renamed to PlaceholderExpectation

pennylane/expval/__init__.py

@@ -58,7 +58,14 @@
 from .cv import __all__ as _cv__all__
 from .qubit import __all__ as _qubit__all__
 
-class PlaceholderOperation:
+class PlaceholderExpectation():
+    r"""pennylane.expval.PlaceholderExpectation()
+    A generic base class for constructing placeholders for operations that
+    exist under the same name in CV and qubit based devices.
+
+    When instantiated inside a QNode context, returns an instance
+    of the respective class in expval.cv or expval.qubit.
+    """
     def __new__(cls, *args, **kwargs):
         if QNode._current_context is None:
             raise QuantumFunctionError("Quantum operations can only be used inside a qfunc.")
@@ -69,14 +76,14 @@ def __new__(cls, *args, **kwargs):
         elif supported_expectations.intersection([cls for cls in _qubit__all__]):
             return getattr(qubit, cls.__name__)(*args, **kwargs)
         else:
-            raise QuantumFunctionError("Unable to determine whether this device supports CV or qubit operations when constructing this "+cls.__name__+" expectation.")
+            raise QuantumFunctionError("Unable to determine whether this device supports CV or qubit Operations when constructing this "+cls.__name__+" Expectation.")
 
     num_wires = 0
     num_params = 0
     par_domain = 'A'
     grad_method = None
 
-class Identity(PlaceholderOperation): #pylint: disable=too-few-public-methods,function-redefined
+class Identity(PlaceholderExpectation): #pylint: disable=too-few-public-methods,function-redefined
     r"""pennylane.expval.Identity(wires)
     Expectation value of the identity observable :math:`\I`.
 

tests/test_expval.py

@@ -53,7 +53,7 @@ def test_identiy_raises_exception_if_cannot_guess_device_type(self):
         def circuit():
             return qml.expval.Identity(wires=0)
 
-        with self.assertRaisesRegex(QuantumFunctionError, 'Unable to determine whether this device supports CV or qubit operations'):
+        with self.assertRaisesRegex(QuantumFunctionError, 'Unable to determine whether this device supports CV or qubit'):
             circuit()