API: concatting DataFrames does not skip empty objects (#39035)

* Revert "BUG: casting on concat with empties (#38907)"

This reverts commit 04282c7.

* Revert "BUG: inconsistent concat casting EA vs non-EA (#38843)"

This reverts commit 2362df9.

Author: jorisvandenbossche

doc/source/whatsnew/v1.3.0.rst

@@ -312,8 +312,8 @@ Reshaping
 - Bug in :func:`merge` raising error when performing an inner join with partial index and ``right_index`` when no overlap between indices (:issue:`33814`)
 - Bug in :meth:`DataFrame.unstack` with missing levels led to incorrect index names (:issue:`37510`)
 - Bug in :func:`join` over :class:`MultiIndex` returned wrong result, when one of both indexes had only one level (:issue:`36909`)
-- Bug in :func:`concat` incorrectly casting to ``object`` dtype in some cases when one or more of the operands is empty (:issue:`38843`, :issue:`38907`)
 - :meth:`merge_asof` raises ``ValueError`` instead of cryptic ``TypeError`` in case of non-numerical merge columns (:issue:`29130`)
+-
 
 Sparse
 ^^^^^^

pandas/core/dtypes/concat.py

@@ -127,7 +127,7 @@ def is_nonempty(x) -> bool:
     # marginal given that it would still require shape & dtype calculation and
     # np.concatenate which has them both implemented is compiled.
     non_empties = [x for x in to_concat if is_nonempty(x)]
-    if non_empties:
+    if non_empties and axis == 0:
         to_concat = non_empties
 
     typs = _get_dtype_kinds(to_concat)

pandas/core/internals/concat.py

@@ -318,12 +318,6 @@ def _concatenate_join_units(
         # Concatenating join units along ax0 is handled in _merge_blocks.
         raise AssertionError("Concatenating join units along axis0")
 
-    nonempties = [
-        x for x in join_units if x.block is None or x.block.shape[concat_axis] > 0
-    ]
-    if nonempties:
-        join_units = nonempties
-
     empty_dtype, upcasted_na = _get_empty_dtype_and_na(join_units)
 
     to_concat = [

pandas/tests/indexing/test_partial.py

@@ -154,8 +154,7 @@ def test_partial_setting_mixed_dtype(self):
         # columns will align
         df = DataFrame(columns=["A", "B"])
         df.loc[0] = Series(1, index=range(4))
-        expected = DataFrame(columns=["A", "B"], index=[0], dtype=int)
-        tm.assert_frame_equal(df, expected)
+        tm.assert_frame_equal(df, DataFrame(columns=["A", "B"], index=[0]))
 
         # columns will align
         df = DataFrame(columns=["A", "B"])
@@ -171,21 +170,11 @@ def test_partial_setting_mixed_dtype(self):
         with pytest.raises(ValueError, match=msg):
             df.loc[0] = [1, 2, 3]
 
-    @pytest.mark.parametrize("dtype", [None, "int64", "Int64"])
-    def test_loc_setitem_expanding_empty(self, dtype):
+        # TODO: #15657, these are left as object and not coerced
         df = DataFrame(columns=["A", "B"])
+        df.loc[3] = [6, 7]
 
-        value = [6, 7]
-        if dtype == "int64":
-            value = np.array(value, dtype=dtype)
-        elif dtype == "Int64":
-            value = pd.array(value, dtype=dtype)
-
-        df.loc[3] = value
-
-        exp = DataFrame([[6, 7]], index=[3], columns=["A", "B"], dtype=dtype)
-        if dtype is not None:
-            exp = exp.astype(dtype)
+        exp = DataFrame([[6, 7]], index=[3], columns=["A", "B"], dtype="object")
         tm.assert_frame_equal(df, exp)
 
     def test_series_partial_set(self):

pandas/tests/reshape/concat/test_append.py

@@ -82,7 +82,6 @@ def test_append_length0_frame(self, sort):
         df5 = df.append(df3, sort=sort)
 
         expected = DataFrame(index=[0, 1], columns=["A", "B", "C"])
-        expected["C"] = expected["C"].astype(np.float64)
         tm.assert_frame_equal(df5, expected)
 
     def test_append_records(self):
@@ -341,11 +340,16 @@ def test_append_empty_frame_to_series_with_dateutil_tz(self):
         expected = DataFrame(
             [[np.nan, np.nan, 1.0, 2.0, date]], columns=["c", "d", "a", "b", "date"]
         )
+        # These columns get cast to object after append
+        expected["c"] = expected["c"].astype(object)
+        expected["d"] = expected["d"].astype(object)
         tm.assert_frame_equal(result_a, expected)
 
         expected = DataFrame(
             [[np.nan, np.nan, 1.0, 2.0, date]] * 2, columns=["c", "d", "a", "b", "date"]
         )
+        expected["c"] = expected["c"].astype(object)
+        expected["d"] = expected["d"].astype(object)
 
         result_b = result_a.append(s, ignore_index=True)
         tm.assert_frame_equal(result_b, expected)

pandas/tests/reshape/concat/test_concat.py

@@ -474,12 +474,11 @@ def test_concat_will_upcast(dt, pdt):
         assert x.values.dtype == "float64"
 
 
-@pytest.mark.parametrize("dtype", ["int64", "Int64"])
-def test_concat_empty_and_non_empty_frame_regression(dtype):
+def test_concat_empty_and_non_empty_frame_regression():
     # GH 18178 regression test
-    df1 = DataFrame({"foo": [1]}).astype(dtype)
+    df1 = DataFrame({"foo": [1]})
     df2 = DataFrame({"foo": []})
-    expected = df1
+    expected = DataFrame({"foo": [1.0]})
     result = pd.concat([df1, df2])
     tm.assert_frame_equal(result, expected)
 

pandas/tests/reshape/concat/test_empty.py

@@ -202,14 +202,12 @@ def test_concat_empty_series_dtypes_sparse(self):
         expected = pd.SparseDtype("object")
         assert result.dtype == expected
 
-    @pytest.mark.parametrize("dtype", ["int64", "Int64"])
-    def test_concat_empty_df_object_dtype(self, dtype):
+    def test_concat_empty_df_object_dtype(self):
         # GH 9149
         df_1 = DataFrame({"Row": [0, 1, 1], "EmptyCol": np.nan, "NumberCol": [1, 2, 3]})
-        df_1["Row"] = df_1["Row"].astype(dtype)
         df_2 = DataFrame(columns=df_1.columns)
         result = pd.concat([df_1, df_2], axis=0)
-        expected = df_1.copy()
+        expected = df_1.astype(object)
         tm.assert_frame_equal(result, expected)
 
     def test_concat_empty_dataframe_dtypes(self):