The effective value of *attr_name* on this placeholder shape; its
        directly-applied value if it has one, otherwise the value on the
        layout placeholder it inherits from.

def _effective_value(self, attr_name):
        """
        The effective value of *attr_name* on this placeholder shape; its
        directly-applied value if it has one, otherwise the value on the
        layout placeholder it inherits from.
        """
        directly_applied_value = getattr(
            super(_InheritsDimensions, self), attr_name
        )
        if directly_applied_value is not None:
            return directly_applied_value
        return self._inherited_value(attr_name)