'Set a callable function for this attribute on the given object.
This callable will be executed when the attribute is next
accessed, and is assumed to construct part of the instances
previously stored state. When its value or values are loaded,
they will be established as part of the instance\'s *committed
state*.  While *trackparent* information will be assembled for
these instances, attribute-level event handlers will not be
fired.
The callable overrides the class level callable set in the
``InstrumentedAttribute`` constructor.'

def set_callable(self, state, callable_):

    state.callables[self.key] = callable_
