Small fixes.

Author: babacry

cirq-core/cirq/ops/pauli_string.py

@@ -41,7 +41,6 @@
 import numpy as np
 import sympy
 
-import cirq
 from cirq import value, protocols, linalg, qis, _compat
 from cirq._doc import document
 from cirq._import import LazyLoader
@@ -496,7 +495,7 @@ def matrix(self, qubits: Optional[Iterable[TKey]] = None) -> np.ndarray:
         """
         qubits = self.qubits if qubits is None else qubits
         factors = [self.get(q, default=identity.I) for q in qubits]
-        if cirq.is_parameterized(self):
+        if protocols.is_parameterized(self):
             raise NotImplementedError('Cannot express as matrix when parameterized')
         assert isinstance(self.coefficient, complex)
         return linalg.kron(self.coefficient, *[protocols.unitary(f) for f in factors])
@@ -980,7 +979,9 @@ def conjugated_by(self, clifford: 'cirq.OP_TREE') -> 'PauliString':
         # Initialize the ps the same as self.
         ps = PauliString(qubit_pauli_map=self._qubit_pauli_map, coefficient=self.coefficient)
         all_ops = list(op_tree.flatten_to_ops(clifford))
-        all_qubits = set.union(set(self.qubits), [q for op in all_ops for q in op.qubits])
+        all_qubits: set[TKey] = set.union(
+            set(self.qubits), [q for op in all_ops for q in op.qubits]
+        )
 
         # Iteratively calculate the conjugation in reverse order of ops.
         for op in all_ops[::-1]:
@@ -989,11 +990,9 @@ def conjugated_by(self, clifford: 'cirq.OP_TREE') -> 'PauliString':
             # Then the conjugation = (C^{-1}⊗I·Pc⊗R·C⊗I) = (C^{-1}·Pc·C)⊗R.
 
             # Isolate R
-            remain: 'cirq.PauliString' = PauliString()
-            for q in all_qubits:
-                pauli = ps.get(q)
-                if pauli is not None and not q in op.qubits:
-                    remain *= pauli(q)
+            remain: 'cirq.PauliString' = PauliString(
+                *(pauli(q) for q in all_qubits - set(op.qubits) if (pauli := ps.get(q)) is not None)
+            )
 
             # Initialize the conjugation of Pc.
             conjugated: 'cirq.DensePauliString' = (
@@ -1005,7 +1004,7 @@ def conjugated_by(self, clifford: 'cirq.OP_TREE') -> 'PauliString':
             # Note the clifford_tableau in CliffordGate represents C·P·C^-1 instead of C^-1·P·C.
             # So we take the inverse of the tableau to match the definition of the conjugation here.
             gate_in_clifford: 'cirq.CliffordGate'
-            if isinstance(op.gate, cirq.CliffordGate):
+            if isinstance(op.gate, clifford_gate.CliffordGate):
                 gate_in_clifford = op.gate
             else:
                 # Convert the clifford gate to CliffordGate type.
@@ -1020,7 +1019,7 @@ def conjugated_by(self, clifford: 'cirq.OP_TREE') -> 'PauliString':
             #   Puali X_k's conjugation is from the destabilzer table;
             #   Puali Z_k's conjugation is from the stabilzer table;
             #   Puali Y_k's conjugation is calcluated according to Y = iXZ. E.g., for the kth qubit,
-            #     C^{-1}·Y_k⊗I·C = C^{-1}·(iX_k⊗I·Z_k⊗I)·C = i (C^{-1}·X_k⊗I·C)·(C^{-1}·Z_k⊗I·C)
+            #     C^{-1}·Y_k⊗I·C = C^{-1}·(iX_k⊗I·Z_k⊗I)·C = i (C^{-1}·X_k⊗I·C)·(C^{-1}·Z_k⊗I·C).
             for qid, qubit in enumerate(op.qubits):
                 pauli = ps.get(qubit)
                 match pauli:
@@ -1179,7 +1178,7 @@ def _try_interpret_as_pauli_string(op: Any):
         if (pauli := gates.get(type(op.gate), None)) is not None:
             exponent = op.gate.exponent  # type: ignore
             if exponent % 2 == 0:
-                return cirq.PauliString()
+                return PauliString()
             if exponent % 2 == 1:
                 return pauli.on(op.qubits[0])
     return None