BUG: Fix implicit conversion to float64 with isin()

pandas/core/algorithms.py

@@ -47,15 +47,13 @@
     is_bool_dtype,
     is_complex_dtype,
     is_dict_like,
-    is_dtype_equal,
     is_extension_array_dtype,
     is_float,
     is_float_dtype,
     is_integer,
     is_integer_dtype,
     is_list_like,
     is_object_dtype,
-    is_signed_integer_dtype,
     needs_i8_conversion,
 )
 from pandas.core.dtypes.concat import concat_compat
@@ -508,16 +506,6 @@ def isin(comps: ListLike, values: ListLike) -> npt.NDArray[np.bool_]:
         orig_values = list(values)
         values = _ensure_arraylike(orig_values, func_name="isin-targets")
 
-        if (
-            len(values) > 0
-            and values.dtype.kind in "iufcb"
-            and not is_signed_integer_dtype(comps)
-            and not is_dtype_equal(values, comps)
-        ):
-            # GH#46485 Use object to avoid upcast to float64 later
-            # TODO: Share with _find_common_type_compat
-            values = construct_1d_object_array_from_listlike(orig_values)
-
     elif isinstance(values, ABCMultiIndex):
         # Avoid raising in extract_array
         values = np.array(values)
@@ -569,6 +557,16 @@ def f(c, v):
 
     else:
         common = np_find_common_type(values.dtype, comps_array.dtype)
+        if (
+            values.dtype.kind in "iu"
+            and comps_array.dtype.kind in "iu"
+            and common == np.float64
+        ):
+            # GH#46485
+            # Let's np_find_common_type do the job and return float64
+            # when it cannot do otherwise with integers
+            # We replace it by an object
+            common = np.dtype("O")
         values = values.astype(common, copy=False)
         comps_array = comps_array.astype(common, copy=False)
         f = htable.ismember

pandas/tests/test_algos.py

@@ -1197,6 +1197,13 @@ def test_isin_unsigned_dtype(self):
         expected = Series(False)
         tm.assert_series_equal(result, expected)
 
+    def test_isin_unsigned_dtype_other_side(self):
+        # GH#46485
+        ser = Series([1378774140726870442], dtype=np.int64)
+        result = ser.isin([np.uint64(1378774140726870528)])
+        expected = Series(False)
+        tm.assert_series_equal(result, expected)
+
 
 class TestValueCounts:
     def test_value_counts(self):