python-sqlobject 0.12.4-2 / usr / share / pyshared / sqlobject / include / pydispatch / saferef.py

"""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