code: def add_method_callback(self, classname, methodname, method_callback,
                              namespace=None,):
          
  
          if namespace is None:
              namespace = self.default_namespace
  
          # Validate namespace
          method_repo = self._get_method_repo(namespace)
  
          if classname not in method_repo:
              method_repo[classname] = NocaseDict()
          if methodname in method_repo[classname]:
              raise ValueError("Duplicate method specification")
  
          method_repo[classname][methodname] = method_callback
docs: Register a callback function for a CIM method that will be called when
          the CIM method is invoked via `InvokeMethod`.
  
          If the namespace does not exist, :exc:`~pywbem.CIMError` with status
          CIM_ERR_INVALID_NAMESPACE is raised.
  
          Parameters:
  
            classname (:term:`string`):
              The CIM class name for which the callback function is registered.
  
              The faked `InvokeMethod` implementation uses this information to
              look up the callback function from its parameters.
  
              For method invocations on a target instance, this must be the class
              name of the creation class of the target instance.
  
              For method invocations on a target class, this must be the class
              name of the target class.
  
            methodname (:term:`string`):
              The CIM method name for which the callback function is registered.
  
              The faked `InvokeMethod` implementation uses this information to
              look up the callback function from its parameters.
  
            method_callback (:func:`~pywbem_mock.method_callback_interface`):
              The callback function.
  
            namespace (:term:`string`):
              The CIM namespace for which the callback function is registered.
  
              If `None`, the callback function is registered for the default
              namespace of the connection.
  
              The faked `InvokeMethod` implementation uses this information to
              look up the callback function from its parameters.
  
          Raises:
  
            ValueError: Duplicate method specification.
            :exc:`~pywbem.CIMError`: CIM_ERR_INVALID_NAMESPACE: Namespace does
              not exist.