Fix removal of shared polar axes.

There's really two separate fixes here:

- Move isDefault_{maj,min}{loc,fmt} tracking to the Ticker instances,
  where they logically belong (note the previous need to additionally
  track them manually on axes removal, when that info was tracked on
  the Axis).  This has the side effect of fixing removal of sharex'd
  polar axes, as ThetaLocators rely on _AxisWrappers which don't have
  that isDefault attribute.  (Note that the patch would have resulted
  in a net decrease of lines of code if it didn't need to maintain
  backcompat on isDefault_foos).

- Ensure that RadialLocator correctly propagates Axis information to
  the linear locator it wraps (consistently with ThetaLocator), so that
  when an axes is removed the wrapped linear locator doesn't stay
  pointing at an obsolete axes.  This, together with the first patch,
  fixes removal of sharey'd polar axes.

Author: anntzer

lib/matplotlib/axis.py

@@ -552,6 +552,8 @@ class Ticker:
     def __init__(self):
         self._locator = None
         self._formatter = None
+        self._locator_is_default = True
+        self._formatter_is_default = True
 
     @property
     def locator(self):
@@ -689,6 +691,38 @@ def __init__(self, axes, pickradius=15):
         self.clear()
         self._set_scale('linear')
 
+    @property
+    def isDefault_majloc(self):
+        return self.major._locator_is_default
+
+    @isDefault_majloc.setter
+    def isDefault_majloc(self, value):
+        self.major._locator_is_default = value
+
+    @property
+    def isDefault_majfmt(self):
+        return self.major._formatter_is_default
+
+    @isDefault_majfmt.setter
+    def isDefault_majfmt(self, value):
+        self.major._formatter_is_default = value
+
+    @property
+    def isDefault_minloc(self):
+        return self.minor._locator_is_default
+
+    @isDefault_minloc.setter
+    def isDefault_minloc(self, value):
+        self.minor._locator_is_default = value
+
+    @property
+    def isDefault_minfmt(self):
+        return self.minor._formatter_is_default
+
+    @isDefault_minfmt.setter
+    def isDefault_minfmt(self, value):
+        self.minor._formatter_is_default = value
+
     # During initialization, Axis objects often create ticks that are later
     # unused; this turns out to be a very slow step.  Instead, use a custom
     # descriptor to make the tick lists lazy and instantiate them as needed.

lib/matplotlib/figure.py

@@ -916,33 +916,10 @@ def _reset_locators_and_formatters(axis):
             # Set the formatters and locators to be associated with axis
             # (where previously they may have been associated with another
             # Axis instance)
-            #
-            # Because set_major_formatter() etc. force isDefault_* to be False,
-            # we have to manually check if the original formatter was a
-            # default and manually set isDefault_* if that was the case.
-            majfmt = axis.get_major_formatter()
-            isDefault = majfmt.axis.isDefault_majfmt
-            axis.set_major_formatter(majfmt)
-            if isDefault:
-                majfmt.axis.isDefault_majfmt = True
-
-            majloc = axis.get_major_locator()
-            isDefault = majloc.axis.isDefault_majloc
-            axis.set_major_locator(majloc)
-            if isDefault:
-                majloc.axis.isDefault_majloc = True
-
-            minfmt = axis.get_minor_formatter()
-            isDefault = majloc.axis.isDefault_minfmt
-            axis.set_minor_formatter(minfmt)
-            if isDefault:
-                minfmt.axis.isDefault_minfmt = True
-
-            minloc = axis.get_minor_locator()
-            isDefault = majloc.axis.isDefault_minloc
-            axis.set_minor_locator(minloc)
-            if isDefault:
-                minloc.axis.isDefault_minloc = True
+            axis.get_major_formatter().set_axis(axis)
+            axis.get_major_locator().set_axis(axis)
+            axis.get_minor_formatter().set_axis(axis)
+            axis.get_minor_locator().set_axis(axis)
 
         def _break_share_link(ax, grouper):
             siblings = grouper.get_siblings(ax)

lib/matplotlib/projections/polar.py

@@ -425,6 +425,9 @@ def __init__(self, base, axes=None):
         self.base = base
         self._axes = axes
 
+    def set_axis(self, axis):
+        self.base.set_axis(axis)
+
     def __call__(self):
         # Ensure previous behaviour with full circle non-annular views.
         if self._axes:

lib/matplotlib/tests/test_polar.py

@@ -379,3 +379,18 @@ def test_axvspan():
     ax = plt.subplot(projection="polar")
     span = plt.axvspan(0, np.pi/4)
     assert span.get_path()._interpolation_steps > 1
+
+
+@check_figures_equal(extensions=["png"])
+def test_remove_shared_polar(fig_ref, fig_test):
+    # Removing shared polar axes used to crash.  Test removing them, keeping in
+    # both cases just the lower left axes of a grid to avoid running into a
+    # separate issue (now being fixed) of ticklabel visibility for shared axes.
+    axs = fig_ref.subplots(
+        2, 2, sharex=True, subplot_kw={"projection": "polar"})
+    for i in [0, 1, 3]:
+        axs.flat[i].remove()
+    axs = fig_test.subplots(
+        2, 2, sharey=True, subplot_kw={"projection": "polar"})
+    for i in [0, 1, 3]:
+        axs.flat[i].remove()