TST: Fix xfails for non-box maybe_promote on integer dtypes (#28864)

Author: jbrockmendel

pandas/core/dtypes/cast.py

@@ -424,9 +424,58 @@ def maybe_promote(dtype, fill_value=np.nan):
             dtype = np.dtype(np.object_)
         elif issubclass(dtype.type, np.integer):
             # upcast to prevent overflow
-            arr = np.asarray(fill_value)
-            if arr != arr.astype(dtype):
-                dtype = arr.dtype
+            mst = np.min_scalar_type(fill_value)
+            if mst > dtype:
+                # np.dtype ordering considers:
+                #  int[n] < int[2*n]
+                #  uint[n] < uint[2*n]
+                #  u?int[n] < object_
+                dtype = mst
+
+            elif np.can_cast(fill_value, dtype):
+                pass
+
+            elif dtype.kind == "u" and mst.kind == "i":
+                dtype = np.promote_types(dtype, mst)
+                if dtype.kind == "f":
+                    # Case where we disagree with numpy
+                    dtype = np.dtype(np.object_)
+
+            elif dtype.kind == "i" and mst.kind == "u":
+
+                if fill_value > np.iinfo(np.int64).max:
+                    # object is the only way to represent fill_value and keep
+                    #  the range allowed by the given dtype
+                    dtype = np.dtype(np.object_)
+
+                elif mst.itemsize < dtype.itemsize:
+                    pass
+
+                elif dtype.itemsize == mst.itemsize:
+                    # We never cast signed to unsigned because that loses
+                    #  parts of the original range, so find the smallest signed
+                    #  integer that can hold all of `mst`.
+                    ndt = {
+                        np.int64: np.object_,
+                        np.int32: np.int64,
+                        np.int16: np.int32,
+                        np.int8: np.int16,
+                    }[dtype.type]
+                    dtype = np.dtype(ndt)
+
+                else:
+                    # bump to signed integer dtype that holds all of `mst` range
+                    # Note: we have to use itemsize because some (windows)
+                    #  builds don't satisfiy e.g. np.uint32 == np.uint32
+                    ndt = {
+                        4: np.int64,
+                        2: np.int32,
+                        1: np.int16,  # TODO: Test for this case
+                    }[mst.itemsize]
+                    dtype = np.dtype(ndt)
+
+            fill_value = dtype.type(fill_value)
+
         elif issubclass(dtype.type, np.floating):
             # check if we can cast
             if _check_lossless_cast(fill_value, dtype):

pandas/tests/dtypes/cast/test_promote.py

@@ -151,7 +151,17 @@ def _assert_match(result_fill_value, expected_fill_value):
     # GH#23982/25425 require the same type in addition to equality/NA-ness
     res_type = type(result_fill_value)
     ex_type = type(expected_fill_value)
-    assert res_type == ex_type
+    if res_type.__name__ == "uint64":
+        # No idea why, but these (sometimes) do not compare as equal
+        assert ex_type.__name__ == "uint64"
+    elif res_type.__name__ == "ulonglong":
+        # On some builds we get this instead of np.uint64
+        # Note: cant check res_type.dtype.itemsize directly on numpy 1.18
+        assert res_type(0).itemsize == 8
+        assert ex_type == res_type or ex_type == np.uint64
+    else:
+        # On some builds, type comparison fails, e.g. np.int32 != np.int32
+        assert res_type == ex_type or res_type.__name__ == ex_type.__name__
 
     match_value = result_fill_value == expected_fill_value
 
@@ -275,26 +285,6 @@ def test_maybe_promote_int_with_int(dtype, fill_value, expected_dtype, box):
     expected_dtype = np.dtype(expected_dtype)
     boxed, box_dtype = box  # read from parametrized fixture
 
-    if not boxed:
-        if expected_dtype == object:
-            pytest.xfail("overflow error")
-        if expected_dtype == "int32":
-            pytest.xfail("always upcasts to platform int")
-        if dtype == "int8" and expected_dtype == "int16":
-            pytest.xfail("casts to int32 instead of int16")
-        if (
-            issubclass(dtype.type, np.unsignedinteger)
-            and np.iinfo(dtype).max < fill_value <= np.iinfo("int64").max
-        ):
-            pytest.xfail("falsely casts to signed")
-        if (dtype, expected_dtype) in [
-            ("uint8", "int16"),
-            ("uint32", "int64"),
-        ] and fill_value != np.iinfo("int32").min - 1:
-            pytest.xfail("casts to int32 instead of int8/int16")
-        # this following xfail is "only" a consequence of the - now strictly
-        # enforced - principle that maybe_promote_with_scalar always casts
-        pytest.xfail("wrong return type of fill_value")
     if boxed:
         if expected_dtype != object:
             pytest.xfail("falsely casts to object")