Not found view doesn't work on Pyramid using traversal -

i'm using pyramid (1.5.7) + traversal , following documentation i've tried possible ways "not found exception view" working.

from pyramid.view import notfound_view_config,forbidden_view_config, view_config   @notfound_view_config(renderer="error/not_found.jinja2") def not_found_view(request):     request.response.status = 404     return {}  @forbidden_view_config(renderer="error/forbidden.jinja2") def forbidden_view(request):     return {} 

using contexts:

from pyramid.view import view_config pyramid.httpexceptions import httpforbidden, httpunauthorized   @view_config(context=httpnotfound, renderer="error/not_found.jinja2") def not_found_view(request):     request.response.status = 404     return {}  @view_config(context=httpforbidden, renderer="error/forbidden.jinja2") def forbidden_view(request):     return {} 

i'm using scan mode, i've tried adding custom function configuration:

def main(globals, **settings):     config = configurator()     config.add_notfound_view(notfound) 

not luck either, time getting following unhandled exception:

raise httpnotfound(msg) pyramid.httpexceptions.httpnotfound: /example-url 

ouch... bad! using tween preventing pyramid load exceptions:

def predispatch_factory(handler, registry):     # one-time configuration code goes here      def predispatch(request):         # code executed each request before         # actual application code goes here          response = handler(request)          # code executed each request after         # actual application code goes here          return response      return predispatch 

still don't know why after removing tween seems work expected.