def __init__(
        self,
        sim_state: cirq.SimulationStateBase[cirq.DensityMatrixSimulationState],
        dtype: type[np.complexfloating] = np.complex64,
    ):
        """DensityMatrixStepResult.

        Args:
            sim_state: The qubit:SimulationState lookup for this step.
            dtype: The `numpy.dtype` used by the simulation. One of
                `numpy.complex64` or `numpy.complex128`.
        """
        super().__init__(sim_state)
        self._dtype = dtype
        self._density_matrix: np.ndarray | None = None
DensityMatrixStepResult.

        Args:
            sim_state: The qubit:SimulationState lookup for this step.
            dtype: The `numpy.dtype` used by the simulation. One of
                `numpy.complex64` or `numpy.complex128`.
        
cirq-core/cirq/sim/density_matrix_simulator.py