Use setdefault() because it sets session.modified correctly.

flask.py

@@ -167,7 +167,7 @@ def flash(message):
 
     :param message: the message to be flashed.
     """
-    session['_flashes'] = session.get('_flashes', ()) + (message,)
+    session.setdefault('_flashes', []).append(message)
 
 
 def get_flashed_messages():
@@ -177,7 +177,7 @@ def get_flashed_messages():
     """
     flashes = _request_ctx_stack.top.flashes
     if flashes is None:
-        _request_ctx_stack.top.flashes = flashes = session.pop('_flashes', ())
+        _request_ctx_stack.top.flashes = flashes = session.pop('_flashes', [])
     return flashes
 
 

tests/flask_tests.py

@@ -117,6 +117,18 @@ class BasicFunctionalityTestCase(unittest.TestCase):
             expect_exception(flask.session.__setitem__, 'foo', 42)
             expect_exception(flask.session.pop, 'foo')
 
+    def test_flashes(self):
+        app = flask.Flask(__name__)
+        app.secret_key = 'testkey'
+
+        with app.test_request_context():
+            assert not flask.session.modified
+            flask.flash('Zap')
+            flask.session.modified = False
+            flask.flash('Zip')
+            assert flask.session.modified
+            assert list(flask.get_flashed_messages()) == ['Zap', 'Zip']
+
     def test_request_processing(self):
         app = flask.Flask(__name__)
         evts = []