary will be discarded when there is no
longer a strong reference to the key. This can be used to
associate additional data with an object owned by other parts of
an application without adding attributes to those objects. This
can be especially useful with objects that override attribute
accesses.
"""
def __init__(self, dict=None):
self.data = {}
def remove(k, selfref=ref(self)):
self = selfref()
if self is not None:
try:
del self.data[k]
except KeyError:
pass
self._remove = remove
if dict is not None:
self.update(dict)
def __delitem__(self, key):
del self.data[ref(key)]
def __getitem__(self, key):
return self.data[ref(key)]
def __len__(self):
return len(self.data)
def __repr__(self):
return "<%s at %#x>" % (self.__class__.__name__, id(self))
def __setitem__(self, key, value):
self.data[ref(key, self._remove)] = value
def copy(self):
new = WeakKeyDictionary()
for key, value in self.data.copy().items():
o = key()
if o is not None:
new[o] = value
return new
__copy__ = copy
def __deepcopy__(self, memo):
from copy import deepcopy
new = self.__class__()
for key, value in self.data.copy().items():
o = key()
if o is not None:
new[o] = deepcopy(value, memo)
return new
def get(self, key, default=None):
return self.data.get(ref(key),default)
def __contains__(self, key):
try:
wr = ref(key)
except TypeError:
return False
return wr in self.data
def items(self):
for wr, value in self.data.copy().items():
key = wr()
if key is not None:
yield key, value
def keys(self):
for wr in self.data.copy():
obj = wr()
if obj is not None:
yield obj
__iter__ = keys
def values(self):
for wr, value in self.data.copy().items():
if wr() is not None:
yield value
def keyrefs(self):
"""Return a list of weak references to the keys.
The references are not guaranteed to be 'live' at the time
they are used, so the result of calling the references needs
to be checked before being used. This can be used to avoid
creating references that will cause the garbage collector to
keep the keys around longer than needed.
"""
return list(self.data)
def popitem(self):
while True:
key, value = self.data.popitem()
o = key()
if o is not None:
return o, value
def pop(self, key, *args):
return self.data.pop(ref(key), *args)
def setdefault(self, key, default=None):
return self.data.setdefault(ref(key, self._remove),default)
def update(self, dict=None, /, **kwargs):
d = self.data
if dict is not None:
if not hasattr(dict, "items"):
dict = type({})(dict)
for key, value in dict.items():
d[ref(key, self._remove)] = value
if len(kwargs):
self.update(kwargs)
def __ior__(self, other):
self.update(other)
return self
def __or__(self, other):
if isinstance(other, _collections_abc.Mapping):
c = self.copy()
c.update(other)
return c
return NotImplemented
def __ror__(self, other):
if isinstance(other, _collections_abc.Mapping):
c = self.__class__()
c.update(other)
c.update(self)
return c
return NotImplemented
class finalize:
"""Class for finalization of weakrefable objects
finalize(obj, func, *args, **kwargs) returns a callable finalizer
object which will be called when obj is garbage collected. The
first time the finalizer is called it evaluates func(*arg, **kwargs)
and returns the result. After this the finalizer is dead, and
calling it just returns None.
When the program exits any remaining finalizers for which the
atexit attribute is true will be run in reverse order of creation.
By default atexit is true.
"""
# Finalizer objects don't have any state of their own. They are
# just used as keys to lookup _Info objects in the registry. This
# ensures that they cannot be part of a ref-cycle.
__slots__ = ()
_registry = {}
_shutdown = False
_index_iter = itertools.count()
_dirty = False
_registered_with_atexit = False
class _Info:
__slots__ = ("weakref", "func", "args", "kwargs", "atexit", "index")
def __init__(self, obj, func, /, *args, **kwargs):
if not self._registered_with_atexit:
# We may register the exit function more than once because
# of a thread race, but that is harmless
import atexit
atexit.register(self._exitfunc)
finalize._registered_with_atexit = True
info = self._Info()
info.weakref = ref(obj, self)
info.func = func
info.args = args
info.kwargs = kwargs or None
info.atexit = True
info.index = next(self._index_iter)
self._registry[self] = info
finalize._dirty = True
def __call__(self, _=None):
"""If alive then mark as dead and return func(*args, **kwargs);
otherwise return None"""
info = self._registry.pop(self, None)
if info and not self._shutdown:
return info.func(*info.args, **(info.kwargs or {}))
def detach(self):
"""If alive then mark as dead and return (obj, func, args, kwargs);
otherwise return None"""
info = self._registry.get(self)
obj = info and info.weakref()
if obj is not None and self._registry.pop(self, None):
return (obj, info.func, info.args, info.kwargs or {})
def peek(self):
"""If alive then return (obj, func, args, kwargs);
otherwise return None"""
info = self._registry.get(self)
obj = info and info.weakref()
if obj is not None:
return (obj, info.func, info.args, info.kwargs or {})
@property
def alive(self):
"""Whether finalizer is alive"""
return self in self._registry
@property
def atexit(self):
"""Whether finalizer should be called at exit"""
info = self._registry.get(self)
return bool(info) and info.atexit
@atexit.setter
def atexit(self, value):
info = self._registry.get(self)
if info:
info.atexit = bool(value)
def __repr__(self):
info = self._registry.get(self)
obj = info and info.weakref()
if obj is None:
return '<%s object at %#x; dead>' % (type(self).__name__, id(self))
else:
return '<%s object at %#x; for %r at %#x>' % \
(type(self).__name__, id(self), type(obj).__name__, id(obj))
@classmethod
def _select_for_exit(cls):
# Return live finalizers marked for exit, oldest first
L = [(f,i) for (f,i) in cls._registry.items() if i.atexit]
L.sort(key=lambda item:item[1].index)
return [f for (f,i) in L]
@classmethod
def _exitfunc(cls):
# At shutdown invoke finalizers for which atexit is true.
# This is called once all other non-daemonic threads have been
# joined.
reenable_gc = False
try:
if cls._registry:
import gc
if gc.isenabled():
reenable_gc = True
gc.disable()
pending = None
while True:
if pending is None or finalize._dirty:
pending = cls._select_for_exit()
finalize._dirty = False
if not pending:
break
f = pending.pop()
try:
# gc is disabled, so (assuming no daemonic
# threads) the following is the only line in
# this function which might trigger creation
# of a new finalizer
f()
except Exception:
sys.excepthook(*sys.exc_info())
assert f not in cls._registry
finally:
# prevent any more finalizers from executing during shutdown
finalize._shutdown = True
if reenable_gc:
gc.enable()
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