qutip/qutip-qip

from qutip import Qobj


class MethodError(Exception):
    """When invalid method is chosen, this error is raised."""

    pass


def check_gate(gate, num_qubits):
    """Verifies input is a valid quantum gate.

    Parameters
    ----------
    gate : :class:`qutip.Qobj`
        The matrix that's supposed to be decomposed should be a Qobj.
    num_qubits:
        Total number of qubits in the circuit.
    Raises
    ------
    TypeError
        If the gate is not a Qobj.
    ValueError
        If the gate is not a unitary operator on qubits.
    """
    if not isinstance(gate, Qobj):
        raise TypeError("The input matrix is not a Qobj.")
    if not gate.isunitary:
        raise ValueError("Input is not unitary.")
    if gate.dims != [[2] * num_qubits] * 2:
        raise ValueError(f"Input is not a unitary on {num_qubits} qubits.")