Flask will now give you an error in debug mode if a post request caused a redirect by the routing system.

flask/app.py

@@ -18,7 +18,7 @@ from itertools import chain
 from functools import update_wrapper
 
 from werkzeug.datastructures import ImmutableDict
-from werkzeug.routing import Map, Rule
+from werkzeug.routing import Map, Rule, RequestRedirect
 from werkzeug.exceptions import HTTPException, InternalServerError, \
      MethodNotAllowed, BadRequest
 
@@ -1134,6 +1134,22 @@ class Flask(_PackageBoundObject):
             return InternalServerError()
         return handler(e)
 
+    def raise_routing_exception(self, request):
+        """Exceptions that are recording during routing are reraised with
+        this method.  During debug we are not reraising redirect requests
+        for non ``GET``, ``HEAD``, or ``OPTIONS`` requests and we're raising
+        a different error instead to help debug situations.
+
+        :internal:
+        """
+        if not self.debug \
+           or not isinstance(request.routing_exception, RequestRedirect) \
+           or request.method in ('GET', 'HEAD', 'OPTIONS'):
+            raise request.routing_exception
+
+        from .debughelpers import FormDataRoutingRedirect
+        raise FormDataRoutingRedirect(request)
+
     def dispatch_request(self):
         """Does the request dispatching.  Matches the URL and returns the
         return value of the view or error handler.  This does not have to
@@ -1146,7 +1162,7 @@ class Flask(_PackageBoundObject):
         """
         req = _request_ctx_stack.top.request
         if req.routing_exception is not None:
-            raise req.routing_exception
+            self.raise_routing_exception(req)
         rule = req.url_rule
         # if we provide automatic options for this URL and the
         # request came with the OPTIONS method, reply automatically

flask/debughelpers.py

@@ -33,6 +33,32 @@ class DebugFilesKeyError(KeyError, AssertionError):
         return self.msg
 
 
+class FormDataRoutingRedirect(AssertionError):
+    """This exception is raised by Flask in debug mode if it detects a
+    redirect caused by the routing system when the request method is not
+    GET, HEAD or OPTIONS.  Reasoning: form data will be dropped.
+    """
+
+    def __init__(self, request):
+        exc = request.routing_exception
+        buf = ['A request was sent to this URL (%s) but a redirect was '
+               'issued automatically by the routing system to "%s".'
+               % (request.url, exc.new_url)]
+
+        # In case just a slash was appended we can be extra helpful
+        if request.base_url + '/' == exc.new_url.split('?')[0]:
+            buf.append('  The URL was defined with a trailing slash so '
+                       'Flask will automatically redirect to the URL '
+                       'with the trailing slash if it was accessed '
+                       'without one.')
+
+        buf.append('  Make sure to directly send your %s-request to this URL '
+                   'since we can\'t make browsers or HTTP clients redirect '
+                   'with form data.' % request.method)
+        buf.append('\n\nNote: this exception is only raised in debug mode')
+        AssertionError.__init__(self, ''.join(buf).encode('utf-8'))
+
+
 def attach_enctype_error_multidict(request):
     """Since Flask 0.8 we're monkeypatching the files object in case a
     request is detected that does not use multipart form data but the files

tests/flask_tests.py

@@ -981,6 +981,30 @@ class BasicFunctionalityTestCase(unittest.TestCase):
         self.assertEqual(got, [42])
         self.assert_(app.got_first_request)
 
+    def test_routing_redirect_debugging(self):
+        app = flask.Flask(__name__)
+        app.debug = True
+        @app.route('/foo/', methods=['GET', 'POST'])
+        def foo():
+            return 'success'
+        with app.test_client() as c:
+            try:
+                c.post('/foo', data={})
+            except AssertionError, e:
+                self.assert_('http://localhost/foo/' in str(e))
+                self.assert_('Make sure to directly send your POST-request '
+                             'to this URL' in str(e))
+            else:
+                self.fail('Expected exception')
+
+            rv = c.get('/foo', data={}, follow_redirects=True)
+            self.assertEqual(rv.data, 'success')
+
+        app.debug = False
+        with app.test_client() as c:
+            rv = c.post('/foo', data={}, follow_redirects=True)
+            self.assertEqual(rv.data, 'success')
+
 
 class JSONTestCase(unittest.TestCase):