Fix recent type check failure in the CI (#6699)

Fix failure of the mypy (Type check) job in ci.yml. 
Help mypy detect when optional argument has been provided.

No change in the code function.

cirq-core/cirq/experiments/qubit_characterizations.py

@@ -105,7 +105,7 @@ def plot(self, ax: Optional[plt.Axes] = None, **plot_kwargs: Any) -> plt.Axes:
             The plt.Axes containing the plot.
         """
         show_plot = not ax
-        if not ax:
+        if ax is None:
             fig, ax = plt.subplots(1, 1, figsize=(8, 8))  # pragma: no cover
         ax.set_ylim((0.0, 1.0))  # pragma: no cover
         ax.plot(self._num_cfds_seq, self._gnd_state_probs, 'ro', label='data', **plot_kwargs)

cirq-core/cirq/linalg/decompositions.py

@@ -629,7 +629,7 @@ def scatter_plot_normalized_kak_interaction_coefficients(
         >>> plt.show()
     """
     show_plot = not ax
-    if not ax:
+    if ax is None:
         fig = plt.figure()
         ax = cast(mplot3d.axes3d.Axes3D, fig.add_subplot(1, 1, 1, projection='3d'))
 

cirq-core/cirq/vis/heatmap.py

@@ -296,7 +296,7 @@ def plot(
             is plotted on. ``collection`` is the collection of paths drawn and filled.
         """
         show_plot = not ax
-        if not ax:
+        if ax is None:
             fig, ax = plt.subplots(figsize=(8, 8))
         original_config = copy.deepcopy(self._config)
         self.update_config(**kwargs)
@@ -413,7 +413,7 @@ def plot(
             is plotted on. ``collection`` is the collection of paths drawn and filled.
         """
         show_plot = not ax
-        if not ax:
+        if ax is None:
             fig, ax = plt.subplots(figsize=(8, 8))
         original_config = copy.deepcopy(self._config)
         self.update_config(**kwargs)

cirq-core/cirq/vis/state_histogram.py

@@ -85,7 +85,7 @@ def plot_state_histogram(
         The axis that was plotted on.
     """
     show_fig = not ax
-    if not ax:
+    if ax is None:
         fig, ax = plt.subplots(1, 1)
     if isinstance(data, result.Result):
         values = get_state_histogram(data)

cirq-google/cirq_google/engine/calibration.py

@@ -275,7 +275,7 @@ def plot_histograms(
             ValueError: If the metric values are not single floats.
         """
         show_plot = not ax
-        if not ax:
+        if ax is None:
             fig, ax = plt.subplots(1, 1)
 
         if isinstance(keys, str):
@@ -320,7 +320,7 @@ def plot(
             values are not single floats.
         """
         show_plot = not fig
-        if not fig:
+        if fig is None:
             fig = plt.figure()
         axs = cast(List[plt.Axes], fig.subplots(1, 2))
         self.heatmap(key).plot(axs[0])

docs/dev/plotting.md

@@ -35,7 +35,7 @@ class Foo:
     ...
     def plot(self, ax: Optional[plt.Axes]=None, **plot_kwargs: Any) -> plt.Axes:
         show_plot = not ax
-        if not ax:
+        if ax is None:
             fig, ax = plt.subplots(1, 1)  # or your favorite figure setup
         # Call methods of the ax instance like ax.plot to plot on it.
         ...
@@ -81,7 +81,7 @@ class Foo:
     def plot(self, axes: Optional[List[plt.Axes]]=None,
              **plot_kwargs: Any) -> List[plt.Axes]:
         show_plot = not axes
-        if not axes:
+        if axes is None:
             fig, axes = plt.subplots(1, 2)  # or your favorite figure setup
         elif len(axes) != 2:  # your required number of axes
             raise ValueError('your error message')