Backport PR matplotlib#31758: Fix invalid format string in set_xticklabels error message.

Author: timhoffm

lib/matplotlib/axis.py

@@ -2147,11 +2147,10 @@ def set_ticklabels(self, labels, *, minor=False, fontdict=None, **kwargs):
             labels = [t.get_text() if hasattr(t, 'get_text') else t
                       for t in labels]
         except TypeError:
-            raise TypeError(f"{labels:=} must be a sequence") from None
+            raise TypeError(f"{labels=!r} must be a sequence") from None
         locator = (self.get_minor_locator() if minor
                    else self.get_major_locator())
         if not labels:
-            # eg labels=[]:
             formatter = mticker.NullFormatter()
         elif (isinstance(locator, mticker.FixedLocator) or
               (hasattr(locator, 'base') and

lib/matplotlib/tests/test_axes.py

@@ -6988,6 +6988,8 @@ def test_xticks_bad_args():
     ax = plt.figure().add_subplot()
     with pytest.raises(TypeError, match='must be a sequence'):
         ax.set_xticks([2, 9], 3.1)
+    with pytest.raises(TypeError, match='must be a sequence'):
+        ax.set_xticks([2, 9], ax)
     with pytest.raises(ValueError, match='must be 1D'):
         plt.xticks(np.arange(4).reshape((-1, 1)))
     with pytest.raises(ValueError, match='must be 1D'):