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278 lines
11 KiB
Python
278 lines
11 KiB
Python
"""
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"Safe weakrefs", originally from pyDispatcher.
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Provides a way to safely weakref any function, including bound methods (which
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aren't handled by the core weakref module).
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"""
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import six
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import weakref
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import traceback
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def safeRef(target, onDelete=None):
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"""Return a *safe* weak reference to a callable target
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target -- the object to be weakly referenced, if it's a
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bound method reference, will create a BoundMethodWeakref,
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otherwise creates a simple weakref.
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onDelete -- if provided, will have a hard reference stored
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to the callable to be called after the safe reference
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goes out of scope with the reference object, (either a
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weakref or a BoundMethodWeakref) as argument.
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"""
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if hasattr(target, 'im_self'):
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if im_self(target) is not None:
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# Turn a bound method into a BoundMethodWeakref instance.
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# Keep track of these instances for lookup by disconnect().
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assert hasattr(target, 'im_func'), \
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"safeRef target %r has im_self, but no im_func, " \
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"don't know how to create reference" % (target,)
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reference = get_bound_method_weakref(
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target=target,
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onDelete=onDelete
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)
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return reference
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if callable(onDelete):
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return weakref.ref(target, onDelete)
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else:
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return weakref.ref(target)
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class BoundMethodWeakref(object):
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"""'Safe' and reusable weak references to instance methods
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BoundMethodWeakref objects provide a mechanism for
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referencing a bound method without requiring that the
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method object itself (which is normally a transient
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object) is kept alive. Instead, the BoundMethodWeakref
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object keeps weak references to both the object and the
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function which together define the instance method.
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Attributes:
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key -- the identity key for the reference, calculated
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by the class's calculateKey method applied to the
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target instance method
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deletionMethods -- sequence of callable objects taking
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single argument, a reference to this object which
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will be called when *either* the target object or
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target function is garbage collected (i.e. when
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this object becomes invalid). These are specified
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as the onDelete parameters of safeRef calls.
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weakSelf -- weak reference to the target object
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weakFunc -- weak reference to the target function
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Class Attributes:
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_allInstances -- class attribute pointing to all live
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BoundMethodWeakref objects indexed by the class's
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calculateKey(target) method applied to the target
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objects. This weak value dictionary is used to
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short-circuit creation so that multiple references
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to the same (object, function) pair produce the
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same BoundMethodWeakref instance.
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"""
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_allInstances = weakref.WeakValueDictionary()
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def __new__(cls, target, onDelete=None, *arguments, **named):
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"""Create new instance or return current instance
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Basically this method of construction allows us to
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short-circuit creation of references to already-
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referenced instance methods. The key corresponding
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to the target is calculated, and if there is already
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an existing reference, that is returned, with its
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deletionMethods attribute updated. Otherwise the
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new instance is created and registered in the table
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of already-referenced methods.
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"""
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key = cls.calculateKey(target)
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current = cls._allInstances.get(key)
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if current is not None:
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current.deletionMethods.append(onDelete)
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return current
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else:
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base = super(BoundMethodWeakref, cls).__new__(cls)
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cls._allInstances[key] = base
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base.__init__(target, onDelete, *arguments, **named)
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return base
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def __init__(self, target, onDelete=None):
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"""Return a weak-reference-like instance for a bound method
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target -- the instance-method target for the weak
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reference, must have im_self and im_func attributes
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and be reconstructable via:
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target.im_func.__get__( target.im_self )
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which is true of built-in instance methods.
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onDelete -- optional callback which will be called
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when this weak reference ceases to be valid
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(i.e. either the object or the function is garbage
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collected). Should take a single argument,
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which will be passed a pointer to this object.
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"""
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def remove(weak, self=self):
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"""Set self.isDead to true when method or instance is destroyed"""
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methods = self.deletionMethods[:]
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del self.deletionMethods[:]
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try:
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del self.__class__._allInstances[self.key]
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except KeyError:
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pass
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for function in methods:
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try:
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if callable(function):
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function(self)
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except Exception as e:
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try:
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traceback.print_exc()
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except AttributeError:
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print("Exception during saferef %s cleanup "
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"function %s: %s" % (self, function, e))
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self.deletionMethods = [onDelete]
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self.key = self.calculateKey(target)
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self.weakSelf = weakref.ref(im_self(target), remove)
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self.weakFunc = weakref.ref(im_func(target), remove)
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self.selfName = str(im_self(target))
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self.funcName = str(im_func(target).__name__)
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def calculateKey(cls, target):
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"""Calculate the reference key for this reference
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Currently this is a two-tuple of the id()'s of the
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target object and the target function respectively.
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"""
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return (id(im_self(target)), id(im_func(target)))
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calculateKey = classmethod(calculateKey)
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def __str__(self):
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"""Give a friendly representation of the object"""
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return """%s( %s.%s )""" % (
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self.__class__.__name__,
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self.selfName,
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self.funcName,
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)
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__repr__ = __str__
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def __bool__(self):
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"""Whether we are still a valid reference"""
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return self() is not None
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def __nonzero__(self):
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"""Python2 alternative for __bool__"""
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return self() is not None
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def __cmp__(self, other):
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"""Compare with another reference"""
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if not isinstance(other, self.__class__):
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return ((self.__class__ > type(other)) - (self.__class__ < type(other)))
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return ((self.key > other.key) - (self.key < other.key))
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def __call__(self):
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"""Return a strong reference to the bound method
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If the target cannot be retrieved, then will
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return None, otherwise returns a bound instance
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method for our object and function.
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Note:
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You may call this method any number of times,
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as it does not invalidate the reference.
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"""
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target = self.weakSelf()
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if target is not None:
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function = self.weakFunc()
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if function is not None:
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return function.__get__(target)
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return None
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class BoundNonDescriptorMethodWeakref(BoundMethodWeakref):
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"""A specialized BoundMethodWeakref, for platforms where instance methods
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are not descriptors.
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It assumes that the function name and the target attribute name are the
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same, instead of assuming that the function is a descriptor. This approach
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is equally fast, but not 100% reliable because functions can be stored on
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an attribute named differenty than the function's name such as in:
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class A: pass
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def foo(self): return "foo"
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A.bar = foo
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But this shouldn't be a common use case. So, on platforms where methods
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aren't descriptors (such as Jython) this implementation has the advantage
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of working in the most cases.
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"""
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def __init__(self, target, onDelete=None):
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"""Return a weak-reference-like instance for a bound method
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target -- the instance-method target for the weak
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reference, must have im_self and im_func attributes
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and be reconstructable via:
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target.im_func.__get__( target.im_self )
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which is true of built-in instance methods.
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onDelete -- optional callback which will be called
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when this weak reference ceases to be valid
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(i.e. either the object or the function is garbage
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collected). Should take a single argument,
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which will be passed a pointer to this object.
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"""
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assert getattr(im_self(target), target.__name__) == target, \
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("method %s isn't available as the attribute %s of %s" %
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(target, target.__name__, im_self(target)))
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super(BoundNonDescriptorMethodWeakref, self).__init__(target, onDelete)
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def __call__(self):
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"""Return a strong reference to the bound method
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If the target cannot be retrieved, then will
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return None, otherwise returns a bound instance
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method for our object and function.
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Note:
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You may call this method any number of times,
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as it does not invalidate the reference.
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"""
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target = self.weakSelf()
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if target is not None:
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function = self.weakFunc()
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if function is not None:
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# Using curry() would be another option, but it erases the
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# "signature" of the function. That is, after a function is
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# curried, the inspect module can't be used to determine how
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# many arguments the function expects, nor what keyword
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# arguments it supports, and pydispatcher needs this
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# information.
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return getattr(target, function.__name__)
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return None
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def get_bound_method_weakref(target, onDelete):
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"""Instantiates the appropiate BoundMethodWeakRef, depending on the
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details of the underlying class method implementation"""
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if hasattr(target, '__get__'):
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# target method is a descriptor, so the default implementation works:
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return BoundMethodWeakref(target=target, onDelete=onDelete)
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else:
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# no luck, use the alternative implementation:
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return BoundNonDescriptorMethodWeakref(target=target,
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onDelete=onDelete)
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def im_self(func):
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if six.PY2:
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return func.im_self
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elif six.PY3:
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return func.__self__
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def im_func(func):
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if six.PY2:
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return func.im_func
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elif six.PY3:
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return func.__func__
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