00001 """Refactored "safe reference" from dispatcher.py"""
00002 import weakref, traceback
00003
00004 __version__ = "$Revision: 1.2 $"
00005
00006 def safeRef(target, onDelete = None):
00007 """Return a *safe* weak reference to a callable target
00008
00009 target -- the object to be weakly referenced, if it's a
00010 bound method reference, will create a BoundMethodWeakref,
00011 otherwise creates a simple weakref.
00012 onDelete -- if provided, will have a hard reference stored
00013 to the callable to be called after the safe reference
00014 goes out of scope with the reference object, (either a
00015 weakref or a BoundMethodWeakref) as argument.
00016 """
00017 if hasattr(target, 'im_self'):
00018 if target.im_self is not None:
00019
00020
00021 assert hasattr(target, 'im_func'), """safeRef target %r has im_self, but no im_func, don't know how to create reference"""%( target,)
00022 reference = BoundMethodWeakref(
00023 target=target,
00024 onDelete=onDelete
00025 )
00026 return reference
00027 if callable(onDelete):
00028 return weakref.ref(target, onDelete)
00029 else:
00030 return weakref.ref( target )
00031
00032 class BoundMethodWeakref(object):
00033 """'Safe' and reusable weak references to instance methods
00034
00035 BoundMethodWeakref objects provide a mechanism for
00036 referencing a bound method without requiring that the
00037 method object itself (which is normally a transient
00038 object) is kept alive. Instead, the BoundMethodWeakref
00039 object keeps weak references to both the object and the
00040 function which together define the instance method.
00041
00042 Attributes:
00043 key -- the identity key for the reference, calculated
00044 by the class's calculateKey method applied to the
00045 target instance method
00046 deletionMethods -- sequence of callable objects taking
00047 single argument, a reference to this object which
00048 will be called when *either* the target object or
00049 target function is garbage collected (i.e. when
00050 this object becomes invalid). These are specified
00051 as the onDelete parameters of safeRef calls.
00052 weakSelf -- weak reference to the target object
00053 weakFunc -- weak reference to the target function
00054
00055 Class Attributes:
00056 _allInstances -- class attribute pointing to all live
00057 BoundMethodWeakref objects indexed by the class's
00058 calculateKey(target) method applied to the target
00059 objects. This weak value dictionary is used to
00060 short-circuit creation so that multiple references
00061 to the same (object, function) pair produce the
00062 same BoundMethodWeakref instance.
00063
00064 """
00065 _allInstances = weakref.WeakValueDictionary()
00066 def __new__( cls, target, onDelete=None, *arguments,**named ):
00067 """Create new instance or return current instance
00068
00069 Basically this method of construction allows us to
00070 short-circuit creation of references to already-
00071 referenced instance methods. The key corresponding
00072 to the target is calculated, and if there is already
00073 an existing reference, that is returned, with its
00074 deletionMethods attribute updated. Otherwise the
00075 new instance is created and registered in the table
00076 of already-referenced methods.
00077 """
00078 key = cls.calculateKey(target)
00079 current =cls._allInstances.get(key)
00080 if current is not None:
00081 current.deletionMethods.append( onDelete)
00082 return current
00083 else:
00084 base = super( BoundMethodWeakref, cls).__new__( cls )
00085 cls._allInstances[key] = base
00086 base.__init__( target, onDelete, *arguments,**named)
00087 return base
00088 def __init__(self, target, onDelete=None):
00089 """Return a weak-reference-like instance for a bound method
00090
00091 target -- the instance-method target for the weak
00092 reference, must have im_self and im_func attributes
00093 and be reconstructable via:
00094 target.im_func.__get__( target.im_self )
00095 which is true of built-in instance methods.
00096 onDelete -- optional callback which will be called
00097 when this weak reference ceases to be valid
00098 (i.e. either the object or the function is garbage
00099 collected). Should take a single argument,
00100 which will be passed a pointer to this object.
00101 """
00102 def remove(weak, self=self):
00103 """Set self.isDead to true when method or instance is destroyed"""
00104 methods = self.deletionMethods[:]
00105 del self.deletionMethods[:]
00106 try:
00107 del self.__class__._allInstances[ self.key ]
00108 except KeyError:
00109 pass
00110 for function in methods:
00111 try:
00112 if callable( function ):
00113 function( self )
00114 except Exception:
00115 traceback.print_exc()
00116 self.deletionMethods = [onDelete]
00117 self.key = self.calculateKey( target )
00118 self.weakSelf = weakref.ref(target.im_self, remove)
00119 self.weakFunc = weakref.ref(target.im_func, remove)
00120 def calculateKey( cls, target ):
00121 """Calculate the reference key for this reference
00122
00123 Currently this is a two-tuple of the id()'s of the
00124 target object and the target function respectively.
00125 """
00126 return (id(target.im_self),id(target.im_func))
00127 calculateKey = classmethod( calculateKey )
00128 def __str__(self):
00129 """Give a friendly representation of the object"""
00130 return """%s( %s.%s )"""%(
00131 self.__class__.__name__,
00132 self.weakSelf(),
00133 self.weakFunc().__name__,
00134 )
00135 __repr__ = __str__
00136 def __nonzero__( self ):
00137 """Whether we are still a valid reference"""
00138 return self() is not None
00139 def __cmp__( self, other ):
00140 """Compare with another reference"""
00141 if not isinstance (other,self.__class__):
00142 return cmp( self.__class__, type(other) )
00143 return cmp( self.key, other.key)
00144 def __call__(self):
00145 """Return a strong reference to the bound method
00146
00147 If the target cannot be retrieved, then will
00148 return None, otherwise returns a bound instance
00149 method for our object and function.
00150
00151 Note:
00152 You may call this method any number of times,
00153 as it does not invalidate the reference.
00154 """
00155 target = self.weakSelf()
00156 if target is not None:
00157 function = self.weakFunc()
00158 if function is not None:
00159 return function.__get__(target)
00160 return None