def get_leafs(self) -> list:
    ' return list of leafs ids. Will return empty list, if calculate_leafs() has not been called earlier. '
    return self._leafs[:]
return list of leafs ids. Will return empty list, if calculate_leafs() has not been called earlier.
def get_leafs(self) -> list:
    '  '
    return self._leafs[:]