Updating generic.py error message

pandas/core/generic.py

@@ -2868,9 +2868,17 @@ def groupby(self, by=None, axis=0, level=None, as_index=True, sort=True,
         """
 
         from pandas.core.groupby import groupby
-        axis = self._get_axis_number(axis)
-        return groupby(self, by, axis=axis, level=level, as_index=as_index,
-                       sort=sort, group_keys=group_keys, squeeze=squeeze)
+        if axis is not None:
+            axis = self._get_axis_number(axis)
+            return groupby(self, by, axis=axis, level=level, as_index=as_index,
+                        sort=sort, group_keys=group_keys, squeeze=squeeze)
+        elif level is not None:
+            raise TypeError('You have to specify one of "by" or "level"')
+        elif by is not None:
+            raise TypeError('You have to specify one of "by" or "level"')
+        else:
+            raise TypeError('You have to specify one of "by" or "level"')
+
 
     def asfreq(self, freq, method=None, how=None, normalize=False):
         """

pandas/tests/test_groupby.py

@@ -1961,6 +1961,9 @@ def test_groupby_level(self):
         # raise exception for non-MultiIndex
         self.assertRaises(ValueError, self.df.groupby, level=1)
 
+
+
+
     def test_groupby_level_index_names(self):
         ## GH4014 this used to raise ValueError since 'exp'>1 (in py2)
         df = DataFrame({'exp' : ['A']*3 + ['B']*3, 'var1' : lrange(6),}).set_index('exp')
@@ -1999,6 +2002,27 @@ def test_groupby_level_apply(self):
         result = frame['A'].groupby(level=0).count()
         self.assertEqual(result.index.name, 'first')
 
+
+#PR8618 and issue 8015
+    def test_groupby_args(self):
+        frame = self.mframe
+        def g():
+            frame.groupby(level=None).count()
+            self.assertRaisesRegexp(TypeError, g, "You have to supply one of 'by' or 'level'")
+
+        def k():
+            frame.groupby(by=None).count()
+            self.assertRaisesRegexp(TypeError, k, "You have to supply one of 'by' or 'level'")
+
+        def j():
+            frame.groupby()
+            self.assertRaisesRegexp(TypeError, j, "You have to supply one of 'by' or 'level'")
+
+        def i():
+            frame.groupby(axes=None)
+            self.assertRaisesRegexp(TypeError, i, "You have to supply one of 'by' or 'level'")
+
+
     def test_groupby_level_mapper(self):
         frame = self.mframe
         deleveled = frame.reset_index()