Correctly handle Axes subclasses that override cla

QuLogic · QuLogic · commit 74b2c2d495b2 · 2022-08-25T17:37:26.000-04:00
This fixes, e.g., Cartopy, but probably most other third-party packages
that will subclass `Axes`.
diff --git a/doc/api/next_api_changes/deprecations/23735-ES.rst b/doc/api/next_api_changes/deprecations/23735-ES.rst
@@ -0,0 +1,13 @@
+``AXes`` subclasses should override ``clear`` instead of ``cla``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For clarity, `.axes.Axes.clear` is now preferred over `.Axes.cla`. However, for
+backwards compatibility, the latter will remain as an alias for the former.
+
+For additional compatibility with third-party libraries, Matplotlib will
+continue to call the ``cla`` method of any `~.axes.Axes` subclasses if they
+define it. In the future, this will no longer occur, and Matplotlib will only
+call the ``clear`` method in `~.axes.Axes` subclasses.
+
+It is recommended to define only the ``clear`` method when on Matplotlib 3.6,
+and only ``cla`` for older versions.
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -559,6 +559,8 @@ class _AxesBase(martist.Artist):
     _shared_axes = {name: cbook.Grouper() for name in _axis_names}
     _twinned_axes = cbook.Grouper()
 
+    _subclass_uses_cla = False
+
     @property
     def _axis_map(self):
         """A mapping of axis names, e.g. 'x', to `Axis` instances."""
@@ -699,6 +701,19 @@ def __init__(self, fig, rect,
                         rcParams['ytick.major.right']),
             which='major')
 
+    def __init_subclass__(cls, **kwargs):
+        parent_uses_cla = super(cls, cls)._subclass_uses_cla
+        if 'cla' in cls.__dict__:
+            _api.warn_deprecated(
+                '3.6',
+                pending=True,
+                message=f'Overriding `Axes.cla` in {cls.__qualname__} is '
+                'pending deprecation in %(since)s and will be fully '
+                'deprecated for `Axes.clear` in the future. Please report '
+                f'this to the {cls.__module__!r} author.')
+        cls._subclass_uses_cla = 'cla' in cls.__dict__ or parent_uses_cla
+        super().__init_subclass__(**kwargs)
+
     def __getstate__(self):
         state = super().__getstate__()
         # Prune the sharing & twinning info to only contain the current group.
@@ -1199,7 +1214,7 @@ def sharey(self, other):
         self.set_ylim(y0, y1, emit=False, auto=other.get_autoscaley_on())
         self.yaxis._scale = other.yaxis._scale
 
-    def clear(self):
+    def _clear(self):
         """Clear the Axes."""
         # Note: this is called by Axes.__init__()
 
@@ -1318,6 +1333,24 @@ def clear(self):
 
         self.stale = True
 
+    def clear(self):
+        """Clear the Axes."""
+        # Act as an alias, or as the superclass implementation depending on the
+        # subclass implementation.
+        if self._subclass_uses_cla:
+            self.cla()
+        else:
+            self._clear()
+
+    def cla(self):
+        """Clear the Axes."""
+        # Act as an alias, or as the superclass implementation depending on the
+        # subclass implementation.
+        if self._subclass_uses_cla:
+            self._clear()
+        else:
+            self.clear()
+
     class ArtistList(MutableSequence):
         """
         A sublist of Axes children based on their type.
@@ -1481,10 +1514,6 @@ def texts(self):
         return self.ArtistList(self, 'texts', 'add_artist',
                                valid_types=mtext.Text)
 
-    def cla(self):
-        """Clear the Axes."""
-        self.clear()
-
     def get_facecolor(self):
         """Get the facecolor of the Axes."""
         return self.patch.get_facecolor()
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -486,10 +486,61 @@ def test_inverted_cla():
     plt.close(fig)
 
 
-def test_cla_not_redefined():
+def test_subclass_clear_cla():
+    # Ensure that subclasses of Axes call cla/clear correctly.
+    # Note, we cannot use mocking here as we want to be sure that the
+    # superclass fallback does not recurse.
+
+    with pytest.warns(match='Overriding `Axes.cla`'):
+        class ClaAxes(Axes):
+            def cla(self):
+                nonlocal called
+                called = True
+
+    with pytest.warns(match='Overriding `Axes.cla`'):
+        class ClaSuperAxes(Axes):
+            def cla(self):
+                nonlocal called
+                called = True
+                super().cla()
+
+    class SubClaAxes(ClaAxes):
+        pass
+
+    class ClearAxes(Axes):
+        def clear(self):
+            nonlocal called
+            called = True
+
+    class ClearSuperAxes(Axes):
+        def clear(self):
+            nonlocal called
+            called = True
+            super().clear()
+
+    class SubClearAxes(ClearAxes):
+        pass
+
+    fig = Figure()
+    for axes_class in [ClaAxes, ClaSuperAxes, SubClaAxes,
+                       ClearAxes, ClearSuperAxes, SubClearAxes]:
+        called = False
+        ax = axes_class(fig, [0, 0, 1, 1])
+        # Axes.__init__ has already called clear (which aliases to cla or is in
+        # the subclass).
+        assert called
+
+        called = False
+        ax.cla()
+        assert called
+
+
+def test_cla_not_redefined_internally():
     for klass in Axes.__subclasses__():
-        # check that cla does not get redefined in our Axes subclasses
-        assert 'cla' not in klass.__dict__
+        # Check that cla does not get redefined in our Axes subclasses, except
+        # for in the above test function.
+        if 'test_subclass_clear_cla' not in klass.__qualname__:
+            assert 'cla' not in klass.__dict__
 
 
 @check_figures_equal(extensions=["png"])
