BUG: Fix concat DataFrame and Series with ignore_index=True

doc/source/whatsnew/v3.0.0.rst

@@ -779,6 +779,7 @@ Reshaping
 - Bug in :meth:`DataFrame.pivot_table` incorrectly subaggregating results when called without an ``index`` argument (:issue:`58722`)
 - Bug in :meth:`DataFrame.stack` with the new implementation where ``ValueError`` is raised when ``level=[]`` (:issue:`60740`)
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when manipulating empty :class:`DataFrame` with an :class:`ExtentionDtype` (:issue:`59123`)
+- Bug in :meth:`concat` where concatenating dataframe and series with ignore_index = True drops the series name (:issue:`60723`, :issue:`56257`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/concat.py

@@ -477,6 +477,7 @@ def _sanitize_mixed_ndim(
 
         else:
             name = getattr(obj, "name", None)
+            name_none_flag = False
             if ignore_index or name is None:
                 if axis == 1:
                     # doing a row-wise concatenation so need everything
@@ -485,10 +486,12 @@ def _sanitize_mixed_ndim(
                 else:
                     # doing a column-wise concatenation so need series
                     # to have unique names
-                    name = current_column
+                    if name is None:
+                        name_none_flag = True
+                        name = current_column
                     current_column += 1
                 obj = sample._constructor(obj, copy=False)
-                if isinstance(obj, ABCDataFrame):
+                if isinstance(obj, ABCDataFrame) and name_none_flag:
                     obj.columns = range(name, name + 1, 1)
             else:
                 obj = sample._constructor({name: obj}, copy=False)

pandas/tests/reshape/concat/test_concat.py

@@ -326,6 +326,8 @@ def test_concat_mixed_objs_index(self):
     def test_concat_mixed_objs_index_names(self):
         # Test row-wise concat for mixed series/frames with distinct names
         # GH2385, GH15047
+        # GH #60723 & GH #56257 (Updated the test case,
+        # as the above GH PR ones were incorrect)
 
         index = date_range("01-Jan-2013", periods=10, freq="h")
         arr = np.arange(10, dtype="int64")
@@ -341,8 +343,11 @@ def test_concat_mixed_objs_index_names(self):
         result = concat([s1, df, s2])
         tm.assert_frame_equal(result, expected)
 
-        # Rename all series to 0 when ignore_index=True
-        expected = DataFrame(np.tile(arr, 3).reshape(-1, 1), columns=[0])
+        expected = DataFrame(
+            np.kron(np.where(np.identity(3) == 1, 1, np.nan), arr).T,
+            index=np.arange(30, dtype=np.int64),
+            columns=["foo", 0, "bar"],
+        )
         result = concat([s1, df, s2], ignore_index=True)
         tm.assert_frame_equal(result, expected)
 
@@ -943,3 +948,15 @@ def test_concat_with_moot_ignore_index_and_keys():
     msg = f"Cannot set {ignore_index=} and specify keys. Either should be used."
     with pytest.raises(ValueError, match=msg):
         concat([df1, df2], keys=keys, ignore_index=ignore_index)
+
+
+def test_concat_of_series_and_frame_with_names_for_ignore_index():
+    # GH #60723 and #56257
+    ser = Series([4, 5], name="c")
+    df = DataFrame({"a": [0, 1], "b": [2, 3]})
+
+    result = concat([df, ser], ignore_index=True)
+    expected = DataFrame(
+        {"a": [0, 1, None, None], "b": [2, 3, None, None], "c": [None, None, 4, 5]}
+    )
+    tm.assert_frame_equal(result, expected)