FIX: Polar Radial Tick Warnings Labels Bug (matplotlib#31577)

* Fix polar RadialLocator triggering spurious UserWarning in set_ticklabels

Extend the FixedLocator isinstance check in Axis.set_ticklabels and
_set_formatter to also recognise locators that wrap a FixedLocator via
a .base attribute (e.g. RadialLocator used by polar axes).  Previously
calling set_ticks(ticks, labels) or set_ticklabels(labels) on a polar
axis would always hit the else branch and emit a spurious warning
because RadialAxis.set_major_locator automatically wraps any locator in
a RadialLocator, hiding the inner FixedLocator from the isinstance check.


Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: beelauuu

lib/matplotlib/axis.py

@@ -1999,7 +1999,9 @@ def _set_formatter(self, formatter, level):
 
         if (isinstance(formatter, mticker.FixedFormatter)
                 and len(formatter.seq) > 0
-                and not isinstance(level.locator, mticker.FixedLocator)):
+                and not isinstance(level.locator, mticker.FixedLocator)
+                and not (hasattr(level.locator, 'base') and
+                         isinstance(level.locator.base, mticker.FixedLocator))):
             _api.warn_external('FixedFormatter should only be used together '
                                'with FixedLocator')
 
@@ -2142,16 +2144,21 @@ def set_ticklabels(self, labels, *, minor=False, fontdict=None, **kwargs):
         if not labels:
             # eg labels=[]:
             formatter = mticker.NullFormatter()
-        elif isinstance(locator, mticker.FixedLocator):
+        elif (isinstance(locator, mticker.FixedLocator) or
+              (hasattr(locator, 'base') and
+               isinstance(locator.base, mticker.FixedLocator))):
+            # Also handles locators that wrap a FixedLocator (e.g. RadialLocator).
+            fixed_locator = (locator if isinstance(locator, mticker.FixedLocator)
+                             else locator.base)
             # Passing [] as a list of labels is often used as a way to
             # remove all tick labels, so only error for > 0 labels
-            if len(locator.locs) != len(labels) and len(labels) != 0:
+            if len(fixed_locator.locs) != len(labels) and len(labels) != 0:
                 raise ValueError(
                     "The number of FixedLocator locations"
-                    f" ({len(locator.locs)}), usually from a call to"
+                    f" ({len(fixed_locator.locs)}), usually from a call to"
                     " set_ticks, does not match"
                     f" the number of labels ({len(labels)}).")
-            tickd = {loc: lab for loc, lab in zip(locator.locs, labels)}
+            tickd = {loc: lab for loc, lab in zip(fixed_locator.locs, labels)}
             func = functools.partial(self._format_with_dict, tickd)
             formatter = mticker.FuncFormatter(func)
         else:

lib/matplotlib/tests/test_polar.py

@@ -591,3 +591,13 @@ def test_radial_locator_wrapping():
     assert ax.yaxis.isDefault_majloc
     assert isinstance(ax.yaxis.get_major_locator(), RadialLocator)
     assert isinstance(ax.yaxis.get_major_locator().base, mticker.LogLocator)
+
+
+def test_set_rticks_ticklabels_no_warning():
+    # Regression test: RadialLocator wrapping a FixedLocator must not trigger
+    # the "set_ticklabels() should only be used with a fixed number of ticks"
+    # UserWarning when set_ticks()/set_rticks() was called first.
+
+    fig, ax = plt.subplots(subplot_kw={'projection': 'polar'})
+    ax.set_rticks([0, 1, 2, 3])
+    ax.yaxis.set_ticklabels(['zero', 'one', 'two', 'three'])