Merge nonstring columns

doc/source/whatsnew/v1.5.0.rst

@@ -154,6 +154,8 @@ Other enhancements
 - ``times`` argument in :class:`.ExponentialMovingWindow` now accepts ``np.timedelta64`` (:issue:`47003`)
 - :class:`DataError`, :class:`SpecificationError`, :class:`SettingWithCopyError`, :class:`SettingWithCopyWarning`, and :class:`NumExprClobberingError` are now exposed in ``pandas.errors`` (:issue:`27656`)
 - Added ``check_like`` argument to :func:`testing.assert_series_equal` (:issue:`47247`)
+- :func:`merge` and :meth:`DataFrame.merge` now allows passing ``None`` or ``(None, None)`` for ``suffixes`` argument, keeping column labels unchanged in the resulting :class:`DataFrame` potentially with duplicate column labels (:issue:`46885`)
+- :func:`DataFrame.join` now allows passing empty string for ``lsuffix`` and ``rsuffix`` arguments, keeping column labels unchanged in the resulting :class:`DataFrame` potentially with duplicate column labels (:issue:`46885`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.notable_bug_fixes:

pandas/_typing.py

@@ -111,7 +111,7 @@
 IndexLabel = Union[Hashable, Sequence[Hashable]]
 Level = Union[Hashable, int]
 Shape = Tuple[int, ...]
-Suffixes = Tuple[Optional[str], Optional[str]]
+Suffixes = Optional[Tuple[Optional[str], Optional[str]]]
 Ordered = Optional[bool]
 JSONSerializable = Optional[Union[PythonScalar, List, Dict]]
 Frequency = Union[str, "DateOffset"]

pandas/core/frame.py

@@ -322,13 +322,13 @@
 sort : bool, default False
     Sort the join keys lexicographically in the result DataFrame. If False,
     the order of the join keys depends on the join type (how keyword).
-suffixes : list-like, default is ("_x", "_y")
-    A length-2 sequence where each element is optionally a string
+suffixes : optional list-like, default is ("_x", "_y")
+    An optional length-2 sequence where each element is optionally a string
     indicating the suffix to add to overlapping column names in
     `left` and `right` respectively. Pass a value of `None` instead
     of a string to indicate that the column name from `left` or
-    `right` should be left as-is, with no suffix. At least one of the
-    values must not be None.
+    `right` should be left as-is, with no suffix. Pass `None` to keep
+    both columns labels as-is.
 copy : bool, default True
     If False, avoid copy if possible.
 indicator : bool or str, default False
@@ -412,14 +412,17 @@
 4  bar           2  bar            6
 5  baz           3  baz            7
 
-Merge DataFrames df1 and df2, but raise an exception if the DataFrames have
-any overlapping columns.
-
->>> df1.merge(df2, left_on='lkey', right_on='rkey', suffixes=(False, False))
-Traceback (most recent call last):
-...
-ValueError: columns overlap but no suffix specified:
-    Index(['value'], dtype='object')
+Merge DataFrames df1 and df2 with null as suffix will keep
+the original columns names
+
+>>> df1.merge(df2, left_on='lkey', right_on='rkey', suffixes=None)
+  lkey  value rkey  value
+0  foo  1     foo   5
+1  foo  1     foo   8
+2  foo  5     foo   5
+3  foo  5     foo   8
+4  bar  2     bar   6
+5  baz  3     baz   7
 
 >>> df1 = pd.DataFrame({'a': ['foo', 'bar'], 'b': [1, 2]})
 >>> df2 = pd.DataFrame({'a': ['foo', 'baz'], 'c': [3, 4]})

pandas/core/reshape/merge.py

@@ -2359,7 +2359,7 @@ def _items_overlap_with_suffix(
     If corresponding suffix is empty, the entry is simply converted to string.
 
     """
-    if not is_list_like(suffixes, allow_sets=False):
+    if not (is_list_like(suffixes, allow_sets=False) or suffixes is None):
         warnings.warn(
             f"Passing 'suffixes' as a {type(suffixes)}, is not supported and may give "
             "unexpected results. Provide 'suffixes' as a tuple instead. In the "
@@ -2372,10 +2372,7 @@ def _items_overlap_with_suffix(
     if len(to_rename) == 0:
         return left, right
 
-    lsuffix, rsuffix = suffixes
-
-    if not lsuffix and not rsuffix:
-        raise ValueError(f"columns overlap but no suffix specified: {to_rename}")
+    lsuffix, rsuffix = suffixes if suffixes else (None, None)
 
     def renamer(x, suffix):
         """
@@ -2386,12 +2383,12 @@ def renamer(x, suffix):
 
         Parameters
         ----------
-        x : original column name
+        x : original column label
         suffix : str or None
 
         Returns
         -------
-        x : renamed column name
+        x : renamed column label
         """
         if x in to_rename and suffix is not None:
             return f"{x}{suffix}"

pandas/tests/frame/methods/test_join.py

@@ -280,12 +280,6 @@ def test_join_index(float_frame):
     with pytest.raises(ValueError, match="join method"):
         f.join(f2, how="foo")
 
-    # corner case - overlapping columns
-    msg = "columns overlap but no suffix"
-    for how in ("outer", "left", "inner"):
-        with pytest.raises(ValueError, match=msg):
-            float_frame.join(float_frame, how=how)
-
 
 def test_join_index_more(float_frame):
     af = float_frame.loc[:, ["A", "B"]]

pandas/tests/reshape/merge/test_merge.py

@@ -2209,6 +2209,10 @@ def test_merge_series(on, left_on, right_on, left_index, right_index, nm):
         (0, 0, {"suffixes": ("_a", None)}, ["0_a", 0]),
         ("a", "a", {}, ["a_x", "a_y"]),
         (0, 0, {}, ["0_x", "0_y"]),
+        (0, 0, {"suffixes": None}, [0, 0]),
+        (0, 0, {"suffixes": (None, None)}, [0, 0]),
+        ("a", "a", {"suffixes": None}, ["a", "a"]),
+        ("a", "a", {"suffixes": (None, None)}, ["a", "a"]),
     ],
 )
 def test_merge_suffix(col1, col2, kwargs, expected_cols):
@@ -2255,21 +2259,6 @@ def test_merge_duplicate_suffix(how, expected):
     tm.assert_frame_equal(result, expected)
 
 
-@pytest.mark.parametrize(
-    "col1, col2, suffixes",
-    [("a", "a", (None, None)), ("a", "a", ("", None)), (0, 0, (None, ""))],
-)
-def test_merge_suffix_error(col1, col2, suffixes):
-    # issue: 24782
-    a = DataFrame({col1: [1, 2, 3]})
-    b = DataFrame({col2: [3, 4, 5]})
-
-    # TODO: might reconsider current raise behaviour, see issue 24782
-    msg = "columns overlap but no suffix specified"
-    with pytest.raises(ValueError, match=msg):
-        merge(a, b, left_index=True, right_index=True, suffixes=suffixes)
-
-
 @pytest.mark.parametrize("suffixes", [{"left", "right"}, {"left": 0, "right": 0}])
 def test_merge_suffix_warns(suffixes):
     a = DataFrame({"a": [1, 2, 3]})

pandas/tests/reshape/merge/test_multi.py

@@ -636,12 +636,6 @@ def test_join_multi_levels_invalid(self, portfolio, household):
         ):
             household.join(portfolio, how="inner")
 
-        portfolio2 = portfolio.copy()
-        portfolio2.index.set_names(["household_id", "foo"])
-
-        with pytest.raises(ValueError, match="columns overlap but no suffix specified"):
-            portfolio2.join(portfolio, how="inner")
-
     def test_join_multi_levels2(self):
 
         # some more advanced merges