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Source code for pydispatch.saferef

"""Refactored "safe reference" from dispatcher.py"""
from __future__ import print_function

import weakref, traceback, sys

if sys.hexversion >= 0x3000000:
    im_func = '__func__'
    im_self = '__self__'
else:
    im_func = 'im_func'
    im_self = 'im_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(object): """'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(BoundMethodWeakref, cls).__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

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