allow neg index on str_get (pandas-dev#17741)

Author: bobhaffner

doc/source/whatsnew/v0.21.0.txt

@@ -702,6 +702,7 @@ Indexing
 - Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
 - Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` when no valid entry (:issue:`17400`)
 - Bug in :func:`Series.rename` when called with a `callable`, incorrectly alters the name of the `Series`, rather than the name of the `Index`. (:issue:`17407`)
+- Bug in :func:`String.str_get` raises `index out of range` error instead of inserting NaNs when using a negative index. (:issue:`17704`)
 
 I/O
 ^^^

pandas/core/strings.py

@@ -1260,7 +1260,7 @@ def str_get(arr, i):
     -------
     items : Series/Index of objects
     """
-    f = lambda x: x[i] if len(x) > i else np.nan
+    f = lambda x: x[i] if len(x) > i >= -len(x) else np.nan
     return _na_map(f, arr)
 
 

pandas/tests/test_strings.py

@@ -2484,6 +2484,19 @@ def test_get(self):
         expected = Series([u('b'), u('d'), np.nan, u('g')])
         tm.assert_series_equal(result, expected)
 
+        # bounds testing
+        values = Series(['1_2_3_4_5', '6_7_8_9_10', '11_12'])
+
+        # positive index
+        result = values.str.split('_').str.get(2)
+        expected = Series(['3', '8', np.nan])
+        tm.assert_series_equal(result, expected)
+
+        # negative index
+        result = values.str.split('_').str.get(-3)
+        expected = Series(['3', '8', np.nan])
+        tm.assert_series_equal(result, expected)
+
     def test_more_contains(self):
         # PR #1179
         s = Series(['A', 'B', 'C', 'Aaba', 'Baca', '', NA,