another better patch

Author: pbrochart

doc/source/whatsnew/v3.0.0.rst

@@ -404,6 +404,7 @@ Other API changes
 - Index set operations (like union or intersection) will now ignore the dtype of
   an empty ``RangeIndex`` or empty ``Index`` with object dtype when determining
   the dtype of the resulting Index (:issue:`60797`)
+- ``np_find_common_type`` will now return ``object`` for mixed ``int64`` and ``uint64`` dtypes to avoid precision lost (:issue:`61676`, :issue:`61688`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.deprecations:

pandas/core/algorithms.py

@@ -557,16 +557,6 @@ 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/core/dtypes/cast.py

@@ -1418,6 +1418,13 @@ def np_find_common_type(*dtypes: np.dtype) -> np.dtype:
             # so fall back to object (find_common_dtype did unless there
             # was only one dtype)
             common_dtype = np.dtype("O")
+        elif (
+            # Some precision is lost with float64 when handling uint64/int64
+            # Use object instead for the common type
+            all(np.dtype(x).kind in "iu" and np.dtype(x).itemsize == 8 for x in dtypes)
+            and common_dtype == np.float64
+        ):
+            common_dtype = np.dtype("O")
 
     except TypeError:
         common_dtype = np.dtype("O")

pandas/tests/arrays/integer/test_concat.py

@@ -14,7 +14,7 @@
         (["Int8", "Int16"], "Int16"),
         (["UInt8", "Int8"], "Int16"),
         (["Int32", "UInt32"], "Int64"),
-        (["Int64", "UInt64"], "Float64"),
+        (["Int64", "UInt64"], "object"),
         (["Int64", "boolean"], "object"),
         (["UInt8", "boolean"], "object"),
     ],
@@ -48,7 +48,7 @@ def test_concat_series(to_concat_dtypes, result_dtype):
         (["Int8", "int16"], "Int16"),
         (["UInt8", "int8"], "Int16"),
         (["Int32", "uint32"], "Int64"),
-        (["Int64", "uint64"], "Float64"),
+        (["Int64", "uint64"], "object"),
         (["Int64", "bool"], "object"),
         (["UInt8", "bool"], "object"),
     ],

pandas/tests/dtypes/cast/test_find_common_type.py

@@ -31,7 +31,7 @@
         ((np.float16, np.float32), np.float32),
         ((np.float16, np.int16), np.float32),
         ((np.float32, np.int16), np.float32),
-        ((np.uint64, np.int64), np.float64),
+        ((np.uint64, np.int64), object),
         ((np.int16, np.float64), np.float64),
         ((np.float16, np.int64), np.float64),
         # Into others.
@@ -155,9 +155,16 @@ def test_interval_dtype(left, right):
     elif left.subtype.kind in ["i", "u", "f"]:
         # i.e. numeric
         if right.subtype.kind in ["i", "u", "f"]:
-            # both numeric -> common numeric subtype
-            expected = IntervalDtype(np.float64, "right")
-            assert result == expected
+            if (
+                left.subtype.kind in ["i", "u"]
+                and right.subtype.kind in ["i", "u"]
+                and left.subtype.kind != right.subtype.kind
+            ):
+                assert result == object
+            else:
+                # both numeric -> common numeric subtype
+                expected = IntervalDtype(np.float64, "right")
+                assert result == expected
         else:
             assert result == object
 

pandas/tests/indexes/interval/test_setops.py

@@ -55,6 +55,7 @@ def test_union_empty_result(self, closed, sort):
         tm.assert_index_equal(result, expected)
 
         other = empty_index(dtype="uint64", closed=closed)
+        expected = Index([], dtype='object')
         result = index.union(other, sort=sort)
         tm.assert_index_equal(result, expected)
 
@@ -117,6 +118,7 @@ def test_intersection_empty_result(self, closed, sort):
         tm.assert_index_equal(result, expected)
 
         other = monotonic_index(300, 314, dtype="uint64", closed=closed)
+        expected = Index([], dtype='object')
         result = index.intersection(other, sort=sort)
         tm.assert_index_equal(result, expected)