BUG: Let melt name multiple variable columns for labels from a MultiIndex (pandas-dev#58088)

* Let melt name variable columns for a multiindex

* Respond to comments

* Check is_iterator

Author: rob-sil

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/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/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):