Privatize axis.converter attribute

The replacement is the get/set_converter method.

This includes changes to use the getter and the private setter in the qt
figure options dialog menu.

The choice to use the private setter was a defensive one as the public
setter prevents being called multiple times (though does short circuit
if an identical input is provided, which I think is actually true here,
therefore the public one is probably functional (and a no-op).)

It is not clear to me on analysis how the unit information is or was
lost. A quick test commenting out the two lines which reset
converter/units displayed no obvious detrimental effect to removing
those, suggesting that even if once they were necessary, they may no
longer be.

These lines were last touched in matplotlib#24141, though that really only generalized
the code into a loop rather than copy/pasted x and y behavior.
The original inclusion of resetting was in matplotlib#4909, which indicated that
the dialog reset unit info. AFAICT, that is no longer true, though I
have not rigorously proved that.

lib/matplotlib/axis.py

@@ -600,6 +600,10 @@ class Axis(martist.Artist):
     # The class used in _get_tick() to create tick instances. Must either be
     # overwritten in subclasses, or subclasses must reimplement _get_tick().
     _tick_class = None
+    converter = _api.deprecate_privatize_attribute(
+                    "3.10",
+                    alternative="get_converter and set_converter methods"
+                )
 
     def __str__(self):
         return "{}({},{})".format(
@@ -656,7 +660,7 @@ def __init__(self, axes, *, pickradius=15, clear=True):
         if clear:
             self.clear()
         else:
-            self.converter = None
+            self._converter = None
             self._converter_is_explicit = False
             self.units = None
 
@@ -887,7 +891,7 @@ def clear(self):
                 mpl.rcParams['axes.grid.which'] in ('both', 'minor'))
         self.reset_ticks()
 
-        self.converter = None
+        self._converter = None
         self._converter_is_explicit = False
         self.units = None
         self.stale = True
@@ -1740,20 +1744,20 @@ def grid(self, visible=None, which='major', **kwargs):
     def update_units(self, data):
         """
         Introspect *data* for units converter and update the
-        ``axis.converter`` instance if necessary. Return *True*
+        ``axis.get_converter`` instance if necessary. Return *True*
         if *data* is registered for unit conversion.
         """
         if not self._converter_is_explicit:
             converter = munits.registry.get_converter(data)
         else:
-            converter = self.converter
+            converter = self._converter
 
         if converter is None:
             return False
 
-        neednew = self.converter != converter
+        neednew = self._converter != converter
         self._set_converter(converter)
-        default = self.converter.default_units(data, self)
+        default = self._converter.default_units(data, self)
         if default is not None and self.units is None:
             self.set_units(default)
 
@@ -1767,10 +1771,10 @@ def _update_axisinfo(self):
         Check the axis converter for the stored units to see if the
         axis info needs to be updated.
         """
-        if self.converter is None:
+        if self._converter is None:
             return
 
-        info = self.converter.axisinfo(self.units, self)
+        info = self._converter.axisinfo(self.units, self)
 
         if info is None:
             return
@@ -1797,20 +1801,20 @@ def _update_axisinfo(self):
         self.set_default_intervals()
 
     def have_units(self):
-        return self.converter is not None or self.units is not None
+        return self._converter is not None or self.units is not None
 
     def convert_units(self, x):
         # If x is natively supported by Matplotlib, doesn't need converting
         if munits._is_natively_supported(x):
             return x
 
-        if self.converter is None:
+        if self._converter is None:
             self._set_converter(munits.registry.get_converter(x))
 
-        if self.converter is None:
+        if self._converter is None:
             return x
         try:
-            ret = self.converter.convert(x, self.units, self)
+            ret = self._converter.convert(x, self.units, self)
         except Exception as e:
             raise munits.ConversionError('Failed to convert value(s) to axis '
                                          f'units: {x!r}') from e
@@ -1824,7 +1828,7 @@ def get_converter(self):
         -------
         `~matplotlib.units.ConversionInterface` or None
         """
-        return self.converter
+        return self._converter
 
     def set_converter(self, converter):
         """
@@ -1838,16 +1842,16 @@ def set_converter(self, converter):
         self._converter_is_explicit = True
 
     def _set_converter(self, converter):
-        if self.converter == converter:
+        if self._converter == converter:
             return
         if self._converter_is_explicit:
             raise RuntimeError("Axis already has an explicit converter set")
-        elif self.converter is not None:
+        elif self._converter is not None:
             _api.warn_external(
-                "This axis already has an converter set and "
+                "This axis already has a converter set and "
                 "is updating to a potentially incompatible converter"
             )
-        self.converter = converter
+        self._converter = converter
 
     def set_units(self, u):
         """
@@ -2568,8 +2572,8 @@ def set_default_intervals(self):
         # not changed the view:
         if (not self.axes.dataLim.mutatedx() and
                 not self.axes.viewLim.mutatedx()):
-            if self.converter is not None:
-                info = self.converter.axisinfo(self.units, self)
+            if self._converter is not None:
+                info = self._converter.axisinfo(self.units, self)
                 if info.default_limits is not None:
                     xmin, xmax = self.convert_units(info.default_limits)
                     self.axes.viewLim.intervalx = xmin, xmax
@@ -2798,8 +2802,8 @@ def set_default_intervals(self):
         # not changed the view:
         if (not self.axes.dataLim.mutatedy() and
                 not self.axes.viewLim.mutatedy()):
-            if self.converter is not None:
-                info = self.converter.axisinfo(self.units, self)
+            if self._converter is not None:
+                info = self._converter.axisinfo(self.units, self)
                 if info.default_limits is not None:
                     ymin, ymax = self.convert_units(info.default_limits)
                     self.axes.viewLim.intervaly = ymin, ymax

lib/matplotlib/backends/qt_editor/figureoptions.py

@@ -42,7 +42,7 @@ def convert_limits(lim, converter):
     axis_map = axes._axis_map
     axis_limits = {
         name: tuple(convert_limits(
-            getattr(axes, f'get_{name}lim')(), axis.converter
+            getattr(axes, f'get_{name}lim')(), axis.get_converter()
         ))
         for name, axis in axis_map.items()
     }
@@ -66,7 +66,7 @@ def convert_limits(lim, converter):
 
     # Save the converter and unit data
     axis_converter = {
-        name: axis.converter
+        name: axis.get_converter()
         for name, axis in axis_map.items()
     }
     axis_units = {
@@ -209,7 +209,7 @@ def apply_callback(data):
             axis.set_label_text(axis_label)
 
             # Restore the unit data
-            axis.converter = axis_converter[name]
+            axis._set_converter(axis_converter[name])
             axis.set_units(axis_units[name])
 
         # Set / Curves

lib/matplotlib/tests/test_dates.py

@@ -668,10 +668,12 @@ def test_concise_converter_stays():
     fig, ax = plt.subplots()
     ax.plot(x, y)
     # Bypass Switchable date converter
-    ax.xaxis.converter = conv = mdates.ConciseDateConverter()
+    conv = mdates.ConciseDateConverter()
+    with pytest.warns(UserWarning, match="already has a converter"):
+        ax.xaxis.set_converter(conv)
     assert ax.xaxis.units is None
     ax.set_xlim(*x)
-    assert ax.xaxis.converter == conv
+    assert ax.xaxis.get_converter() == conv
 
 
 def test_offset_changes():

lib/matplotlib/tests/test_units.py

@@ -244,10 +244,10 @@ def test_explicit_converter():
     # Explicit is set
     fig1, ax1 = plt.subplots()
     ax1.xaxis.set_converter(str_cat_converter)
-    assert ax1.xaxis.converter == str_cat_converter
+    assert ax1.xaxis.get_converter() == str_cat_converter
     # Explicit not overridden by implicit
     ax1.plot(d1.keys(), d1.values())
-    assert ax1.xaxis.converter == str_cat_converter
+    assert ax1.xaxis.get_converter() == str_cat_converter
     # No error when called twice with equivalent input
     ax1.xaxis.set_converter(str_cat_converter)
     # Error when explicit called twice