Add fix and test for case of partially named multiindex

Author: Marco Gorelli

pandas/core/frame.py

@@ -6244,8 +6244,6 @@ def explode(self, column: Union[str, Tuple]) -> "DataFrame":
         if not self.columns.is_unique:
             raise ValueError("columns must be unique")
 
-        named_index = any(i for i in self.index.names)
-
         column_with_index = self[column].reset_index()
 
         result = (
@@ -6258,11 +6256,12 @@ def explode(self, column: Union[str, Tuple]) -> "DataFrame":
             result.index = pandas.MultiIndex.from_frame(
                 result.iloc[:, : self.index.nlevels]
             )
+            result.index.rename(
+                [i or None for num, i in enumerate(self.index.names)], inplace=True
+            )
         else:
             result.index = result.iloc[:, 0]
-
-        if not named_index:
-            result.index.names = [None] * self.index.nlevels
+            result.index.rename(self.index.name or None, inplace=True)
 
         result = result.reindex(columns=self.columns, copy=False)
 

pandas/tests/frame/test_explode.py

@@ -176,7 +176,18 @@ def test_usecase():
             ),
             pd.DataFrame(
                 {"col": [1, 2, 3, 4], "other_col": ["a", "a", "b", "b"]},
-                index=pd.MultiIndex.from_tuples([(0, 1), (0, 1), (0, 1), (0, 1)]),
+                pd.MultiIndex.from_tuples([(0, 1), (0, 1), (0, 1), (0, 1)]),
+                dtype=object,
+            ),
+        ),
+        (
+            pd.DataFrame(
+                {"col": [[1, 2], [3, 4]], "other_col": ["a", "b"]},
+                pd.MultiIndex.from_arrays([[0, 0], [1, 1]], names=['foo', None]),
+            ),
+            pd.DataFrame(
+                {"col": [1, 2, 3, 4], "other_col": ["a", "a", "b", "b"]},
+                pd.MultiIndex.from_arrays([[0, 0, 0, 0], [1, 1, 1, 1]], names=['foo', None]),
                 dtype=object,
             ),
         ),