def matrix(self, projector_qids: Iterable[cirq.Qid] | None = None) -> csr_matrix:
        """Returns the matrix of self in computational basis of qubits.

        Args:
            projector_qids: Ordered collection of qubits that determine the subspace
                in which the matrix representation of the ProjectorString is to
                be computed. Qbits absent from self.qubits are acted on by
                the identity. Defaults to the qubits of the projector_dict.

        Returns:
            A sparse matrix that is the projection in the specified basis.
        """
        projector_qids = self._projector_dict.keys() if projector_qids is None else projector_qids
        _check_qids_dimension(projector_qids)
        idx_to_keep = [
            [self._projector_dict[qid]] if qid in self._projector_dict else [0, 1]
            for qid in projector_qids
        ]

        total_d = math.prod(qid.dimension for qid in projector_qids)

        ones_idx = []
        for idx in itertools.product(*idx_to_keep):
            d = total_d
            kron_idx = 0
            for i, qid in zip(idx, projector_qids):
                d //= qid.dimension
                kron_idx += i * d
            ones_idx.append(kron_idx)

        return csr_matrix(
            ([self._coefficient] * len(ones_idx), (ones_idx, ones_idx)), shape=(total_d, total_d)
        )
Returns the matrix of self in computational basis of qubits.

        Args:
            projector_qids: Ordered collection of qubits that determine the subspace
                in which the matrix representation of the ProjectorString is to
                be computed. Qbits absent from self.qubits are acted on by
                the identity. Defaults to the qubits of the projector_dict.

        Returns:
            A sparse matrix that is the projection in the specified basis.
        
cirq-core/cirq/ops/projector.py