"""Refactored "safe reference" from dispatcher.py"""
import weakref
import traceback
import sys
im_func = "__func__"
im_self = "__self__"
[docs]def safeRef(target, onDelete=None):
"""Return a *safe* weak reference to a callable target
target -- the object to be weakly referenced, if it's a
bound method reference, will create a BoundMethodWeakref,
otherwise creates a simple weakref.
onDelete -- if provided, will have a hard reference stored
to the callable to be called after the safe reference
goes out of scope with the reference object, (either a
weakref or a BoundMethodWeakref) as argument.
"""
if hasattr(target, im_self):
if getattr(target, im_self) is not None:
# Turn a bound method into a BoundMethodWeakref instance.
# Keep track of these instances for lookup by disconnect().
assert hasattr(target, im_func), (
"""safeRef target %r has %s, """
"""but no %s, don't know how """
"""to create reference""" % (target, im_self, im_func)
)
reference = BoundMethodWeakref(target=target, onDelete=onDelete)
return reference
if onDelete is not None:
return weakref.ref(target, onDelete)
else:
return weakref.ref(target)
[docs]class BoundMethodWeakref:
"""'Safe' and reusable weak references to instance methods
BoundMethodWeakref objects provide a mechanism for
referencing a bound method without requiring that the
method object itself (which is normally a transient
object) is kept alive. Instead, the BoundMethodWeakref
object keeps weak references to both the object and the
function which together define the instance method.
Attributes:
key -- the identity key for the reference, calculated
by the class's calculateKey method applied to the
target instance method
deletionMethods -- sequence of callable objects taking
single argument, a reference to this object which
will be called when *either* the target object or
target function is garbage collected (i.e. when
this object becomes invalid). These are specified
as the onDelete parameters of safeRef calls.
weakSelf -- weak reference to the target object
weakFunc -- weak reference to the target function
Class Attributes:
_allInstances -- class attribute pointing to all live
BoundMethodWeakref objects indexed by the class's
calculateKey(target) method applied to the target
objects. This weak value dictionary is used to
short-circuit creation so that multiple references
to the same (object, function) pair produce the
same BoundMethodWeakref instance.
"""
_allInstances = weakref.WeakValueDictionary()
def __new__(cls, target, onDelete=None, *arguments, **named):
"""Create new instance or return current instance
Basically this method of construction allows us to
short-circuit creation of references to already-
referenced instance methods. The key corresponding
to the target is calculated, and if there is already
an existing reference, that is returned, with its
deletionMethods attribute updated. Otherwise the
new instance is created and registered in the table
of already-referenced methods.
"""
key = cls.calculateKey(target)
current = cls._allInstances.get(key)
if current is not None:
current.deletionMethods.append(onDelete)
return current
else:
base = super().__new__(cls)
cls._allInstances[key] = base
base.__init__(target, onDelete, *arguments, **named)
return base
def __init__(self, target, onDelete=None):
"""Return a weak-reference-like instance for a bound method
target -- the instance-method target for the weak
reference, must have <im_self> and <im_func> attributes
and be reconstructable via:
target.<im_func>.__get__( target.<im_self> )
which is true of built-in instance methods.
onDelete -- optional callback which will be called
when this weak reference ceases to be valid
(i.e. either the object or the function is garbage
collected). Should take a single argument,
which will be passed a pointer to this object.
"""
def remove(weak, self=self):
"""Set self.isDead to true when method or instance is destroyed"""
methods = self.deletionMethods[:]
del self.deletionMethods[:]
try:
del self.__class__._allInstances[self.key]
except KeyError:
pass
for function in methods:
try:
if hasattr(function, "__call__"):
function(self)
except Exception as e:
try:
traceback.print_exc()
except AttributeError:
print(
"""Exception during saferef %s cleanup """
"""function %s: %s""" % (self, function, e),
file=sys.stderr,
)
self.deletionMethods = [onDelete]
self.key = self.calculateKey(target)
self.weakSelf = weakref.ref(getattr(target, im_self), remove)
self.weakFunc = weakref.ref(getattr(target, im_func), remove)
self.selfName = getattr(target, im_self).__class__.__name__
self.funcName = str(getattr(target, im_func).__name__)
[docs] def calculateKey(cls, target):
"""Calculate the reference key for this reference
Currently this is a two-tuple of the id()'s of the
target object and the target function respectively.
"""
return (id(getattr(target, im_self)), id(getattr(target, im_func)))
calculateKey = classmethod(calculateKey)
def __str__(self):
"""Give a friendly representation of the object"""
return """%s( %s.%s )""" % (
self.__class__.__name__,
self.selfName,
self.funcName,
)
__repr__ = __str__
def __nonzero__(self):
"""Whether we are still a valid reference"""
return self() is not None
def __cmp__(self, other):
"""Compare with another reference"""
if not isinstance(other, self.__class__):
return cmp(self.__class__, type(other))
return cmp(self.key, other.key)
def __call__(self):
"""Return a strong reference to the bound method
If the target cannot be retrieved, then will
return None, otherwise returns a bound instance
method for our object and function.
Note:
You may call this method any number of times,
as it does not invalidate the reference.
"""
target = self.weakSelf()
if target is not None:
function = self.weakFunc()
if function is not None:
return function.__get__(target)
return None
