ENH: Implement MultiIndex.insert_level for inserting levels at specified positions

doc/source/whatsnew/v3.0.0.rst

@@ -214,6 +214,7 @@ Other enhancements
 - :meth:`pandas.concat` will raise a ``ValueError`` when ``ignore_index=True`` and ``keys`` is not ``None`` (:issue:`59274`)
 - :py:class:`frozenset` elements in pandas objects are now natively printed (:issue:`60690`)
 - Add ``"delete_rows"`` option to ``if_exists`` argument in :meth:`DataFrame.to_sql` deleting all records of the table before inserting data (:issue:`37210`).
+- Added :meth:`MultiIndex.insert_level` to insert new levels at specified positions in a MultiIndex (:issue:`62558`)
 - Added half-year offset classes :class:`HalfYearBegin`, :class:`HalfYearEnd`, :class:`BHalfYearBegin` and :class:`BHalfYearEnd` (:issue:`60928`)
 - Added support for ``axis=1`` with ``dict`` or :class:`Series` arguments into :meth:`DataFrame.fillna` (:issue:`4514`)
 - Added support to read and write from and to Apache Iceberg tables with the new :func:`read_iceberg` and :meth:`DataFrame.to_iceberg` functions (:issue:`61383`)

pandas/core/indexes/multi.py

@@ -2710,6 +2710,60 @@ def reorder_levels(self, order) -> MultiIndex:
         result = self._reorder_ilevels(order)
         return result
 
+    def insert_level(self, position: int, value, name=None):
+        """
+        Insert a new level at the specified position in the MultiIndex.
+
+        Parameters
+        ----------
+        position : int
+            The position at which to insert the new level (0-based).
+        value : scalar or array-like
+            Value(s) to use for the new level. If scalar, broadcast to all items.
+            If array-like, length must match the length of the index.
+        name : object, optional
+            Name for the new level.
+
+        Returns
+        -------
+        MultiIndex
+            New MultiIndex with the inserted level.
+
+        Examples
+        --------
+        >>> idx = pd.MultiIndex.from_tuples([("A", 1), ("B", 2)])
+        >>> idx.insert_level(0, "new_value")
+        MultiIndex([('new_value', 'A', 1), ('new_value', 'B', 2)], ...)
+
+        >>> idx.insert_level(1, ["X", "Y"])
+        MultiIndex([('A', 'X', 1), ('B', 'Y', 2)], ...)
+
+        >>> idx.insert_level(0, "new_val", name="new_level")
+        MultiIndex([('new_val', 'A', 1), ('new_val', 'B', 2)], ...)
+        """
+        if not isinstance(position, int):
+            raise TypeError("position must be an integer")
+
+        if position < 0 or position > self.nlevels:
+            raise ValueError(f"position must be between 0 and {self.nlevels}")
+
+        if not hasattr(value, "__iter__") or isinstance(value, str):
+            value = [value] * len(self)
+        else:
+            value = list(value)
+            if len(value) != len(self):
+                raise ValueError("Length of values must match length of index")
+
+        new_tuples = []
+
+        for i, tup in enumerate(self):
+            new_tuple = tup[:position] + (value[i],) + tup[position:]
+            new_tuples.append(new_tuple)
+
+        new_names = self.names[:position] + [name] + self.names[position:]
+
+        return MultiIndex.from_tuples(new_tuples, names=new_names)
+
     def _reorder_ilevels(self, order) -> MultiIndex:
         if len(order) != self.nlevels:
             raise AssertionError(

pandas/tests/indexes/multi/test_constructors.py

@@ -870,3 +870,14 @@ def test_dtype_representation(using_infer_string):
         dtype=object,
     )
     tm.assert_series_equal(result, expected)
+
+
+def test_insert_level_integration():
+    idx = MultiIndex.from_tuples([("A", 1), ("B", 2)])
+
+    df = pd.DataFrame({"data": [10, 20]}, index=idx)
+    new_idx = idx.insert_level(0, "group1")
+    df_new = df.set_index(new_idx)
+
+    assert df_new.index.nlevels == 3
+    assert len(df_new) == 2

pandas/tests/indexes/multi/test_insert_level.py

@@ -0,0 +1,149 @@
+import pytest
+
+import pandas as pd
+import pandas._testing as tm
+
+
+class TestMultiIndexInsertLevel:
+    @pytest.mark.parametrize(
+        "position, value, name, expected_tuples, expected_names",
+        [
+            (
+                0,
+                "new_value",
+                None,
+                [("new_value", "A", 1), ("new_value", "B", 2), ("new_value", "C", 3)],
+                [None, "level1", "level2"],
+            ),
+            (
+                1,
+                "middle",
+                None,
+                [("A", "middle", 1), ("B", "middle", 2), ("C", "middle", 3)],
+                ["level1", None, "level2"],
+            ),
+            (
+                0,
+                "new_val",
+                "new_level",
+                [("new_val", "A", 1), ("new_val", "B", 2), ("new_val", "C", 3)],
+                ["new_level", "level1", "level2"],
+            ),
+            (
+                1,
+                "middle",
+                "custom_name",
+                [("A", "middle", 1), ("B", "middle", 2), ("C", "middle", 3)],
+                ["level1", "custom_name", "level2"],
+            ),
+            (
+                0,
+                "start",
+                None,
+                [("start", "A", 1), ("start", "B", 2), ("start", "C", 3)],
+                [None, "level1", "level2"],
+            ),
+            (
+                2,
+                "end",
+                None,
+                [("A", 1, "end"), ("B", 2, "end"), ("C", 3, "end")],
+                ["level1", "level2", None],
+            ),
+            (
+                1,
+                100,
+                None,
+                [("A", 100, 1), ("B", 100, 2), ("C", 100, 3)],
+                ["level1", None, "level2"],
+            ),
+            (
+                1,
+                1.5,
+                None,
+                [("A", 1.5, 1), ("B", 1.5, 2), ("C", 1.5, 3)],
+                ["level1", None, "level2"],
+            ),
+            (
+                1,
+                None,
+                None,
+                [("A", None, 1), ("B", None, 2), ("C", None, 3)],
+                ["level1", None, "level2"],
+            ),
+            (
+                1,
+                ["X", "Y", "Z"],
+                None,
+                [("A", "X", 1), ("B", "Y", 2), ("C", "Z", 3)],
+                ["level1", None, "level2"],
+            ),
+            (
+                0,
+                "",
+                "empty_string",
+                [("", "A", 1), ("", "B", 2), ("", "C", 3)],
+                ["empty_string", "level1", "level2"],
+            ),
+            (
+                1,
+                True,
+                None,
+                [("A", True, 1), ("B", True, 2), ("C", True, 3)],
+                ["level1", None, "level2"],
+            ),
+        ],
+    )
+    def test_insert_level_basic(
+        self, position, value, name, expected_tuples, expected_names
+    ):
+        simple_idx = pd.MultiIndex.from_tuples(
+            [("A", 1), ("B", 2), ("C", 3)], names=["level1", "level2"]
+        )
+
+        result = simple_idx.insert_level(position, value, name=name)
+        expected = pd.MultiIndex.from_tuples(expected_tuples, names=expected_names)
+        tm.assert_index_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "position, value, expected_error",
+        [
+            (5, "invalid", "position must be between"),
+            (-1, "invalid", "position must be between"),
+            (1, ["too", "few"], "Length of values must match"),
+            (3, "value", "position must be between"),
+        ],
+    )
+    def test_insert_level_error_cases(self, position, value, expected_error):
+        simple_idx = pd.MultiIndex.from_tuples(
+            [("A", 1), ("B", 2), ("C", 3)], names=["level1", "level2"]
+        )
+
+        with pytest.raises(ValueError, match=expected_error):
+            simple_idx.insert_level(position, value)
+
+    def test_insert_level_preserves_original(self):
+        simple_idx = pd.MultiIndex.from_tuples(
+            [("A", 1), ("B", 2), ("C", 3)], names=["level1", "level2"]
+        )
+
+        original = simple_idx.copy()
+        simple_idx.insert_level(1, "temp")
+
+        tm.assert_index_equal(original, simple_idx)
+
+    def test_insert_level_empty_index(self):
+        empty_idx = pd.MultiIndex.from_tuples([], names=["level1", "level2"])
+
+        result = empty_idx.insert_level(0, [])
+        expected = pd.MultiIndex.from_tuples([], names=[None, "level1", "level2"])
+        tm.assert_index_equal(result, expected)
+
+    def test_insert_level_single_element(self):
+        single_idx = pd.MultiIndex.from_tuples([("A", 1)], names=["level1", "level2"])
+
+        result = single_idx.insert_level(1, "middle")
+        expected = pd.MultiIndex.from_tuples(
+            [("A", "middle", 1)], names=["level1", None, "level2"]
+        )
+        tm.assert_index_equal(result, expected)