BUG: casting on concat with empties (#38907)

Author: jbrockmendel

doc/source/whatsnew/v1.3.0.rst

@@ -296,7 +296,7 @@ Groupby/resample/rolling
 Reshaping
 ^^^^^^^^^
 - Bug in :meth:`DataFrame.unstack` with missing levels led to incorrect index names (:issue:`37510`)
-- Bug in :func:`concat` incorrectly casting to ``object`` dtype in some cases when one or more of the operands is empty (:issue:`38843`)
+- 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`)
 -
 
 

pandas/core/internals/concat.py

@@ -318,6 +318,12 @@ 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

@@ -154,7 +154,8 @@ def test_partial_setting_mixed_dtype(self):
         # columns will align
         df = DataFrame(columns=["A", "B"])
         df.loc[0] = Series(1, index=range(4))
-        tm.assert_frame_equal(df, DataFrame(columns=["A", "B"], index=[0]))
+        expected = DataFrame(columns=["A", "B"], index=[0], dtype=int)
+        tm.assert_frame_equal(df, expected)
 
         # columns will align
         df = DataFrame(columns=["A", "B"])

pandas/tests/reshape/concat/test_append.py

@@ -82,6 +82,7 @@ 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):
@@ -340,16 +341,11 @@ 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_empty.py

@@ -210,7 +210,6 @@ def test_concat_empty_df_object_dtype(self, dtype):
         df_2 = DataFrame(columns=df_1.columns)
         result = pd.concat([df_1, df_2], axis=0)
         expected = df_1.copy()
-        expected["EmptyCol"] = expected["EmptyCol"].astype(object)  # TODO: why?
         tm.assert_frame_equal(result, expected)
 
     def test_concat_empty_dataframe_dtypes(self):