Keep join columns untouched

Author: kopytjuk

pandas/core/reshape/merge.py

@@ -1095,8 +1095,12 @@ def _reindex_and_concat(
         left = self.left[:]
         right = self.right[:]
 
+        keep_left = [x for x in self.left_on if isinstance(x, str)]
+        keep_right = [x for x in self.right_on if isinstance(x, str)]
+        
         llabels, rlabels = _items_overlap_with_suffix(
-            self.left._info_axis, self.right._info_axis, self.suffixes, self.force_suffixes
+            self.left._info_axis, self.right._info_axis, self.suffixes,
+            self.force_suffixes, keep_left, keep_right
         )
 
         if left_indexer is not None and not is_range_indexer(left_indexer, len(left)):
@@ -3020,7 +3024,8 @@ def _validate_operand(obj: DataFrame | Series) -> DataFrame:
 
 
 def _items_overlap_with_suffix(
-    left: Index, right: Index, suffixes: Suffixes, force_suffixes: bool = False
+    left: Index, right: Index, suffixes: Suffixes, force_suffixes: bool,
+    keep_left: list, keep_right: list
 ) -> tuple[Index, Index]:
     """
     Suffixes type validation.
@@ -3035,11 +3040,12 @@ def _items_overlap_with_suffix(
             f"Passing 'suffixes' as a {type(suffixes)}, is not supported. "
             "Provide 'suffixes' as a tuple instead."
         )
-
-    if not force_suffixes:
-        to_rename = left.intersection(right)
-    else:
+    
+    if force_suffixes:
         to_rename = left.union(right)
+    else:
+        to_rename = left.intersection(right)
+        keep_left, keep_right = [], []
 
     if len(to_rename) == 0:
         return left, right
@@ -3049,7 +3055,7 @@ def _items_overlap_with_suffix(
     if not lsuffix and not rsuffix:
         raise ValueError(f"columns overlap but no suffix specified: {to_rename}")
 
-    def renamer(x, suffix: str | None):
+    def renamer(x, suffix: str | None, keep: list):
         """
         Rename the left and right indices.
 
@@ -3065,12 +3071,12 @@ def renamer(x, suffix: str | None):
         -------
         x : renamed column name
         """
-        if x in to_rename and suffix is not None:
+        if x in to_rename and suffix is not None and (x not in keep):
             return f"{x}{suffix}"
         return x
 
-    lrenamer = partial(renamer, suffix=lsuffix)
-    rrenamer = partial(renamer, suffix=rsuffix)
+    lrenamer = partial(renamer, suffix=lsuffix, keep=keep_left)
+    rrenamer = partial(renamer, suffix=rsuffix, keep=keep_right)
 
     llabels = left._transform_index(lrenamer)
     rlabels = right._transform_index(rrenamer)

pandas/tests/reshape/merge/test_merge.py

@@ -2382,8 +2382,8 @@ def test_merge_suffix_with_force_simple(force_suffixes):
                 })
 
     if force_suffixes:
-        expected = DataFrame([[2, 2, "B", "D"], [3, 3, "C", "E"]],
-                             columns=["ID_left", "Value_left", "ID_right", "Value_right"])
+        expected = DataFrame([[2, "B", "D"], [3, "C", "E"]],
+                             columns=["ID", "Value_left", "Value_right"])
     else:
         expected = DataFrame([[2, "B", "D"], [3, "C", "E"]],
                              columns=["ID", "Value_left", "Value_right"])
@@ -2399,7 +2399,7 @@ def test_merge_suffix_with_force_multi_column(force_suffixes):
 
     if force_suffixes:
         expected = DataFrame([[1, 4, "A", 1, 4, "a"], [2, 5, "B", 2, 5, "b"], [3, 6, "C", 3, 6, "c"]],
-                             columns=["A_x", "B_x", "ALPHABET_x", "a_y", "b_y", "alphabet_y"])
+                             columns=["A", "B", "ALPHABET_x", "a", "b", "alphabet_y"])
     else:
         expected = DataFrame([[1, 4, "A", 1, 4, "a"], [2, 5, "B", 2, 5, "b"], [3, 6, "C", 3, 6, "c"]],
                              columns=["A", "B", "ALPHABET", "a", "b", "alphabet"])