BUG: Check types in Index.__contains__ (#22085)

doc/source/whatsnew/v0.24.0.txt

@@ -658,6 +658,7 @@ Indexing
 - Bug when indexing :class:`DatetimeIndex` with nanosecond resolution dates and timezones (:issue:`11679`)
 - Bug where indexing with a Numpy array containing negative values would mutate the indexer (:issue:`21867`)
 - ``Float64Index.get_loc`` now raises ``KeyError`` when boolean key passed. (:issue:`19087`)
+- Bug in `scalar in Index` if scalar is a float while the ``Index`` is of integer dtype (:issue:`22085`)
 
 Missing
 ^^^^^^^

pandas/_libs/index.pyx

@@ -93,6 +93,9 @@ cdef class IndexEngine:
     def __contains__(self, object val):
         self._ensure_mapping_populated()
         hash(val)
+        if (util.is_float_object(val) and isinstance(self, Int64Engine) and
+                int(val) != val):
+            return False
         return val in self.mapping
 
     cpdef get_value(self, ndarray arr, object key, object tz=None):

pandas/tests/indexes/test_base.py

@@ -2483,6 +2483,19 @@ def test_comparison_tzawareness_compat(self, op):
         # TODO: implement _assert_tzawareness_compat for the reverse
         # comparison with the Series on the left-hand side
 
+    def test_contains_with_float_val(self):
+        # GH#22085
+        index1 = pd.Index([0, 1, 2, 3])
+        index2 = pd.Index([0.1, 1.1, 2.2, 3.3])
+
+        assert 1.1 not in index1
+        assert 1.0 in index1
+        assert 1 in index1
+
+        assert 1.1 in index2
+        assert 1.0 not in index2
+        assert 1 not in index2
+
 
 class TestIndexUtils(object):