<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def schema(self):
 """The nested Schema object. .. versionchanged:: 1.0.0 Renamed from `serializer` to `schema` """

        if not self.__schema:
            # Inherit context from parent.
            context = getattr(self.parent, 'context', {})
            if isinstance(self.nested, SchemaABC):
                self.__schema = self.nested
                self.__schema.context.update(context)
            else:
                if isinstance(self.nested, type) and issubclass(self.nested, SchemaABC):
                    schema_class = self.nested
                elif not isinstance(self.nested, basestring):
                    raise ValueError(
                        'Nested fields must be passed a '
                        'Schema, not {}.'.format(self.nested.__class__),
                    )
                elif self.nested == 'self':
                    schema_class = self.parent.__class__
                else:
                    schema_class = class_registry.get_class(self.nested)
                self.__schema = schema_class(
                    many=self.many,
                    only=self.only,
                    exclude=self.exclude,
                    context=context,
                    load_only=self._nested_normalized_option('load_only'),
                    dump_only=self._nested_normalized_option('dump_only'),
                )
        return self.__schema