diff --git a/flask/sessions.py b/flask/sessions.py
index 82b588bc..8f111718 100644
--- a/flask/sessions.py
+++ b/flask/sessions.py
@@ -10,6 +10,7 @@
 """
 import hashlib
 import warnings
+from collections import MutableMapping
 from datetime import datetime
 
 from itsdangerous import BadSignature, URLSafeTimedSerializer
@@ -19,43 +20,55 @@ from flask.helpers import is_ip, total_seconds
 from flask.json.tag import TaggedJSONSerializer
 
 
-class SessionMixin(object):
-    """Expands a basic dictionary with an accessors that are expected
-    by Flask extensions and users for the session.
-    """
+class SessionMixin(MutableMapping):
+    """Expands a basic dictionary with session attributes."""
 
-    def _get_permanent(self):
+    @property
+    def permanent(self):
+        """This reflects the ``'_permanent'`` key in the dict."""
         return self.get('_permanent', False)
 
-    def _set_permanent(self, value):
+    @permanent.setter
+    def permanent(self, value):
         self['_permanent'] = bool(value)
 
-    #: this reflects the ``'_permanent'`` key in the dict.
-    permanent = property(_get_permanent, _set_permanent)
-    del _get_permanent, _set_permanent
-
-    #: some session backends can tell you if a session is new, but that is
-    #: not necessarily guaranteed.  Use with caution.  The default mixin
-    #: implementation just hardcodes ``False`` in.
+    #: Some implementations can detect whether a session is newly
+    #: created, but that is not guaranteed. Use with caution. The mixin
+    # default is hard-coded ``False``.
     new = False
 
-    #: for some backends this will always be ``True``, but some backends will
-    #: default this to false and detect changes in the dictionary for as
-    #: long as changes do not happen on mutable structures in the session.
-    #: The default mixin implementation just hardcodes ``True`` in.
+    #: Some implementations can detect changes to the session and set
+    #: this when that happens. The mixin default is hard coded to
+    #: ``True``.
     modified = True
 
-    #: the accessed variable indicates whether or not the session object has
-    #: been accessed in that request. This allows flask to append a `Vary:
-    #: Cookie` header to the response if the session is being accessed. This
-    #: allows caching proxy servers, like Varnish, to use both the URL and the
-    #: session cookie as keys when caching pages, preventing multiple users
-    #: from being served the same cache.
+    #: Some implementations can detect when session data is read or
+    #: written and set this when that happens. The mixin default is hard
+    #: coded to ``True``.
     accessed = True
 
 
 class SecureCookieSession(CallbackDict, SessionMixin):
-    """Base class for sessions based on signed cookies."""
+    """Base class for sessions based on signed cookies.
+
+    This session backend will set the :attr:`modified` and
+    :attr:`accessed` attributes. It cannot reliably track whether a
+    session is new (vs. empty), so :attr:`new` remains hard coded to
+    ``False``.
+    """
+
+    #: When data is changed, this is set to ``True``. Only the session
+    #: dictionary itself is tracked; if the session contains mutable
+    #: data (for example a nested dict) then this must be set to
+    #: ``True`` manually when modifying that data. The session cookie
+    #: will only be written to the response if this is ``True``.
+    modified = False
+
+    #: When data is read or written, this is set to ``True``. Used by
+    # :class:`.SecureCookieSessionInterface` to add a ``Vary: Cookie``
+    #: header, which allows caching proxies to cache different pages for
+    #: different users.
+    accessed = False
 
     def __init__(self, initial=None):
         def on_update(self):
@@ -63,8 +76,6 @@ class SecureCookieSession(CallbackDict, SessionMixin):
             self.accessed = True
 
         super(SecureCookieSession, self).__init__(initial, on_update)
-        self.modified = False
-        self.accessed = False
 
     def __getitem__(self, key):
         self.accessed = True
diff --git a/tests/test_basic.py b/tests/test_basic.py
index 0e3076df..d7406baf 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -221,12 +221,21 @@ def test_endpoint_decorator(app, client):
 def test_session(app, client):
     @app.route('/set', methods=['POST'])
     def set():
+        assert not flask.session.accessed
+        assert not flask.session.modified
         flask.session['value'] = flask.request.form['value']
+        assert flask.session.accessed
+        assert flask.session.modified
         return 'value set'
 
     @app.route('/get')
     def get():
-        return flask.session['value']
+        assert not flask.session.accessed
+        assert not flask.session.modified
+        v = flask.session.get('value', 'None')
+        assert flask.session.accessed
+        assert not flask.session.modified
+        return v
 
     assert client.post('/set', data={'value': '42'}).data == b'value set'
     assert client.get('/get').data == b'42'