From 97b64d3b736f6abf0d78126c75d5e56e774bd234 Mon Sep 17 00:00:00 2001
From: Paul Winkler <slinkp@gmail.com>
Date: Mon, 30 Jan 2012 17:34:51 -0500
Subject: Hello world with traversal, linked from various places; plus some
 'what this chapter is for' notes on the other traversal chapters. Hope this
 helps.

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+.. _hello_traversal_chapter:
+
+Hello Traversal World
+======================
+
+
+.. index::
+   single: traversal quick example
+
+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:
+
+.. literalinclude:: hellotraversal.py
+   :linenos:
+
+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 8-9, we hard-code a :term:`resource tree` in our :term:`root
+factory` function.
+
+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.
+
+Example requests
+----------------
+
+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.
+
+See Also
+---------
+
+Full technical details may be found in :doc:`traversal`.
+
+For more about *why* you might use traversal, see :doc:`muchadoabouttraversal`.
+
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