TST: Add test to check category dtype remains unchanged after concat. (pandas-dev#29352)

Author: louishuynh

pandas/tests/reshape/test_concat.py

@@ -2747,6 +2747,22 @@ def test_concat_categorical_tz():
     tm.assert_series_equal(result, expected)
 
 
+def test_concat_categorical_unchanged():
+    # GH-12007
+    # test fix for when concat on categorical and float
+    # coerces dtype categorical -> float
+    df = pd.DataFrame(pd.Series(["a", "b", "c"], dtype="category", name="A"))
+    ser = pd.Series([0, 1, 2], index=[0, 1, 3], name="B")
+    result = pd.concat([df, ser], axis=1)
+    expected = pd.DataFrame(
+        {
+            "A": pd.Series(["a", "b", "c", np.nan], dtype="category"),
+            "B": pd.Series([0, 1, np.nan, 2], dtype="float"),
+        }
+    )
+    tm.assert_equal(result, expected)
+
+
 def test_concat_datetimeindex_freq():
     # GH 3232
     # Monotonic index result