BUG: Fix .iat indexing with a PeriodIndex GH4390

     Series.get with negative indexers returns the same as []

Author: jreback

doc/source/release.rst

@@ -62,6 +62,7 @@ pandas 0.13
   - deprecated ``iterkv``, which will be removed in a future release (was just
     an alias of iteritems used to get around ``2to3``'s changes).
     (:issue:`4384`, :issue:`4375`, :issue:`4372`)
+  - ``Series.get`` with negative indexers now returns the same as ``[]`` (:issue:`4390`)
 
 **Experimental Features**
 
@@ -87,6 +88,7 @@ pandas 0.13
     dtypes, surfaced in (:issue:`4377`)
   - Fixed bug with duplicate columns and type conversion in ``read_json`` when
     ``orient='split'`` (:issue:`4377`)
+  - Fix ``.iat`` indexing with a ``PeriodIndex`` (:issue:`4390`)
 
 pandas 0.12
 ===========

doc/source/v0.13.0.txt

@@ -29,6 +29,7 @@ API changes
   - deprecated ``iterkv``, which will be removed in a future release (was just
     an alias of iteritems used to get around ``2to3``'s changes).
     (:issue:`4384`, :issue:`4375`, :issue:`4372`)
+  - ``Series.get`` with negative indexers now returns the same as ``[]`` (:issue:`4390`)
 
 Enhancements
 ~~~~~~~~~~~~

pandas/core/series.py

@@ -1018,7 +1018,7 @@ def get_value(self, label):
         -------
         value : scalar value
         """
-        return self.index._engine.get_value(self, label)
+        return self.index.get_value(self, label)
 
     def set_value(self, label, value):
         """

pandas/tests/test_series.py

@@ -629,7 +629,7 @@ def test_getitem_get(self):
         self.assertEqual(self.series[idx1], self.series[5])
         self.assertEqual(self.objSeries[idx2], self.objSeries[5])
 
-        self.assert_(self.series.get(-1) is None)
+        self.assertEqual(self.series.get(-1), self.series.get(self.series.index[-1]))
         self.assertEqual(self.series[5], self.series.get(self.series.index[5]))
 
         # missing

pandas/tseries/tests/test_period.py

@@ -1323,6 +1323,15 @@ def test_as_frame_columns(self):
         ts = df['1/1/2000']
         assert_series_equal(ts, df.ix[:, 0])
 
+    def test_indexing(self):
+
+        # GH 4390, iat incorrectly indexing
+        index = period_range('1/1/2001', periods=10)
+        s = Series(randn(10), index=index)
+        expected = s[index[0]]
+        result = s.iat[0]
+        self.assert_(expected == result)
+
     def test_frame_setitem(self):
         rng = period_range('1/1/2000', periods=5)
         rng.name = 'index'