refactor: Raise error if observable qubits do not feature in circuit. (…

…#182)

* Add check that observable is correct

* Add test

* Remove prints

* Minor tidy

qermit/taskgraph/mitex.py

@@ -134,6 +134,18 @@ def task(
             symbols = ansatz_circuit[2]
             observable_tracker = measurement_wire[1]
 
+            non_existant_qubits = (
+                observable_tracker.qubit_pauli_operator.all_qubits.difference(
+                    set(circuit.qubits)
+                )
+            )
+            # TODO: In a future refactor, this check should be done
+            # inside ObservableExperiment.
+            if len(non_existant_qubits) > 0:
+                raise Exception(
+                    f"ObservableTracker qubits {non_existant_qubits} are not found in circuit."
+                )
+
             # first make sure all observable has some measurement circuit
             strings_for_circuits = observable_tracker.get_empty_strings()
             for string in strings_for_circuits:

tests/mitex_test.py

@@ -15,6 +15,7 @@
 
 import copy
 
+import pytest
 from pytket.circuit import Circuit, OpType, Qubit, fresh_symbol  # type: ignore
 from pytket.extensions.qiskit import AerBackend  # type: ignore
 from pytket.pauli import Pauli, QubitPauliString  # type: ignore
@@ -37,6 +38,44 @@
 )
 
 
+def test_valid_observable():
+    backend = AerBackend(n_qubits=2)
+    mitex = MitEx(backend=backend)
+
+    circuit = Circuit()
+    q_reg = circuit.add_q_register(name="my_quibts", size=2)
+    circuit.CX(q_reg[0], q_reg[1])
+
+    qps_one = QubitPauliString(
+        [Qubit(name="my_quibts", index=0), Qubit(name="my_quibts", index=1)],
+        [Pauli.Z, Pauli.Z],
+    )
+    qps_two = QubitPauliString([Qubit(1), Qubit(2)], [Pauli.Z, Pauli.Z])
+
+    obs_exp = ObservableExperiment(
+        AnsatzCircuit(circuit, 2, SymbolsDict()),
+        ObservableTracker(
+            QubitPauliOperator(
+                {
+                    qps_one: 1,
+                }
+            )
+        ),
+    )
+
+    assert mitex.run([obs_exp])[0]._dict[qps_one] == 1.0
+
+    obs_exp = ObservableExperiment(
+        AnsatzCircuit(circuit, 2, SymbolsDict()),
+        ObservableTracker(QubitPauliOperator({qps_one: 1, qps_two: 1})),
+    )
+    with pytest.raises(
+        Exception,
+        match="ObservableTracker qubits \{q\[2\], q\[1\]\} are not found in circuit.",
+    ):
+        mitex.run([obs_exp])
+
+
 def test_mitex_cache():
     circuit = Circuit(1).X(0)
     backend = AerBackend()