DEPR: Enforce disallowed merging scenarios

doc/source/whatsnew/v2.0.0.rst

@@ -269,6 +269,9 @@ Removal of prior version deprecations/changes
 - Enforced disallowing a string column label into ``times`` in :meth:`DataFrame.ewm` (:issue:`43265`)
 - Enforced disallowing a tuple of column labels into :meth:`.DataFrameGroupBy.__getitem__` (:issue:`30546`)
 - Enforced disallowing setting values with ``.loc`` using a positional slice. Use ``.loc`` with labels or ``.iloc`` with positions instead (:issue:`31840`)
+- Enforced disallowing ``dict`` or ``set`` objects in ``suffixes`` in :func:`merge` (:issue:`34810`)
+- Enforced disallowing :func:`merge` to produce duplicated columns through the ``suffixes`` keyword and already existing columns (:issue:`22818`)
+- Enforced disallowing using :func:`merge` or :func:`join` on a different number of levels (:issue:`34862`)
 - Removed setting Categorical._codes directly (:issue:`41429`)
 - Removed setting Categorical.categories directly (:issue:`47834`)
 - Removed argument ``inplace`` from :meth:`Categorical.add_categories`, :meth:`Categorical.remove_categories`, :meth:`Categorical.set_categories`, :meth:`Categorical.rename_categories`, :meth:`Categorical.reorder_categories`, :meth:`Categorical.set_ordered`, :meth:`Categorical.as_ordered`, :meth:`Categorical.as_unordered` (:issue:`37981`, :issue:`41118`, :issue:`41133`, :issue:`47834`)

pandas/core/reshape/merge.py

@@ -676,16 +676,14 @@ def __init__(
                 f"right_index parameter must be of type bool, not {type(right_index)}"
             )
 
-        # warn user when merging between different levels
+        # GH 40993: raise when merging between different levels; enforced in 2.0
         if _left.columns.nlevels != _right.columns.nlevels:
             msg = (
-                "merging between different levels is deprecated and will be removed "
-                f"in a future version. ({_left.columns.nlevels} levels on the left, "
+                "Not allowed to merge between different levels. "
+                f"({_left.columns.nlevels} levels on the left, "
                 f"{_right.columns.nlevels} on the right)"
             )
-            # stacklevel chosen to be correct when this is reached via pd.merge
-            # (and not DataFrame.join)
-            warnings.warn(msg, FutureWarning, stacklevel=find_stack_level())
+            raise MergeError(msg)
 
         self.left_on, self.right_on = self._validate_left_right_on(left_on, right_on)
 
@@ -2475,13 +2473,10 @@ 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):
-        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 "
-            "future a 'TypeError' will be raised.",
-            FutureWarning,
-            stacklevel=find_stack_level(),
+    if not is_list_like(suffixes, allow_sets=False) or isinstance(suffixes, dict):
+        raise TypeError(
+            f"Passing 'suffixes' as a {type(suffixes)}, is not supported. "
+            "Provide 'suffixes' as a tuple instead."
         )
 
     to_rename = left.intersection(right)
@@ -2527,11 +2522,9 @@ def renamer(x, suffix):
     if not rlabels.is_unique:
         dups.extend(rlabels[(rlabels.duplicated()) & (~right.duplicated())].tolist())
     if dups:
-        warnings.warn(
-            f"Passing 'suffixes' which cause duplicate columns {set(dups)} in the "
-            f"result is deprecated and will raise a MergeError in a future version.",
-            FutureWarning,
-            stacklevel=find_stack_level(),
+        raise MergeError(
+            f"Passing 'suffixes' which cause duplicate columns {set(dups)} is "
+            f"not allowed.",
         )
 
     return llabels, rlabels

pandas/tests/frame/methods/test_join.py

@@ -516,8 +516,9 @@ def test_join_multiindex_dates(self):
 
         tm.assert_equal(result, expected)
 
-    def test_merge_join_different_levels(self):
+    def test_merge_join_different_levels_raises(self):
         # GH#9455
+        # GH 40993: For raising, enforced in 2.0
 
         # first dataframe
         df1 = DataFrame(columns=["a", "b"], data=[[1, 11], [0, 22]])
@@ -527,20 +528,16 @@ def test_merge_join_different_levels(self):
         df2 = DataFrame(columns=columns, data=[[1, 33], [0, 44]])
 
         # merge
-        columns = ["a", "b", ("c", "c1")]
-        expected = DataFrame(columns=columns, data=[[1, 11, 33], [0, 22, 44]])
-        with tm.assert_produces_warning(FutureWarning):
-            result = pd.merge(df1, df2, on="a")
-        tm.assert_frame_equal(result, expected)
+        with pytest.raises(
+            MergeError, match="Not allowed to merge between different levels"
+        ):
+            pd.merge(df1, df2, on="a")
 
         # join, see discussion in GH#12219
-        columns = ["a", "b", ("a", ""), ("c", "c1")]
-        expected = DataFrame(columns=columns, data=[[1, 11, 0, 44], [0, 22, 1, 33]])
-        msg = "merging between different levels is deprecated"
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            # stacklevel is chosen to be correct for pd.merge, not DataFrame.join
-            result = df1.join(df2, on="a")
-        tm.assert_frame_equal(result, expected)
+        with pytest.raises(
+            MergeError, match="Not allowed to merge between different levels"
+        ):
+            df1.join(df2, on="a")
 
     def test_frame_join_tzaware(self):
         test1 = DataFrame(

pandas/tests/reshape/merge/test_join.py

@@ -420,18 +420,18 @@ def test_join_inner_multiindex(self, lexsorted_two_level_string_multiindex):
 
         # _assert_same_contents(expected, expected2.loc[:, expected.columns])
 
-    def test_join_hierarchical_mixed(self):
+    def test_join_hierarchical_mixed_raises(self):
         # GH 2024
+        # GH 40993: For raising, enforced in 2.0
         df = DataFrame([(1, 2, 3), (4, 5, 6)], columns=["a", "b", "c"])
         new_df = df.groupby(["a"]).agg({"b": [np.mean, np.sum]})
         other_df = DataFrame([(1, 2, 3), (7, 10, 6)], columns=["a", "b", "d"])
         other_df.set_index("a", inplace=True)
         # GH 9455, 12219
-        msg = "merging between different levels is deprecated"
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            result = merge(new_df, other_df, left_index=True, right_index=True)
-        assert ("b", "mean") in result
-        assert "b" in result
+        with pytest.raises(
+            pd.errors.MergeError, match="Not allowed to merge between different levels"
+        ):
+            merge(new_df, other_df, left_index=True, right_index=True)
 
     def test_join_float64_float32(self):
 
@@ -642,11 +642,12 @@ def test_join_dups(self):
         dta = x.merge(y, left_index=True, right_index=True).merge(
             z, left_index=True, right_index=True, how="outer"
         )
-        with tm.assert_produces_warning(FutureWarning):
-            dta = dta.merge(w, left_index=True, right_index=True)
-        expected = concat([x, y, z, w], axis=1)
-        expected.columns = ["x_x", "y_x", "x_y", "y_y", "x_x", "y_x", "x_y", "y_y"]
-        tm.assert_frame_equal(dta, expected)
+        # GH 40991: As of 2.0 causes duplicate columns
+        with pytest.raises(
+            pd.errors.MergeError,
+            match="Passing 'suffixes' which cause duplicate columns",
+        ):
+            dta.merge(w, left_index=True, right_index=True)
 
     def test_join_multi_to_multi(self, join_type):
         # GH 20475

pandas/tests/reshape/merge/test_merge.py

@@ -2303,12 +2303,12 @@ def test_merge_suffix_error(col1, col2, suffixes):
 
 
 @pytest.mark.parametrize("suffixes", [{"left", "right"}, {"left": 0, "right": 0}])
-def test_merge_suffix_warns(suffixes):
+def test_merge_suffix_raises(suffixes):
     a = DataFrame({"a": [1, 2, 3]})
     b = DataFrame({"b": [3, 4, 5]})
 
-    with tm.assert_produces_warning(FutureWarning):
-        merge(a, b, left_index=True, right_index=True, suffixes={"left", "right"})
+    with pytest.raises(TypeError, match="Passing 'suffixes' as a"):
+        merge(a, b, left_index=True, right_index=True, suffixes=suffixes)
 
 
 @pytest.mark.parametrize(
@@ -2609,20 +2609,16 @@ def test_merge_result_empty_index_and_on():
     tm.assert_frame_equal(result, expected)
 
 
-def test_merge_suffixes_produce_dup_columns_warns():
-    # GH#22818
+def test_merge_suffixes_produce_dup_columns_raises():
+    # GH#22818; Enforced in 2.0
     left = DataFrame({"a": [1, 2, 3], "b": 1, "b_x": 2})
     right = DataFrame({"a": [1, 2, 3], "b": 2})
-    expected = DataFrame(
-        [[1, 1, 2, 2], [2, 1, 2, 2], [3, 1, 2, 2]], columns=["a", "b_x", "b_x", "b_y"]
-    )
-    with tm.assert_produces_warning(FutureWarning):
-        result = merge(left, right, on="a")
-    tm.assert_frame_equal(result, expected)
 
-    with tm.assert_produces_warning(FutureWarning):
+    with pytest.raises(MergeError, match="Passing 'suffixes' which cause duplicate"):
+        merge(left, right, on="a")
+
+    with pytest.raises(MergeError, match="Passing 'suffixes' which cause duplicate"):
         merge(right, left, on="a", suffixes=("_y", "_x"))
-    tm.assert_frame_equal(result, expected)
 
 
 def test_merge_duplicate_columns_with_suffix_no_warning():
@@ -2635,15 +2631,13 @@ def test_merge_duplicate_columns_with_suffix_no_warning():
     tm.assert_frame_equal(result, expected)
 
 
-def test_merge_duplicate_columns_with_suffix_causing_another_duplicate():
-    # GH#22818
+def test_merge_duplicate_columns_with_suffix_causing_another_duplicate_raises():
+    # GH#22818, Enforced in 2.0
     # This should raise warning because suffixes cause another collision
     left = DataFrame([[1, 1, 1, 1], [2, 2, 2, 2]], columns=["a", "b", "b", "b_x"])
     right = DataFrame({"a": [1, 3], "b": 2})
-    with tm.assert_produces_warning(FutureWarning):
-        result = merge(left, right, on="a")
-    expected = DataFrame([[1, 1, 1, 1, 2]], columns=["a", "b_x", "b_x", "b_x", "b_y"])
-    tm.assert_frame_equal(result, expected)
+    with pytest.raises(MergeError, match="Passing 'suffixes' which cause duplicate"):
+        merge(left, right, on="a")
 
 
 def test_merge_string_float_column_result():