Session falls back to a dummy object now if secret_key is missing.

This makes it possible to still read-only access the empty session but
requires the secret key to be set for write access.  The error message
raised explains that.  This closes #10.

flask.py

@@ -68,6 +68,22 @@ class _RequestGlobals(object):
     pass
 
 
+class _NullSession(SecureCookie):
+    """Class used to generate nicer error messages if sessions are not
+    available.  Will still allow read-only access to the empty session
+    but fail on setting.
+    """
+
+    def _fail(self, *args, **kwargs):
+        raise RuntimeError('the session is unavailable because no secret '
+                           'key was set.  Set the secret_key on the '
+                           'application to something unique and secret')
+    __setitem__ = __delitem__ = clear = pop = popitem = \
+        update = setdefault = _fail
+    del _fail
+
+
+
 class _RequestContext(object):
     """The request context contains all request relevant information.  It is
     created at the beginning of the request and pushed to the
@@ -80,6 +96,8 @@ class _RequestContext(object):
         self.url_adapter = app.url_map.bind_to_environ(environ)
         self.request = app.request_class(environ)
         self.session = app.open_session(self.request)
+        if self.session is None:
+            self.session = _NullSession()
         self.g = _RequestGlobals()
         self.flashes = None
 
@@ -384,7 +402,6 @@ class Flask(object):
                         object)
         :param response: an instance of :attr:`response_class`
         """
-        if session is not None:
         session.save_cookie(response, self.session_cookie_name)
 
     def add_url_rule(self, rule, endpoint, **options):
@@ -603,7 +620,7 @@ class Flask(object):
                  instance of :attr:`response_class`.
         """
         session = _request_ctx_stack.top.session
-        if session is not None:
+        if not isinstance(session, _NullSession):
             self.save_session(session, response)
         for handler in self.after_request_funcs:
             response = handler(response)

tests/flask_tests.py

@@ -72,6 +72,20 @@ class BasicFunctionality(unittest.TestCase):
         assert c.post('/set', data={'value': '42'}).data == 'value set'
         assert c.get('/get').data == '42'
 
+    def test_missing_session(self):
+        app = flask.Flask(__name__)
+        def expect_exception(f, *args, **kwargs):
+            try:
+                f(*args, **kwargs)
+            except RuntimeError, e:
+                assert e.args and 'session is unavailable' in e.args[0]
+            else:
+                assert False, 'expected exception'
+        with app.test_request_context():
+            assert flask.session.get('missing_key') is None
+            expect_exception(flask.session.__setitem__, 'foo', 42)
+            expect_exception(flask.session.pop, 'foo')
+
     def test_request_processing(self):
         app = flask.Flask(__name__)
         evts = []