fix suffix with none

Author: charlesdong1991

pandas/core/reshape/merge.py

@@ -488,7 +488,15 @@ def __init__(self, left, right, how='inner', on=None,
         self.right_on = com.maybe_make_list(right_on)
 
         self.copy = copy
+
+        if suffixes == (None, None):
+            raise ValueError('Column names cannot be identical after merging.')
+        if suffixes[0] is None:
+            suffixes = ('', suffixes[1])
+        elif suffixes[1] is None:
+            suffixes = (suffixes[0], '')
         self.suffixes = suffixes
+
         self.sort = sort
 
         self.left_index = left_index

pandas/tests/reshape/merge/test_merge.py

@@ -22,7 +22,7 @@
 from pandas.core.reshape.concat import concat
 from pandas.core.reshape.merge import MergeError, merge
 import pandas.util.testing as tm
-from pandas.util.testing import assert_frame_equal, assert_series_equal
+from pandas.util.testing import assert_frame_equal, assert_series_equal, assert_index_equal
 
 N = 50
 NGROUPS = 8
@@ -1442,3 +1442,20 @@ def test_merge_series(on, left_on, right_on, left_index, right_index, nm):
         with pytest.raises(ValueError, match='a Series without a name'):
             result = pd.merge(a, b, on=on, left_on=left_on, right_on=right_on,
                               left_index=left_index, right_index=right_index)
+
+
+@pytest.mark.parametrize("col1, col2, suffixes, expected_cols", [
+    (0, 0, ("", "_dup"), ["0", "0_dup"]),
+    (0, 0, (None, "_dup"), ["0", "0_dup"]),
+    (0, 0, ("_x", "_y"), ["0_x", "0_y"]),
+    ("a", 0, (None, "_y"), ["a", 0])
+])
+def test_merge_suffix(col1, col2, suffixes, expected_cols):
+    a = pd.DataFrame({col1: [1, 2, 3]})
+    b = pd.DataFrame({col2: [4, 5, 6]})
+
+    suffix_df = a.merge(b, left_index=True, right_index=True, suffixes=suffixes)
+    assert suffix_df.columns.tolist() == expected_cols
+
+    with pytest.raises(ValueError):
+        a.merge(b, left_index=True, right_index=True, suffixes=(None, None))