Deprecate setting Axis.major.locator to non-Locator; idem for Formatters

anntzer · anntzer · commit eaf4437f828f · 2019-04-02T23:16:15.000+02:00
See changelog.

This is so that we can actually add new APIs to the base class (e.g.
format_ticks; tick_deconflict (however that one ends up being named)
and actually benefit from it in the rest of the codebase without
worrying that third-party locators/formatters do not inherit from the
base class (note that inheriting from the base class does not preclude
third-party locators/formatters from completely redefining all methods,
so this does not limit them).
diff --git a/doc/api/next_api_changes/2019-04-02-AL.rst b/doc/api/next_api_changes/2019-04-02-AL.rst
@@ -0,0 +1,8 @@
+Deprecations
+````````````
+
+Setting ``Axis.major.locator``, ``Axis.minor.locator``, ``Axis.major.formatter``
+or ``Axis.minor.formatter`` to an object that is not a subclass of `Locator` or
+`Formatter` (respectively) is deprecated.  Note that these attributes should
+usually be set using `Axis.set_major_locator`, `Axis.set_minor_locator`, etc.
+which already raise an exception when an object of the wrong class is passed.
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -640,8 +640,36 @@ class Ticker(object):
     formatter : `matplotlib.ticker.Formatter` subclass
         Determines the format of the tick labels.
     """
-    locator = None
-    formatter = None
+
+    def __init__(self):
+        self._locator = None
+        self._formatter = None
+
+    @property
+    def locator(self):
+        return self._locator
+
+    @locator.setter
+    def locator(self, locator):
+        if not isinstance(locator, mticker.Locator):
+            cbook.warn_deprecated(
+                "3.2", message="Support for locators that do not subclass "
+                "matplotlib.ticker.Locator is deprecated since %(since)s and "
+                "support for them will be removed %(removal)s.")
+        self._locator = locator
+
+    @property
+    def formatter(self):
+        return self._formatter
+
+    @formatter.setter
+    def formatter(self, formatter):
+        if not isinstance(formatter, mticker.Formatter):
+            cbook.warn_deprecated(
+                "3.2", message="Support for formatters that do not subclass "
+                "matplotlib.ticker.Formatter is deprecated since %(since)s "
+                "and support for them will be removed %(removal)s.")
+        self._formatter = formatter
 
 
 class _LazyTickList(object):
