code: def get_func_signature(self, hsh: bytes) -> Optional[str]:
          
          if not isinstance(hsh, (bytes, bytearray)):
              raise TypeError('The selector argument must be a concrete byte array')
          return self._function_signatures_by_selector.get(hsh)
docs: Returns the signature of the normal function with the selector ``hsh``,
          or ``None`` if no such function exists.
  
          This function returns ``None`` for any selector that will be dispatched to a fallback function.