Python metaclass to ensure custom types are immutable

Question

After searching for a way to ensure the invariability of custom types and not getting a satisfactory answer, I came up with my own shot in the solution in the form of a metaclass:

class ImmutableTypeException( Exception ): pass

class Immutable( type ):
   '''
   Enforce some aspects of the immutability contract for new-style classes:
    - attributes must not be created, modified or deleted after object construction
    - immutable types must implement __eq__ and __hash__
   '''

   def __new__( meta, classname, bases, classDict ):
      instance = type.__new__( meta, classname, bases, classDict )

      # Make sure __eq__ and __hash__ have been implemented by the immutable type.
      # In the case of __hash__ also make sure the object default implementation has been overridden. 
      # TODO: the check for eq and hash functions could probably be done more directly and thus more efficiently
      #       (hasattr does not seem to traverse the type hierarchy)
      if not '__eq__' in dir( instance ):
         raise ImmutableTypeException( 'Immutable types must implement __eq__.' )

      if not '__hash__'  in dir( instance ):
         raise ImmutableTypeException( 'Immutable types must implement __hash__.' )

      if _methodFromObjectType( instance.__hash__ ):
         raise ImmutableTypeException( 'Immutable types must override object.__hash__.' )

      instance.__setattr__ = _setattr
      instance.__delattr__ = _delattr

      return instance

   def __call__( self, *args, **kwargs ):

      obj = type.__call__( self, *args, **kwargs )
      obj.__immutable__ = True

      return obj

def _setattr( self, attr, value ):

   if '__immutable__' in self.__dict__ and self.__immutable__:
      raise AttributeError( "'%s' must not be modified because '%s' is immutable" % ( attr, self ) )

   object.__setattr__( self, attr, value )

def _delattr( self, attr ):
   raise AttributeError( "'%s' must not be deleted because '%s' is immutable" % ( attr, self ) )

def _methodFromObjectType( method ):
   '''
   Return True if the given method has been defined by object, False otherwise.
   '''
   try:
      # TODO: Are we exploiting an implementation detail here? Find better solution! 
      return isinstance( method.__objclass__, object )
   except:
      return False

However, although the general approach works pretty well, there are still some details of the iffy implementation (also see the TODO comments in the code):

• How to check if a particular method has been implemented in a type hierarchy?
• How to check which type is the source of the method declaration (i.e., as part of the type by which the method was defined)?

Answer 1

, . hasattr . :.

>>> class A(object): pass
... 
>>> class B(A): __eq__ = lambda *_: 1
... 
>>> class C(B): pass
... 
>>> c = C()
>>> hasattr(type(c), '__eq__')
True

hasattr(c, '__eq__') , "" per-instance __eq__, c, ( , __eq__ True hasattr, object, - "" , "" ; -).

, (, , , ):

import inspect

def whichancestor(c, attname):
  for ancestor in inspect.getmro(type(c)):
    if attname in ancestor.__dict__:
      return ancestor
  return None

inspect , , __mro__ type(c).

Answer 2

"" . ,

immutable_obj.attr.attrs_attr = new_value
immutable_obj.attr[2] = new_value

, attrs_attr , . . , :

>>> a = ImmutableClass(value)
>>> b = ImmutableClass(value)
>>> c = a
>>> a == b
True
>>> b == c
True
>>> a.attr.attrs_attr = new_value
>>> b == c
False

, , getattr, - . . setattr , , ? , , .

, , :

class Tuple(list):
    __metaclass__ = Immutable

, .

>>> t = Tuple([1,2,3])
>>> t.append(4)
>>> t
[1, 2, 3, 4]
>>> u = t
>>> t += (5,)
>>> t
[1, 2, 3, 4, 5]
>>> u
[1, 2, 3, 4, 5]

, C, , .