Merge branch 'pandas-dev:main' into Fix#58748

doc/source/development/maintaining.rst

@@ -84,7 +84,7 @@ Here's a typical workflow for triaging a newly opened issue.
    example. See https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
    for a good explanation. If the example is not reproducible, or if it's
    *clearly* not minimal, feel free to ask the reporter if they can provide
-   and example or simplify the provided one. Do acknowledge that writing
+   an example or simplify the provided one. Do acknowledge that writing
    minimal reproducible examples is hard work. If the reporter is struggling,
    you can try to write one yourself and we'll edit the original post to include it.
 
@@ -93,6 +93,9 @@ Here's a typical workflow for triaging a newly opened issue.
    If a reproducible example is provided, but you see a simplification,
    edit the original post with your simpler reproducible example.
 
+   If this is a regression report, post the result of a ``git bisect`` run.
+   More info on this can be found in the :ref:`maintaining.regressions` section.
+
    Ensure the issue exists on the main branch and that it has the "Needs Triage" tag
    until all steps have been completed. Add a comment to the issue once you have
    verified it exists on the main branch, so others know it has been confirmed.
@@ -125,7 +128,10 @@ Here's a typical workflow for triaging a newly opened issue.
    If the issue is clearly defined and the fix seems relatively straightforward,
    label the issue as "Good first issue".
 
-   Once you have completed the above, make sure to remove the "needs triage" label.
+   If the issue is a regression report, add the "Regression" label and the next patch
+   release milestone.
+
+   Once you have completed the above, make sure to remove the "Needs Triage" label.
 
 .. _maintaining.regressions:
 

doc/source/whatsnew/v3.0.0.rst

@@ -440,6 +440,7 @@ Missing
 MultiIndex
 ^^^^^^^^^^
 - :func:`DataFrame.loc` with ``axis=0``  and :class:`MultiIndex` when setting a value adds extra columns (:issue:`58116`)
+- :meth:`DataFrame.melt` would not accept multiple names in ``var_name`` when the columns were a :class:`MultiIndex` (:issue:`58033`)
 -
 
 I/O

pandas/core/apply.py

@@ -664,7 +664,7 @@ def _apply_str(self, obj, func: str, *args, **kwargs):
             # people may aggregate on a non-callable attribute
             # but don't let them think they can pass args to it
             assert len(args) == 0
-            assert len([kwarg for kwarg in kwargs if kwarg not in ["axis"]]) == 0
+            assert not any(kwarg == "axis" for kwarg in kwargs)
             return f
         elif hasattr(np, func) and hasattr(obj, "__array__"):
             # in particular exclude Window

pandas/core/computation/eval.py

@@ -193,6 +193,8 @@ def eval(
     corresponding bitwise operators.  :class:`~pandas.Series` and
     :class:`~pandas.DataFrame` objects are supported and behave as they would
     with plain ol' Python evaluation.
+    `eval` can run arbitrary code which can make you vulnerable to code
+    injection if you pass user input to this function.
 
     Parameters
     ----------

pandas/core/frame.py

@@ -4472,6 +4472,9 @@ def query(self, expr: str, *, inplace: bool = False, **kwargs) -> DataFrame | No
         """
         Query the columns of a DataFrame with a boolean expression.
 
+        This method can run arbitrary code which can make you vulnerable to code
+        injection if you pass user input to this function.
+
         Parameters
         ----------
         expr : str

pandas/core/generic.py

@@ -1750,19 +1750,25 @@ def _get_label_or_level_values(self, key: Level, axis: AxisInt = 0) -> ArrayLike
             if `key` matches multiple labels
         """
         axis = self._get_axis_number(axis)
-        other_axes = [ax for ax in range(self._AXIS_LEN) if ax != axis]
+        first_other_axes = next(
+            (ax for ax in range(self._AXIS_LEN) if ax != axis), None
+        )
 
         if self._is_label_reference(key, axis=axis):
             self._check_label_or_level_ambiguity(key, axis=axis)
-            values = self.xs(key, axis=other_axes[0])._values
+            if first_other_axes is None:
+                raise ValueError("axis matched all axes")
+            values = self.xs(key, axis=first_other_axes)._values
         elif self._is_level_reference(key, axis=axis):
             values = self.axes[axis].get_level_values(key)._values
         else:
             raise KeyError(key)
 
         # Check for duplicates
         if values.ndim > 1:
-            if other_axes and isinstance(self._get_axis(other_axes[0]), MultiIndex):
+            if first_other_axes is not None and isinstance(
+                self._get_axis(first_other_axes), MultiIndex
+            ):
                 multi_message = (
                     "\n"
                     "For a multi-index, the label must be a "

pandas/core/reshape/melt.py

@@ -5,7 +5,10 @@
 
 import numpy as np
 
-from pandas.core.dtypes.common import is_list_like
+from pandas.core.dtypes.common import (
+    is_iterator,
+    is_list_like,
+)
 from pandas.core.dtypes.concat import concat_compat
 from pandas.core.dtypes.missing import notna
 
@@ -64,9 +67,10 @@ def melt(
     value_vars : scalar, tuple, list, or ndarray, optional
         Column(s) to unpivot. If not specified, uses all columns that
         are not set as `id_vars`.
-    var_name : scalar, default None
+    var_name : scalar, tuple, list, or ndarray, optional
         Name to use for the 'variable' column. If None it uses
-        ``frame.columns.name`` or 'variable'.
+        ``frame.columns.name`` or 'variable'. Must be a scalar if columns are a
+        MultiIndex.
     value_name : scalar, default 'value'
         Name to use for the 'value' column, can't be an existing column label.
     col_level : scalar, optional
@@ -217,7 +221,16 @@ def melt(
                 frame.columns.name if frame.columns.name is not None else "variable"
             ]
     elif is_list_like(var_name):
-        raise ValueError(f"{var_name=} must be a scalar.")
+        if isinstance(frame.columns, MultiIndex):
+            if is_iterator(var_name):
+                var_name = list(var_name)
+            if len(var_name) > len(frame.columns):
+                raise ValueError(
+                    f"{var_name=} has {len(var_name)} items, "
+                    f"but the dataframe columns only have {len(frame.columns)} levels."
+                )
+        else:
+            raise ValueError(f"{var_name=} must be a scalar.")
     else:
         var_name = [var_name]
 

pandas/io/excel/_base.py

@@ -857,24 +857,23 @@ def _parse_sheet(
         # a row containing just the index name(s)
         has_index_names = False
         if is_list_header and not is_len_one_list_header and index_col is not None:
-            index_col_list: Sequence[int]
+            index_col_set: set[int]
             if isinstance(index_col, int):
-                index_col_list = [index_col]
+                index_col_set = {index_col}
             else:
                 assert isinstance(index_col, Sequence)
-                index_col_list = index_col
+                index_col_set = set(index_col)
 
             # We have to handle mi without names. If any of the entries in the data
             # columns are not empty, this is a regular row
             assert isinstance(header, Sequence)
             if len(header) < len(data):
                 potential_index_names = data[len(header)]
-                potential_data = [
-                    x
+                has_index_names = all(
+                    x == "" or x is None
                     for i, x in enumerate(potential_index_names)
-                    if not control_row[i] and i not in index_col_list
-                ]
-                has_index_names = all(x == "" or x is None for x in potential_data)
+                    if not control_row[i] and i not in index_col_set
+                )
 
         if is_list_like(index_col):
             # Forward fill values for MultiIndex index.
@@ -1457,9 +1456,9 @@ def inspect_excel_format(
         with zipfile.ZipFile(stream) as zf:
             # Workaround for some third party files that use forward slashes and
             # lower case names.
-            component_names = [
+            component_names = {
                 name.replace("\\", "/").lower() for name in zf.namelist()
-            ]
+            }
 
         if "xl/workbook.xml" in component_names:
             return "xlsx"

pandas/io/excel/_odfreader.py

@@ -122,29 +122,25 @@ def get_sheet_data(
         table: list[list[Scalar | NaTType]] = []
 
         for sheet_row in sheet_rows:
-            sheet_cells = [
-                x
-                for x in sheet_row.childNodes
-                if hasattr(x, "qname") and x.qname in cell_names
-            ]
             empty_cells = 0
             table_row: list[Scalar | NaTType] = []
 
-            for sheet_cell in sheet_cells:
-                if sheet_cell.qname == table_cell_name:
-                    value = self._get_cell_value(sheet_cell)
-                else:
-                    value = self.empty_value
-
-                column_repeat = self._get_column_repeat(sheet_cell)
-
-                # Queue up empty values, writing only if content succeeds them
-                if value == self.empty_value:
-                    empty_cells += column_repeat
-                else:
-                    table_row.extend([self.empty_value] * empty_cells)
-                    empty_cells = 0
-                    table_row.extend([value] * column_repeat)
+            for sheet_cell in sheet_row.childNodes:
+                if hasattr(sheet_cell, "qname") and sheet_cell.qname in cell_names:
+                    if sheet_cell.qname == table_cell_name:
+                        value = self._get_cell_value(sheet_cell)
+                    else:
+                        value = self.empty_value
+
+                    column_repeat = self._get_column_repeat(sheet_cell)
+
+                    # Queue up empty values, writing only if content succeeds them
+                    if value == self.empty_value:
+                        empty_cells += column_repeat
+                    else:
+                        table_row.extend([self.empty_value] * empty_cells)
+                        empty_cells = 0
+                        table_row.extend([value] * column_repeat)
 
             if max_row_len < len(table_row):
                 max_row_len = len(table_row)

pandas/io/excel/_xlrd.py

@@ -128,16 +128,13 @@ def _parse_cell(cell_contents, cell_typ):
                         cell_contents = val
             return cell_contents
 
-        data = []
-
         nrows = sheet.nrows
         if file_rows_needed is not None:
             nrows = min(nrows, file_rows_needed)
-        for i in range(nrows):
-            row = [
+        return [
+            [
                 _parse_cell(value, typ)
                 for value, typ in zip(sheet.row_values(i), sheet.row_types(i))
             ]
-            data.append(row)
-
-        return data
+            for i in range(nrows)
+        ]

pandas/io/sql.py

@@ -157,6 +157,7 @@ def _convert_arrays_to_dataframe(
     dtype_backend: DtypeBackend | Literal["numpy"] = "numpy",
 ) -> DataFrame:
     content = lib.to_object_array_tuples(data)
+    idx_len = content.shape[0]
     arrays = convert_object_array(
         list(content.T),
         dtype=None,
@@ -177,9 +178,9 @@ def _convert_arrays_to_dataframe(
             result_arrays.append(ArrowExtensionArray(pa_array))
         arrays = result_arrays  # type: ignore[assignment]
     if arrays:
-        df = DataFrame(dict(zip(range(len(columns)), arrays)))
-        df.columns = columns
-        return df
+        return DataFrame._from_arrays(
+            arrays, columns=columns, index=range(idx_len), verify_integrity=False
+        )
     else:
         return DataFrame(columns=columns)
 

pandas/tests/reshape/test_melt.py

@@ -533,6 +533,26 @@ def test_melt_non_scalar_var_name_raises(self):
         with pytest.raises(ValueError, match=r".* must be a scalar."):
             df.melt(id_vars=["a"], var_name=[1, 2])
 
+    def test_melt_multiindex_columns_var_name(self):
+        # GH 58033
+        df = DataFrame({("A", "a"): [1], ("A", "b"): [2]})
+
+        expected = DataFrame(
+            [("A", "a", 1), ("A", "b", 2)], columns=["first", "second", "value"]
+        )
+
+        tm.assert_frame_equal(df.melt(var_name=["first", "second"]), expected)
+        tm.assert_frame_equal(df.melt(var_name=["first"]), expected[["first", "value"]])
+
+    def test_melt_multiindex_columns_var_name_too_many(self):
+        # GH 58033
+        df = DataFrame({("A", "a"): [1], ("A", "b"): [2]})
+
+        with pytest.raises(
+            ValueError, match="but the dataframe columns only have 2 levels"
+        ):
+            df.melt(var_name=["first", "second", "third"])
+
 
 class TestLreshape:
     def test_pairs(self):