Fix flakes in pauli_string_measurement_with_readout_mitigation_test (quantumlib#7185)

* Deflake pauli_string_measurement_with_readout_mitigation_test

Ensure random Pauli strings have a non-identity operator for real.

* Add missing spaces to exception message

* Simplify - make do without an extra local variable

Author: pavoljuhas

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py

@@ -83,8 +83,8 @@ def _validate_input(
 
             if all(q == ops.I for q in pauli_str):
                 raise ValueError(
-                    "Empty Pauli strings or Pauli strings consisting"
-                    "only of Pauli I are not allowed. Please provide"
+                    "Empty Pauli strings or Pauli strings consisting "
+                    "only of Pauli I are not allowed. Please provide "
                     "valid input Pauli strings."
                 )
             if pauli_str.coefficient.imag != 0:

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation_test.py

@@ -36,13 +36,9 @@ def _create_ghz(number_of_qubits: int, qubits: Sequence[cirq.Qid]) -> cirq.Circu
 def _generate_random_pauli_string(qubits: Sequence[cirq.Qid], enable_coeff: bool = False):
     pauli_ops = [cirq.I, cirq.X, cirq.Y, cirq.Z]
 
-    # Ensure at least one non-identity.
-    operators = {q: cirq.I(q) for q in qubits}  # Start with all identities
-    # Choose a random subset of qubits to have non-identity operators
-    non_identity_qubits = random.sample(qubits, random.randint(1, len(qubits)))
-    for q in non_identity_qubits:
-        operators[q] = random.choice([cirq.X, cirq.Y, cirq.Z])(q)  # Only non-identity ops
     operators = {q: random.choice(pauli_ops) for q in qubits}
+    # Ensure at least one non-identity.
+    operators[random.choice(qubits)] = random.choice(pauli_ops[1:])
 
     if enable_coeff:
         coefficient = (2 * random.random() - 1) * 100
@@ -461,8 +457,8 @@ def test_all_pauli_strings_are_pauli_i() -> None:
 
     with pytest.raises(
         ValueError,
-        match="Empty Pauli strings or Pauli strings consisting"
-        "only of Pauli I are not allowed. Please provide"
+        match="Empty Pauli strings or Pauli strings consisting "
+        "only of Pauli I are not allowed. Please provide "
         "valid input Pauli strings.",
     ):
         measure_pauli_strings(