BUG: Fix pandas-dev#10355, std() groupby calculation

Author: alexcwatt

pandas/core/groupby/groupby.py

@@ -1187,8 +1187,9 @@ def std(self, ddof=1, *args, **kwargs):
         """
 
         # TODO: implement at Cython level?
-        nv.validate_groupby_func('std', args, kwargs)
-        return np.sqrt(self.var(ddof=ddof, **kwargs))
+        with _group_selection_context(self):
+            f = lambda x: x.std(axis=self.axis, **kwargs)
+            return self._python_agg_general(f)
 
     @Substitution(name='groupby')
     @Appender(_common_see_also)

pandas/tests/groupby/test_groupby.py

@@ -26,6 +26,31 @@ def test_repr():
     assert result == expected
 
 
+def test_groupby_std():
+    # GH10355
+    df = pd.DataFrame({
+        'a': [1, 1, 1, 2, 2, 2, 3, 3, 3],
+        'b': [1, 2, 3, 3, 5, 7, 7, 8, 9],
+    })
+    result = df.groupby('a', as_index=False).std()
+    expected = pd.DataFrame({
+        'a': [1, 2, 3],
+        'b': [1, 2, 1]
+    })
+    assert_frame_equal(result, expected)
+
+    df = pd.DataFrame({
+        'a': [1, 1, 1, 2, 2, 2, 3, 3, 3],
+        'b': [1, 2, 3, 3, 5, 7, 7, 8, 9],
+    })
+    result = df.groupby('a', as_index=True).std()
+    expected = pd.DataFrame({
+        'a': [1, 2, 3],
+        'b': [1, 2, 1]
+    }).set_index('a')
+    assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize('dtype', ['int64', 'int32', 'float64', 'float32'])
 def test_basic(dtype):