diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index 671142a98a6b36..08ab621f5e3cef 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -6,6 +6,7 @@
 import warnings
 import copy
 from textwrap import dedent
+from contextlib import contextmanager
 
 from pandas.compat import (
     zip, range, lzip,
@@ -549,6 +550,16 @@ def f(self):
         return attr
 
 
+@contextmanager
+def _group_selection_context(groupby):
+    """
+    set / reset the _group_selection_context
+    """
+    groupby._set_group_selection()
+    yield groupby
+    groupby._reset_group_selection()
+
+
 class _GroupBy(PandasObject, SelectionMixin):
     _group_selection = None
     _apply_whitelist = frozenset([])
@@ -704,6 +715,8 @@ def _set_group_selection(self):
         """
         Create group based selection. Used when selection is not passed
         directly but instead via a grouper.
+
+        NOTE: this should be paired with a call to _reset_group_selection
         """
         grp = self.grouper
         if not (self.as_index and
@@ -785,10 +798,10 @@ def _make_wrapper(self, name):
                                                      type(self).__name__))
             raise AttributeError(msg)
 
-        # need to setup the selection
-        # as are not passed directly but in the grouper
         self._set_group_selection()
 
+        # need to setup the selection
+        # as are not passed directly but in the grouper
         f = getattr(self._selected_obj, name)
         if not isinstance(f, types.MethodType):
             return self.apply(lambda self: getattr(self, name))
@@ -913,9 +926,8 @@ def f(g):
                 # fails on *some* columns, e.g. a numeric operation
                 # on a string grouper column
 
-                self._set_group_selection()
-                result = self._python_apply_general(f)
-                self._reset_group_selection()
+                with _group_selection_context(self):
+                    return self._python_apply_general(f)
 
         return result
 
@@ -1295,9 +1307,9 @@ def mean(self, *args, **kwargs):
         except GroupByError:
             raise
         except Exception:  # pragma: no cover
-            self._set_group_selection()
-            f = lambda x: x.mean(axis=self.axis, **kwargs)
-            return self._python_agg_general(f)
+            with _group_selection_context(self):
+                f = lambda x: x.mean(axis=self.axis, **kwargs)
+                return self._python_agg_general(f)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
@@ -1313,13 +1325,12 @@ def median(self, **kwargs):
             raise
         except Exception:  # pragma: no cover
 
-            self._set_group_selection()
-
             def f(x):
                 if isinstance(x, np.ndarray):
                     x = Series(x)
                 return x.median(axis=self.axis, **kwargs)
-            return self._python_agg_general(f)
+            with _group_selection_context(self):
+                return self._python_agg_general(f)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
@@ -1356,9 +1367,9 @@ def var(self, ddof=1, *args, **kwargs):
         if ddof == 1:
             return self._cython_agg_general('var', **kwargs)
         else:
-            self._set_group_selection()
             f = lambda x: x.var(ddof=ddof, **kwargs)
-            return self._python_agg_general(f)
+            with _group_selection_context(self):
+                return self._python_agg_general(f)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
@@ -1404,6 +1415,7 @@ def f(self, **kwargs):
                     kwargs['numeric_only'] = numeric_only
                 if 'min_count' not in kwargs:
                     kwargs['min_count'] = min_count
+
                 self._set_group_selection()
                 try:
                     return self._cython_agg_general(
@@ -1797,13 +1809,12 @@ def ngroup(self, ascending=True):
         .cumcount : Number the rows in each group.
         """
 
-        self._set_group_selection()
-
-        index = self._selected_obj.index
-        result = Series(self.grouper.group_info[0], index)
-        if not ascending:
-            result = self.ngroups - 1 - result
-        return result
+        with _group_selection_context(self):
+            index = self._selected_obj.index
+            result = Series(self.grouper.group_info[0], index)
+            if not ascending:
+                result = self.ngroups - 1 - result
+            return result
 
     @Substitution(name='groupby')
     def cumcount(self, ascending=True):
@@ -1854,11 +1865,10 @@ def cumcount(self, ascending=True):
         .ngroup : Number the groups themselves.
         """
 
-        self._set_group_selection()
-
-        index = self._selected_obj.index
-        cumcounts = self._cumcount_array(ascending=ascending)
-        return Series(cumcounts, index)
+        with _group_selection_context(self):
+            index = self._selected_obj.index
+            cumcounts = self._cumcount_array(ascending=ascending)
+            return Series(cumcounts, index)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
diff --git a/pandas/tests/groupby/test_apply.py b/pandas/tests/groupby/test_apply.py
index ecb4e2af530424..07eef2d87feb30 100644
--- a/pandas/tests/groupby/test_apply.py
+++ b/pandas/tests/groupby/test_apply.py
@@ -519,11 +519,11 @@ def test_func(x):
 
 def test_apply_with_mixed_types():
     # gh-20949
-    df = pd.DataFrame({'A': 'a a b'.split(), 'B': [1,2,3], 'C': [4, 6, 5]})
+    df = pd.DataFrame({'A': 'a a b'.split(), 'B': [1, 2, 3], 'C': [4, 6, 5]})
     g = df.groupby('A')
 
     result = g.transform(lambda x: x / x.sum())
-    expected = pd.DataFrame({'B': [1/3., 2/3., 1], 'C': [0.4, 0.6, 1.0]})
+    expected = pd.DataFrame({'B': [1 / 3., 2 / 3., 1], 'C': [0.4, 0.6, 1.0]})
     tm.assert_frame_equal(result, expected)
 
     result = g.apply(lambda x: x / x.sum())