DEPR: Change .ix DeprecationWarning -> FutureWarning (pandas-dev#26438)

Author: jorisvandenbossche

doc/source/whatsnew/v0.25.0.rst

@@ -257,6 +257,7 @@ Other API Changes
 Deprecations
 ~~~~~~~~~~~~
 
+- The deprecated ``.ix[]`` indexer now raises a more visible FutureWarning instead of DeprecationWarning (:issue:`26438`).
 - Deprecated the ``units=M`` (months) and ``units=Y`` (year) parameters for ``units`` of :func:`pandas.to_timedelta`, :func:`pandas.Timedelta` and :func:`pandas.TimedeltaIndex` (:issue:`16344`)
 - The functions :func:`pandas.to_datetime` and :func:`pandas.to_timedelta` have deprecated the ``box`` keyword. Instead, use :meth:`to_numpy` or :meth:`Timestamp.to_datetime64` or :meth:`Timedelta.to_timedelta64`. (:issue:`24416`)
 - The :meth:`DataFrame.compound` and :meth:`Series.compound` methods are deprecated and will be removed in a future version.

pandas/core/indexing.py

@@ -1419,7 +1419,7 @@ class _IXIndexer(_NDFrameIndexer):
 
     def __init__(self, name, obj):
         warnings.warn(self._ix_deprecation_warning,
-                      DeprecationWarning, stacklevel=2)
+                      FutureWarning, stacklevel=2)
         super().__init__(name, obj)
 
     @Appender(_NDFrameIndexer._validate_key.__doc__)