def _get_base_value(self, entity):
return self._apply_to_values(entity, self._opt_call_to_base_type)<EOL>
Return the base value for this property of the given entity.

        This implies calling all _to_base_type() methods, in the method
        resolution order of the property's class, and adding a
        _BaseValue() wrapper, if one is not already present.  (If one
        is present, no work is done.)  It also handles default values and
        repeated properties.