Raise an error if qml.probs is called without wires or obs

doc/releases/changelog-dev.md

@@ -366,6 +366,10 @@ the `decimals` and `show_matrices` keywords are added. `qml.drawer.tape_text(tap
 
 <h3>Bug fixes</h3>
 
+* Fixes a bug by adding a comprehensible error message for calling `qml.probs`
+  without passing wires or an observable.
+  [(#2438)](https://github.com/PennyLaneAI/pennylane/pull/2438)
+
 * Call `pip show` with the subprocess library to avoid outputting a common warning.
   [(#2422)](https://github.com/PennyLaneAI/pennylane/pull/2422)
 

pennylane/measurements.py

@@ -133,6 +133,7 @@ def diagonalizing_gates(self):
             List[.Operation]: the operations that diagonalize the observables
         """
         try:
+            # pylint: disable=no-member
             return self.expand().operations
         except qml.operation.DecompositionUndefinedError:
             return []
@@ -517,6 +518,12 @@ def circuit():
          the computational basis
     """
     # pylint: disable=protected-access
+
+    if wires is None and op is None:
+        raise qml.QuantumFunctionError(
+            "qml.probs requires either the wires or the observable to be passed."
+        )
+
     if isinstance(op, qml.Hamiltonian):
         raise qml.QuantumFunctionError("Hamiltonians are not supported for rotating probabilities.")
 

tests/test_prob.py

@@ -437,6 +437,25 @@ def circuit():
         circuit()
 
 
+def test_probs_no_wires_obs_raises():
+    """Test that an informative error is raised when no wires or observables
+    are passed to qml.probs."""
+    num_wires = 1
+
+    dev = qml.device("default.qubit", wires=num_wires, shots=None)
+
+    @qml.qnode(dev)
+    def circuit_probs():
+        qml.RY(0.34, wires=0)
+        return qml.probs()
+
+    with pytest.raises(
+        qml.QuantumFunctionError,
+        match="qml.probs requires either the wires or the observable to be passed.",
+    ):
+        circuit_probs()
+
+
 @pytest.mark.parametrize(
     "operation", [qml.SingleExcitation, qml.SingleExcitationPlus, qml.SingleExcitationMinus]
 )