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python/packages/sqlobject/include/pydispatch/saferef.py

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+"""Refactored "safe reference" from dispatcher.py"""
+import weakref, traceback
+
+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 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 im_self, but no im_func, don't know how to create reference"""%( target,)
+			reference = BoundMethodWeakref(
+				target=target,
+				onDelete=onDelete
+			)
+			return reference
+	if callable(onDelete):
+		return weakref.ref(target, onDelete)
+	else:
+		return weakref.ref( target )
+
+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 callable( function ):
+						function( self )
+				except Exception, e:
+					try:
+						traceback.print_exc()
+					except AttributeError, err:
+						print '''Exception during saferef %s cleanup function %s: %s'''%(
+							self, function, e
+						)
+		self.deletionMethods = [onDelete]
+		self.key = self.calculateKey( target )
+		self.weakSelf = weakref.ref(target.im_self, remove)
+		self.weakFunc = weakref.ref(target.im_func, remove)
+		self.selfName = str(target.im_self)
+		self.funcName = str(target.im_func.__name__)
+	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(target.im_self),id(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