'Identifies a field of protocol. One can look up symbolic
field name in a `.proto` file by this field tag.'

@property
def field_tag(self):

    if hasattr(self, '_m_field_tag'):
        return (self._m_field_tag if hasattr(self, '_m_field_tag') else None)
    self._m_field_tag = (self.key.value >> 3)
    return (self._m_field_tag if hasattr(self, '_m_field_tag') else None)
