How to add optional URL parameters with Python Django?

To add optional URL parameters with Python Django, we call add multiple rules for the same URL.

For instance, we write

urlpatterns = patterns('',
    url(r'^project_config/$', views.foo),
    url(r'^project_config/(?P<product>\w+)/$', views.foo),
    url(r'^project_config/(?P<product>\w+)/(?P<project_id>\w+)/$', views.foo),
)

to add 3 URL patterns that has no URL parameters, the product parameter only, and the product and project_id` parameters.

And we map all 3 to the same view.