Python metaclass for enforcing immutability of custom types
- by Mark Lehmacher
Having searched for a way to enforce immutability of custom types and not having found a satisfactory answer I came up with my own shot at a solution in 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, while the general approach seems to be working rather well there are still some iffy implementation details (also see TODO comments in code):
How do I check if a particular method has been implemented anywhere in the type hierarchy?
How do I check which type is the origin of a method declaration (i.e. as part of which type a method has been defined)?
