Hello Traversal World
Traversal is an alternative to URL dispatch which allows Pyramid applications to map URLs to code.
If code speaks louder than words, maybe this will help. Here is a single-file Pyramid application that uses traversal:
You may notice that this application is intentionally very similar to the "hello world" app from :doc:`firstapp`.
On lines 5-6, we create a trivial :term:`resource` class that's just a dictionary subclass.
On lines 11-13 we define a single :term:`view callable` that can display a single instance of our Resource class, passed as the context argument.
The rest of the file sets up and serves our pyramid WSGI app. Line 18 is where our view gets configured for use whenever the traversal ends with an instance of our Resource class.
Interestingly, there are no URLs explicitly configured in this application. Instead, the URL space is defined entirely by the keys in the resource tree.
If this example is running on http://localhost:8080, and the user browses to http://localhost:8080/a/b, Pyramid will call get_root(request) to get the root resource, then traverse the tree from there by key; starting from the root, it will find the child with key "a", then its child with key "b"; then use that as the context argument for calling hello_world_of_resources.
Or, if the user browses to http://localhost:8080/ , Pyramid will stop at the root - the outermost Resource instance, in this case - and use that as the context argument to the same view.
Or, if the user browses to a key that doesn't exist in this resource tree, like http://localhost:8080/xyz or http://localhost:8080/a/b/c/d, the traversal will end by raising a KeyError, and Pyramid will turn that into a 404 HTTP response.
A more complicated application could have many types of resources, with different view callables defined for each type, and even multiple views for each type.
Full technical details may be found in :doc:`traversal`.
For more about why you might use traversal, see :doc:`muchadoabouttraversal`.