How to determine if an operation has a gate?

It used to be that you could check whether an operation had a gate using a type check isinstance(gate_op, cirq.GateOperation) But if you tag an op it loses that signature. isinstance(gate_op.with_tags('you are it'), cirq.GateOperation) == False

Note that there are a lot of these checks throughout Cirq that will presumably break when you tag an operation. For example look at cirq.EjectPhasedPaulis().

eject = cirq.EjectPhasedPaulis()
q = cirq.NamedQubit('a')
circuit = cirq.Circuit(cirq.PhasedXPowGate(phase_exponent=0.125)(q), cirq.Z(q))
eject.optimize_circuit(circuit)
print(circuit)

does the correct optimization

a: ───PhX(0.625)───────

Put if you tag the Z, it doesn’t do the optimization

circuit = cirq.Circuit(cirq.PhasedXPowGate(phase_exponent=0.125)(q), cirq.Z(q).with_tags('you are it')
eject.optimize_circuit(circuit)
print(circuit)

prints

a: ───PhX(0.125)───Z['you are it']───

[95-martin-orion]

A similar issue exists for CircuitOperations (#3580), which do not populate the gate field but instead have a circuit field not present in the base Operation class.

[maffoo]

cirq.Operation has a gate property of type Optional[Gate] so you can check it against None on any operation:

if op.gate is not None:
    ...

Note that TaggedOperation delegates gate to the underlying operation (https://github.com/quantumlib/Cirq/blob/master/cirq/ops/raw_types.py#L534) so it also works fine to access the gate of a tagged operation directly without going through the sub operation.