Documentation updates for GH 5677

Author: Jon M. Mease

doc/source/groupby.rst

@@ -94,6 +94,9 @@ The mapping can be specified many different ways:
   - For DataFrame objects, a string indicating a column to be used to group. Of
     course ``df.groupby('A')`` is just syntactic sugar for
     ``df.groupby(df['A'])``, but it makes life simpler
+  - For DataFrame objects, a string indicating an index level to be used to group.
+    If a string matches both an index level name and a column name then the column
+    takes precedence
   - A list of any of the above things
 
 Collectively we refer to the grouping objects as the **keys**. For example,

doc/source/whatsnew/v0.20.0.txt

@@ -29,8 +29,7 @@ New features
 
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
-
-
+- Strings passed to ``DataFrame.groupby()`` as the ``by`` parameter may now reference either column names or index level names (:issue:`5677`)
 
 
 .. _whatsnew_0200.api_breaking:

pandas/core/generic.py

@@ -3926,7 +3926,7 @@ def groupby(self, by=None, axis=0, level=None, as_index=True, sort=True,
         Parameters
         ----------
         by : mapping function / list of functions, dict, Series, or tuple /
-            list of column names.
+            list of column names or index level names.
             Called on each element of the object index to determine the groups.
             If a dict or Series is passed, the Series or dict VALUES will be
             used to determine the groups