fix groupby with tuple bug

Author: TomAugspurger

doc/source/v0.15.0.txt

@@ -640,6 +640,7 @@ Bug Fixes
 
 
 
-
+- Bug in accessing groups from a ``GroupBy`` when the original grouper
+  was a tuple (:issue:`8121`).
 
 

pandas/core/groupby.py

@@ -431,9 +431,18 @@ def convert(key, s):
         sample = next(iter(self.indices))
         if isinstance(sample, tuple):
             if not isinstance(name, tuple):
-                raise ValueError("must supply a tuple to get_group with multiple grouping keys")
+                msg = ("must supply a tuple to get_group with multiple"
+                       " grouping keys")
+                raise ValueError(msg)
             if not len(name) == len(sample):
-                raise ValueError("must supply a a same-length tuple to get_group with multiple grouping keys")
+                try:
+                    # If the original grouper was a tuple
+                    return self.indices[name]
+                except KeyError:
+                    # turns out it wasn't a tuple
+                    msg = ("must supply a a same-length tuple to get_group"
+                           " with multiple grouping keys")
+                    raise ValueError(msg)
 
             name = tuple([ convert(n, k) for n, k in zip(name,sample) ])
 

pandas/tests/test_groupby.py

@@ -598,6 +598,23 @@ def test_get_group(self):
         self.assertRaises(ValueError, lambda : g.get_group(('foo')))
         self.assertRaises(ValueError, lambda : g.get_group(('foo','bar','baz')))
 
+    def test_get_group_grouped_by_tuple(self):
+        # GH 8121
+        df = DataFrame([[(1,), (1, 2), (1,), (1, 2)]],
+                       index=['ids']).T
+        gr = df.groupby('ids')
+        expected = DataFrame({'ids': [(1,), (1,)]}, index=[0, 2])
+        result = gr.get_group((1,))
+        assert_frame_equal(result, expected)
+
+        dt = pd.to_datetime(['2010-01-01', '2010-01-02', '2010-01-01',
+                            '2010-01-02'])
+        df = DataFrame({'ids': [(x,) for x in dt]}) 
+        gr = df.groupby('ids')
+        result = gr.get_group(('2010-01-01',))
+        expected = DataFrame({'ids': [(dt[0],), (dt[0],)]}, index=[0, 2])
+        assert_frame_equal(result, expected)
+
     def test_agg_apply_corner(self):
         # nothing to group, all NA
         grouped = self.ts.groupby(self.ts * np.nan)