Improved code coverage (#2537)

* improve code coverage

* Update doc/releases/changelog-dev.md

* Update tests/ops/functions/test_generator.py

Co-authored-by: antalszava <antalszava@gmail.com>

Co-authored-by: antalszava <antalszava@gmail.com>

doc/releases/changelog-dev.md

@@ -45,6 +45,9 @@
 * `Operator.decomposition()` is now an instance method, and no longer accepts parameters.
   [(#2498)](https://github.com/PennyLaneAI/pennylane/pull/2498)
 
+* Adds tests, adds no-coverage directives, and removes inaccessible logic to improve code coverage.
+  [(#2537)](https://github.com/PennyLaneAI/pennylane/pull/2537)
+
 <h3>Bug fixes</h3>
 
 * Fixed a bug where `QNGOptimizer` did not work with operators

pennylane/drawer/mpldrawer.py

@@ -20,7 +20,7 @@
 try:
     import matplotlib.pyplot as plt
     from matplotlib import patches
-except (ModuleNotFoundError, ImportError) as e:
+except (ModuleNotFoundError, ImportError) as e:  # pragma: no cover
     has_mpl = False
 
 
@@ -247,7 +247,7 @@ class MPLDrawer:
 
     def __init__(self, n_layers, n_wires, wire_options=None, figsize=None):
 
-        if not has_mpl:
+        if not has_mpl:  # pragma: no cover
             raise ImportError(
                 "Module matplotlib is required for ``MPLDrawer`` class. "
                 "You can install matplotlib via \n\n   pip install matplotlib"

pennylane/fourier/visualize.py

@@ -21,7 +21,7 @@
 # a hard requirement for everything in this module.
 try:
     from matplotlib.colors import to_rgb
-except (ModuleNotFoundError, ImportError) as e:
+except (ModuleNotFoundError, ImportError) as e:  # pragma: no cover
     raise ImportError(
         "Module matplotlib is required for visualization in the Fourier module. "
         "You can install matplolib via \n\n   pip install matplotlib"

pennylane/ops/qubit/hamiltonian.py

@@ -621,8 +621,6 @@ def queue(self, context=qml.QueuingContext):
             except QueuingError:
                 o.queue(context=context)
                 context.update_info(o, owner=self)
-            except NotImplementedError:
-                pass
 
         context.append(self, owns=tuple(self.ops))
         return self

tests/fourier/test_coefficients.py

@@ -258,7 +258,7 @@ def test_anti_aliasing(self, circuit, inpt, degree):
         """Test that the coefficients obtained through anti-aliasing are the
         same as the ones when we don't anti-alias at the correct degree."""
         coeffs_regular = coefficients(circuit, len(inpt), degree, lowpass_filter=False)
-        coeffs_anti_aliased = coefficients(circuit, len(inpt), degree)
+        coeffs_anti_aliased = coefficients(circuit, len(inpt), degree, lowpass_filter=True)
 
         assert np.allclose(coeffs_regular, coeffs_anti_aliased)
 

tests/ops/functions/test_generator.py

@@ -176,6 +176,18 @@ class DeprecatedClassOp(CustomOp):
         assert gen.name == "Hermitian"
         assert gen.wires.tolist() == ["a"]
 
+    def test_generator_property_old_default(self):
+        """Test that if the old-style generator property is the default,
+        a GeneratorUndefinedError is raised and a warning is raised about the old syntax."""
+
+        class DeprecatedClassOp(CustomOp):
+            generator = [None, 1]
+
+        op = DeprecatedClassOp(0.5, wires="a")
+        with pytest.warns(UserWarning, match=r"The Operator\.generator property is deprecated"):
+            with pytest.raises(qml.operation.GeneratorUndefinedError):
+                qml.generator(op)
+
 
 class TestPrefactorReturn:
     """Tests for format="prefactor". This format attempts to isolate a prefactor

tests/ops/qubit/test_matrix_ops.py

@@ -252,11 +252,16 @@ def test_matrix_representation(self, tol):
         assert np.allclose(res_static, expected, atol=tol)
         assert np.allclose(res_dynamic, expected, atol=tol)
 
-    def test_error_not_unitary(self):
-        """Tests that error is raised if diagonal does not lead to a unitary"""
+    def test_error_matrix_not_unitary(self):
+        """Tests that error is raised if diagonal by `compute_matrix` does not lead to a unitary"""
         with pytest.raises(ValueError, match="Operator must be unitary"):
             qml.DiagonalQubitUnitary.compute_matrix(np.array([1, 2]))
 
+    def test_error_eigvals_not_unitary(self):
+        """Tests that error is raised by `compute_eigvals` if diagonal does not lead to a unitary"""
+        with pytest.raises(ValueError, match="Operator must be unitary"):
+            qml.DiagonalQubitUnitary.compute_eigvals(np.array([1, 2]))
+
     def test_jax_jit(self):
         """Test that the diagonal matrix unitary operation works
         within a QNode that uses the JAX JIT"""