Add test

Author: yuanx749

pandas/tests/frame/methods/test_combine_first.py

@@ -380,7 +380,7 @@ def test_combine_first_with_asymmetric_other(self, val):
         df2 = DataFrame({"isBool": [True]})
 
         res = df1.combine_first(df2)
-        exp = DataFrame({"isBool": [True], "isNum": [val]})
+        exp = DataFrame({"isNum": [val], "isBool": [True]})
 
         tm.assert_frame_equal(res, exp)
 
@@ -555,3 +555,13 @@ def test_combine_first_empty_columns():
     result = left.combine_first(right)
     expected = DataFrame(columns=["a", "b", "c"])
     tm.assert_frame_equal(result, expected)
+
+
+def test_combine_first_preserve_column_order():
+    # GH#60427
+    df1 = DataFrame({"B": [1, 2, 3], "A": [4, None, 6]})
+    df2 = DataFrame({"A": [5]}, index=[1])
+
+    result = df1.combine_first(df2)
+    expected = DataFrame({"B": [1, 2, 3], "A": [4.0, 5.0, 6.0]})
+    tm.assert_frame_equal(result, expected)