Backport PR #48058 on branch 1.4.x (REGR: fix reset_index (Index.insert) regression with custom Index subclasses)

doc/source/whatsnew/v1.4.4.rst

@@ -22,6 +22,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.loc` not aligning index in some cases when setting a :class:`DataFrame` (:issue:`47578`)
 - Fixed regression in :meth:`DataFrame.loc` setting a length-1 array like value to a single value in the DataFrame (:issue:`46268`)
 - Fixed regression in setting ``None`` or non-string value into a ``string``-dtype Series using a mask (:issue:`47628`)
+- Fixed regression using custom Index subclasses (for example, used in xarray) with :meth:`~DataFrame.reset_index` or :meth:`Index.insert` (:issue:`47071`)
 - Fixed regression in :meth:`DatetimeIndex.intersection` when the :class:`DatetimeIndex` has dates crossing daylight savings time (:issue:`46702`)
 - Fixed regression in :func:`merge` throwing an error when passing a :class:`Series` with a multi-level name (:issue:`47946`)
 - Fixed regression in :meth:`DataFrame.eval` creating a copy when updating inplace (:issue:`47449`)

pandas/core/indexes/base.py

@@ -6614,9 +6614,12 @@ def insert(self, loc: int, item) -> Index:
             loc = loc if loc >= 0 else loc - 1
             new_values[loc] = item
 
-        # Use self._constructor instead of Index to retain NumericIndex GH#43921
-        # TODO(2.0) can use Index instead of self._constructor
-        return self._constructor._with_infer(new_values, name=self.name)
+        if self._typ == "numericindex":
+            # Use self._constructor instead of Index to retain NumericIndex GH#43921
+            # TODO(2.0) can use Index instead of self._constructor
+            return self._constructor._with_infer(new_values, name=self.name)
+        else:
+            return Index._with_infer(new_values, name=self.name)
 
     def drop(self, labels, errors: str_t = "raise") -> Index:
         """

pandas/tests/indexes/test_subclass.py

@@ -0,0 +1,38 @@
+"""
+Tests involving custom Index subclasses
+"""
+import numpy as np
+
+from pandas import (
+    DataFrame,
+    Index,
+)
+import pandas._testing as tm
+
+
+class CustomIndex(Index):
+    def __new__(cls, data, name=None):
+        # assert that this index class cannot hold strings
+        if any(isinstance(val, str) for val in data):
+            raise TypeError("CustomIndex cannot hold strings")
+
+        if name is None and hasattr(data, "name"):
+            name = data.name
+        data = np.array(data, dtype="O")
+
+        return cls._simple_new(data, name)
+
+
+def test_insert_fallback_to_base_index():
+    # https://github.com/pandas-dev/pandas/issues/47071
+
+    idx = CustomIndex([1, 2, 3])
+    result = idx.insert(0, "string")
+    expected = Index(["string", 1, 2, 3], dtype=object)
+    tm.assert_index_equal(result, expected)
+
+    df = DataFrame(
+        np.random.randn(2, 3), columns=idx, index=Index([1, 2], name="string")
+    )
+    result = df.reset_index()
+    tm.assert_index_equal(result.columns, expected)