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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def set_density_matrix(self, density_matrix_repr: Union[int, np.ndarray]):
 """Set the density matrix to a new density matrix. Args: density_matrix_repr: If this is an int, the density matrix is set to the computational basis state corresponding to this state. Otherwise if this is a np.ndarray it is the full state, either a pure state or the full density matrix. If it is the pure state it must be the correct size, be normalized (an L2 norm of 1), and be safely castable to an appropriate dtype for the simulator. If it is a mixed state it must be correctly sized and positive semidefinite with trace one. """

        density_matrix = density_matrix_utils.to_valid_density_matrix(
            density_matrix_repr, len(self._qubit_map), self._dtype)
        density_matrix = np.reshape(density_matrix,
                                    self.simulator_state().density_matrix.shape)
        np.copyto(dst=self.simulator_state().density_matrix, src=density_matrix)