BUG: Respect ignore_index in Series.explode (pandas-dev#40507)

Author: dsaxton

doc/source/whatsnew/v1.3.0.rst

@@ -651,6 +651,7 @@ Reshaping
 - Allow :class:`Index` to be passed to the :func:`numpy.all` function (:issue:`40180`)
 - Bug in :meth:`DataFrame.stack` not preserving ``CategoricalDtype`` in a ``MultiIndex`` (:issue:`36991`)
 - Bug in :func:`to_datetime` raising error when input sequence contains unhashable items (:issue:`39756`)
+- Bug in :meth:`Series.explode` preserving index when ``ignore_index`` was ``True`` and values were scalars (:issue:`40487`)
 
 Sparse
 ^^^^^^

pandas/core/series.py

@@ -3857,7 +3857,8 @@ def explode(self, ignore_index: bool = False) -> Series:
         dtype: object
         """
         if not len(self) or not is_object_dtype(self):
-            return self.copy()
+            result = self.copy()
+            return result.reset_index(drop=True) if ignore_index else result
 
         values, counts = reshape.explode(np.asarray(self._values))
 

pandas/tests/series/methods/test_explode.py

@@ -134,3 +134,11 @@ def test_explode_sets():
     result = s.explode().sort_values()
     expected = pd.Series(["a", "b", "c"], index=[1, 1, 1])
     tm.assert_series_equal(result, expected)
+
+
+def test_explode_scalars_can_ignore_index():
+    # https://github.com/pandas-dev/pandas/issues/40487
+    s = pd.Series([1, 2, 3], index=["a", "b", "c"])
+    result = s.explode(ignore_index=True)
+    expected = pd.Series([1, 2, 3])
+    tm.assert_series_equal(result, expected)