Deprecate old and little used formatters.

anntzer · anntzer · commit 74fd787ab846 · 2020-01-09T11:14:27.000+01:00
- The new ScalarFormatter replaced OldScalarFormatter in 3910bb9 (2005). - IndexFormatter appeared in e66091d (2009) but was never used in the examples; IndexDateFormatter is unused in the docs since 4ea4b96 (2009); both can be fairly easily implemented in terms of FuncFormatter. I'm willing to back off the deprecations if there's demand to keep them, though.
diff --git a/doc/api/next_api_changes/deprecations.rst b/doc/api/next_api_changes/deprecations.rst
@@ -131,3 +131,8 @@ This method is deprecated.  Use
 ``SymmetricalScale.SymmetricalTransform`` and
 ``SymmetricalScale.InvertedSymmetricalTransform`` are deprecated.  Directly
 access the transform classes from the :mod:`.scale` module.
+
+``OldScalarFormatter``, ``IndexFormatter`` and ``DateIndexFormatter``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+These formatters are deprecated.  Their functionality can be implemented using
+e.g. `.FuncFormatter`.
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -604,6 +604,7 @@ def set_tzinfo(self, tz):
         self.tz = tz
 
 
+@cbook.deprecated("3.3")
 class IndexDateFormatter(ticker.Formatter):
     """Use with `.IndexLocator` to cycle format strings by index."""
 
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
@@ -10,6 +10,7 @@
 import pytest
 
 import matplotlib
+from matplotlib import cbook
 import matplotlib.pyplot as plt
 import matplotlib.ticker as mticker
 
@@ -457,7 +458,8 @@ class TestIndexFormatter:
                                           (2, 'label2'),
                                           (2.5, '')])
     def test_formatting(self, x, label):
-        formatter = mticker.IndexFormatter(['label0', 'label1', 'label2'])
+        with cbook._suppress_matplotlib_deprecation_warning():
+            formatter = mticker.IndexFormatter(['label0', 'label1', 'label2'])
         assert formatter(x) == label
 
 
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -299,6 +299,7 @@ def _set_locator(self, locator):
         pass
 
 
+@cbook.deprecated("3.3")
 class IndexFormatter(Formatter):
     """
     Format the position x to the nearest i-th label where ``i = int(x + 0.5)``.
@@ -328,13 +329,10 @@ def __call__(self, x, pos=None):
 
 
 class NullFormatter(Formatter):
-    """
-    Always return the empty string.
-    """
+    """Always return the empty string."""
+
     def __call__(self, x, pos=None):
-        """
-        Returns an empty string for all inputs.
-        """
+        # docstring inherited
         return ''
 
 
