matplotlib · ksunden · Apr 11, 2023
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -97,6 +97,7 @@ def __init__(
 
         self.set_figure(axes.figure)
         self.axes = axes
+        self.converter = None
 
         self._loc = loc
         self._major = major
@@ -1821,6 +1822,13 @@ def _set_formatter(self, formatter, level):
             _api.warn_external('FixedFormatter should only be used together '
                                'with FixedLocator')
 
+        if hasattr(self, "converter") and self.converter is not None:
+            if not self.converter.validate_formatter(formatter):
+                _api.warn_external(
+                    f'Converter {self.converter.__class__.__name__!r} got unexpected '
+                    f'formatter {formatter.__class__.__name__!r}'
+                )
+
         if level == self.major:
             self.isDefault_majfmt = False
         else:

diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
@@ -114,6 +114,10 @@ def _validate_unit(unit):
                 f'Provided unit "{unit}" is not valid for a categorical '
                 'converter, as it does not have a _mapping attribute.')
 
+    @staticmethod
+    def validate_formatter(formatter):
+        return isinstance(formatter, (StrCategoryFormatter, ticker.NullFormatter))
+
 
 class StrCategoryLocator(ticker.Locator):
     """Tick at every integer mapping of the string data."""

diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -1836,6 +1836,13 @@ def default_units(x, axis):
             pass
         return None
 
+    @staticmethod
+    def validate_formatter(formatter):
+        return isinstance(formatter, (DateFormatter,
+                                      ConciseDateFormatter,
+                                      AutoDateFormatter,
+                                      ticker.NullFormatter))
+
 
 class ConciseDateConverter(DateConverter):
     # docstring inherited
@@ -1888,6 +1895,9 @@ def default_units(self, *args, **kwargs):
     def convert(self, *args, **kwargs):
         return self._get_converter().convert(*args, **kwargs)
 
+    def validate_formatter(self, formatter):
+        return self._get_converter().validate_formatter(formatter)
+
 
 units.registry[np.datetime64] = \
     units.registry[datetime.date] = \

diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -421,6 +421,7 @@ def test_autofmt_xdate(which):
         warnings.filterwarnings(
             'ignore',
             'FixedFormatter should only be used together with FixedLocator')
+        warnings.filterwarnings('ignore', 'Converter .* got unexpected formatter .*')
         ax.xaxis.set_minor_formatter(FixedFormatter(minors))
 
     fig.autofmt_xdate(0.2, angle, 'right', which)

diff --git a/lib/matplotlib/units.py b/lib/matplotlib/units.py
@@ -36,6 +36,13 @@ def default_units(x, axis):
             "Return the default unit for x or None."
             return 'date'
 
+        @staticmethod
+        def validate_formatter(formatter):
+            "Return whether the formatter works with this converter"
+            return isinstance(formatter, (dates.DateFormatter,
+                                          dates.ConciseDateFormatter,
+                                          dates.AutoDateFormatter))
+
     # Finally we register our object type with the Matplotlib units registry.
     units.registry[datetime.date] = DateConverter()
 """
@@ -131,6 +138,22 @@ def convert(obj, unit, axis):
         """
         return obj
 
+    @staticmethod
+    def validate_formatter(formatter):
+        """Return whether a given formatter is valid for the converter
+
+        Parameters
+        ----------
+        formatter: `~.Formatter`
+            The formatter instance to validate.
+
+        Returns
+        -------
+        bool: if the formatter is valid
+        """
+        # For backwards compatibility, the default is to accept any formatter
+        return True
+
 
 class DecimalConverter(ConversionInterface):
     """Converter for decimal.Decimal data to float."""