DEPR: deprecate Index.__getitem__ with float key

doc/source/whatsnew/v1.1.0.rst

@@ -590,6 +590,9 @@ Deprecations
 - :func:`pandas.api.types.is_categorical` is deprecated and will be removed in a future version; use `:func:pandas.api.types.is_categorical_dtype` instead (:issue:`33385`)
 - :meth:`Index.get_value` is deprecated and will be removed in a future version (:issue:`19728`)
 - :meth:`DateOffset.__call__` is deprecated and will be removed in a future version, use ``offset + other`` instead (:issue:`34171`)
+- Indexing an :class:`Index` object with a float key is deprecated, and will
+  raise an ``IndexError`` in the future. You can manually convert to an integer key
+  instead (:issue:`34191`).
 - The ``squeeze`` keyword in the ``groupby`` function is deprecated and will be removed in a future version (:issue:`32380`)
 
 .. ---------------------------------------------------------------------------

pandas/core/common.py

@@ -9,6 +9,7 @@
 from functools import partial
 import inspect
 from typing import Any, Collection, Iterable, List, Union
+import warnings
 
 import numpy as np
 
@@ -144,20 +145,29 @@ def is_bool_indexer(key: Any) -> bool:
     return False
 
 
-def cast_scalar_indexer(val):
+def cast_scalar_indexer(val, warn_float=False):
     """
     To avoid numpy DeprecationWarnings, cast float to integer where valid.
 
     Parameters
     ----------
     val : scalar
+    warn_float : bool, default False
+        If True, issue deprecation warning for a float indexer.
 
     Returns
     -------
     outval : scalar
     """
     # assumes lib.is_scalar(val)
     if lib.is_float(val) and val.is_integer():
+        if warn_float:
+            warnings.warn(
+                "Indexing with a float is deprecated, and will raise an IndexError "
+                "in pandas 2.0. You can manually convert to an integer key instead.",
+                FutureWarning,
+                stacklevel=3,
+            )
         return int(val)
     return val
 

pandas/core/indexes/base.py

@@ -4041,7 +4041,7 @@ def __getitem__(self, key):
         promote = self._shallow_copy
 
         if is_scalar(key):
-            key = com.cast_scalar_indexer(key)
+            key = com.cast_scalar_indexer(key, warn_float=True)
             return getitem(key)
 
         if isinstance(key, slice):

pandas/core/indexes/multi.py

@@ -1896,7 +1896,7 @@ def __reduce__(self):
 
     def __getitem__(self, key):
         if is_scalar(key):
-            key = com.cast_scalar_indexer(key)
+            key = com.cast_scalar_indexer(key, warn_float=True)
 
             retval = []
             for lev, level_codes in zip(self.levels, self.codes):

pandas/plotting/_matplotlib/core.py

@@ -8,6 +8,7 @@
 from pandas.util._decorators import cache_readonly
 
 from pandas.core.dtypes.common import (
+    is_float,
     is_hashable,
     is_integer,
     is_iterator,
@@ -1188,6 +1189,8 @@ def _post_plot_logic(self, ax, data):
         from matplotlib.ticker import FixedLocator
 
         def get_label(i):
+            if is_float(i) and i.is_integer():
+                i = int(i)
             try:
                 return pprint_thing(data.index[i])
             except Exception:

pandas/tests/indexes/test_indexing.py

@@ -17,6 +17,7 @@
 import pytest
 
 from pandas import Float64Index, Index, Int64Index, UInt64Index
+import pandas._testing as tm
 
 
 class TestContains:
@@ -82,3 +83,16 @@ def test_contains_with_float_index(self):
         assert 1.1 in float_index
         assert 1.0 not in float_index
         assert 1 not in float_index
+
+
+@pytest.mark.parametrize(
+    "idx", [Index([1, 2, 3]), Index([0.1, 0.2, 0.3]), Index(["a", "b", "c"])]
+)
+def test_getitem_deprecated_float(idx):
+    # https://github.com/pandas-dev/pandas/issues/34191
+
+    with tm.assert_produces_warning(FutureWarning):
+        result = idx[1.0]
+
+    expected = idx[1]
+    assert result == expected