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Traversal

A :term:`traversal` uses the URL (Universal Resource Locator) to find a :term:`resource` located in a :term:`resource tree`, which is a set of nested dictionary-like objects. Traversal is done by using each segment of the path portion of the URL to navigate through the :term:`resource tree`. You might think of this as looking up files and directories in a file system. Traversal walks down the path until it finds a published resource, analogous to a file system "directory" or "file". The resource found as the result of a traversal becomes the :term:`context` of the :term:`request`. Then, the :term:`view lookup` subsystem is used to find some view code willing to "publish" this resource by generating a :term:`response`.

Using :term:`Traversal` to map a URL to code is optional. It is often less easy to understand than :term:`URL dispatch`, so if you're a rank beginner, it probably makes sense to use URL dispatch to map URLs to code instead of traversal. In that case, you can skip this chapter.

Traversal Details

:term:`Traversal` is dependent on information in a :term:`request` object. Every :term:`request` object contains URL path information in the PATH_INFO portion of the :term:`WSGI` environment. The PATH_INFO string is the portion of a request's URL following the hostname and port number, but before any query string elements or fragment element. For example the PATH_INFO portion of the URL http://example.com:8080/a/b/c?foo=1 is /a/b/c.

Traversal treats the PATH_INFO segment of a URL as a sequence of path segments. For example, the PATH_INFO string /a/b/c is converted to the sequence ['a', 'b', 'c'].

This path sequence is then used to descend through the :term:`resource tree`, looking up a resource for each path segment. Each lookup uses the __getitem__ method of a resource in the tree.

For example, if the path info sequence is ['a', 'b', 'c']:

  • :term:`Traversal` starts by acquiring the :term:`root` resource of the application by calling the :term:`root factory`. The :term:`root factory` can be configured to return whatever object is appropriate as the traversal root of your application.
  • Next, the first element ('a') is popped from the path segment sequence and is used as a key to lookup the corresponding resource in the root. This invokes the root resource's __getitem__ method using that value ('a') as an argument.
  • If the root resource "contains" a resource with key 'a', its __getitem__ method will return it. The :term:`context` temporarily becomes the "A" resource.
  • The next segment ('b') is popped from the path sequence, and the "A" resource's __getitem__ is called with that value ('b') as an argument; we'll presume it succeeds.
  • The "A" resource's __getitem__ returns another resource, which we'll call "B". The :term:`context` temporarily becomes the "B" resource.

Traversal continues until the path segment sequence is exhausted or a path element cannot be resolved to a resource. In either case, the :term:`context` resource is the last object that the traversal successfully resolved. If any resource found during traversal lacks a __getitem__ method, or if its __getitem__ method raises a :exc:`KeyError`, traversal ends immediately, and that resource becomes the :term:`context`.

The results of a :term:`traversal` also include a :term:`view name`. If traversal ends before the path segment sequence is exhausted, the :term:`view name` is the next remaining path segment element. If the :term:`traversal` expends all of the path segments, then the :term:`view name` is the empty string ('').

The combination of the context resource and the :term:`view name` found via traversal is used later in the same request by the :term:`view lookup` subsystem to find a :term:`view callable`. How :app:`Pyramid` performs view lookup is explained within the :ref:`view_config_chapter` chapter.

The Resource Tree

The resource tree is a set of nested dictionary-like resource objects that begins with a :term:`root` resource. In order to use :term:`traversal` to resolve URLs to code, your application must supply a :term:`resource tree` to :app:`Pyramid`.

In order to supply a root resource for an application the :app:`Pyramid` :term:`Router` is configured with a callback known as a :term:`root factory`. The root factory is supplied by the application, at startup time, as the root_factory argument to the :term:`Configurator`.

The root factory is a Python callable that accepts a :term:`request` object, and returns the root object of the :term:`resource tree`. A function, or class is typically used as an application's root factory. Here's an example of a simple root factory class:

Here's an example of using this root factory within startup configuration, by passing it to an instance of a :term:`Configurator` named config:

The root_factory argument to the :class:`~pyramid.config.Configurator` constructor registers this root factory to be called to generate a root resource whenever a request enters the application. The root factory registered this way is also known as the global root factory. A root factory can alternately be passed to the Configurator as a :term:`dotted Python name` which can refer to a root factory defined in a different module.

If no :term:`root factory` is passed to the :app:`Pyramid` :term:`Configurator` constructor, or if the root_factory value specified is None, a default root factory is used. The default root factory always returns a resource that has no child resources; it is effectively empty.

Usually a root factory for a traversal-based application will be more complicated than the above Root class; in particular it may be associated with a database connection or another persistence mechanism.

Emulating the Default Root Factory

For purposes of understanding the default root factory better, we'll note that you can emulate the default root factory by using this code as an explicit root factory in your application setup:

The default root factory is just a really stupid object that has no behavior or state. Using :term:`traversal` against an application that uses the resource tree supplied by the default root resource is not very interesting, because the default root resource has no children. Its availability is more useful when you're developing an application using :term:`URL dispatch`.

Note

If the items contained within the resource tree are "persistent" (they have state that lasts longer than the execution of a single process), they become analogous to the concept of :term:`domain model` objects used by many other frameworks.

The resource tree consists of container resources and leaf resources. There is only one difference between a container resource and a leaf resource: container resources possess a __getitem__ method (making it "dictionary-like") while leaf resources do not. The __getitem__ method was chosen as the signifying difference between the two types of resources because the presence of this method is how Python itself typically determines whether an object is "containerish" or not (dictionary objects are "containerish").

Each container resource is presumed to be willing to return a child resource or raise a KeyError based on a name passed to its __getitem__.

Leaf-level instances must not have a __getitem__. If instances that you'd like to be leaves already happen to have a __getitem__ through some historical inequity, you should subclass these resource types and cause their __getitem__ methods to simply raise a KeyError. Or just disuse them and think up another strategy.

Usually, the traversal root is a container resource, and as such it contains other resources. However, it doesn't need to be a container. Your resource tree can be as shallow or as deep as you require.

In general, the resource tree is traversed beginning at its root resource using a sequence of path elements described by the PATH_INFO of the current request; if there are path segments, the root resource's __getitem__ is called with the next path segment, and it is expected to return another resource. The resulting resource's __getitem__ is called with the very next path segment, and it is expected to return another resource. This happens ad infinitum until all path segments are exhausted.

The Traversal Algorithm

This section will attempt to explain the :app:`Pyramid` traversal algorithm. We'll provide a description of the algorithm, a diagram of how the algorithm works, and some example traversal scenarios that might help you understand how the algorithm operates against a specific resource tree.

We'll also talk a bit about :term:`view lookup`. The :ref:`view_config_chapter` chapter discusses :term:`view lookup` in detail, and it is the canonical source for information about views. Technically, :term:`view lookup` is a :app:`Pyramid` subsystem that is separated from traversal entirely. However, we'll describe the fundamental behavior of view lookup in the examples in the next few sections to give you an idea of how traversal and view lookup cooperate, because they are almost always used together.

A Description of The Traversal Algorithm

When a user requests a page from your traversal-powered application, the system uses this algorithm to find a :term:`context` resource and a :term:`view name`.

  1. The request for the page is presented to the :app:`Pyramid` :term:`router` in terms of a standard :term:`WSGI` request, which is represented by a WSGI environment and a WSGI start_response callable.

  2. The router creates a :term:`request` object based on the WSGI environment.

  3. The :term:`root factory` is called with the :term:`request`. It returns a :term:`root` resource.

  4. The router uses the WSGI environment's PATH_INFO information to determine the path segments to traverse. The leading slash is stripped off PATH_INFO, and the remaining path segments are split on the slash character to form a traversal sequence.

    The traversal algorithm by default attempts to first URL-unquote and then Unicode-decode each path segment derived from PATH_INFO from its natural byte string (str type) representation. URL unquoting is performed using the Python standard library urllib.unquote function. Conversion from a URL-decoded string into Unicode is attempted using the UTF-8 encoding. If any URL-unquoted path segment in PATH_INFO is not decodeable using the UTF-8 decoding, a :exc:`TypeError` is raised. A segment will be fully URL-unquoted and UTF8-decoded before it is passed in to the __getitem__ of any resource during traversal.

    Thus, a request with a PATH_INFO variable of /a/b/c maps to the traversal sequence [u'a', u'b', u'c'].

  5. :term:`Traversal` begins at the root resource returned by the root factory. For the traversal sequence [u'a', u'b', u'c'], the root resource's __getitem__ is called with the name 'a'. Traversal continues through the sequence. In our example, if the root resource's __getitem__ called with the name a returns a resource (aka resource "A"), that resource's __getitem__ is called with the name 'b'. If resource "A" returns a resource "B" when asked for 'b', resource B's __getitem__ is then asked for the name 'c', and may return resource "C".

  6. Traversal ends when a) the entire path is exhausted or b) when any resouce raises a :exc:`KeyError` from its __getitem__ or c) when any non-final path element traversal does not have a __getitem__ method (resulting in a :exc:`AttributeError`) or d) when any path element is prefixed with the set of characters @@ (indicating that the characters following the @@ token should be treated as a :term:`view name`).

  7. When traversal ends for any of the reasons in the previous step, the last resource found during traversal is deemed to be the :term:`context`. If the path has been exhausted when traversal ends, the :term:`view name` is deemed to be the empty string (''). However, if the path was not exhausted before traversal terminated, the first remaining path segment is treated as the view name.

  8. Any subsequent path elements after the :term:`view name` is found are deemed the :term:`subpath`. The subpath is always a sequence of path segments that come from PATH_INFO that are "left over" after traversal has completed.

Once the :term:`context` resource, the :term:`view name`, and associated attributes such as the :term:`subpath` are located, the job of :term:`traversal` is finished. It passes back the information it obtained to its caller, the :app:`Pyramid` :term:`Router`, which subsequently invokes :term:`view lookup` with the context and view name information.

The traversal algorithm exposes two special cases:

  • You will often end up with a :term:`view name` that is the empty string as the result of a particular traversal. This indicates that the view lookup machinery should look up the :term:`default view`. The default view is a view that is registered with no name or a view which is registered with a name that equals the empty string.
  • If any path segment element begins with the special characters @@ (think of them as goggles), the value of that segment minus the goggle characters is considered the :term:`view name` immediately and traversal stops there. This allows you to address views that may have the same names as resource names in the tree unambiguously.

Finally, traversal is responsible for locating a :term:`virtual root`. A virtual root is used during "virtual hosting"; see the :ref:`vhosting_chapter` chapter for information. We won't speak more about it in this chapter.

resourcetreetraverser.png

Traversal Algorithm Examples

No one can be expected to understand the traversal algorithm by analogy and description alone, so let's examine some traversal scenarios that use concrete URLs and resource tree compositions.

Let's pretend the user asks for http://example.com/foo/bar/baz/biz/buz.txt. The request's PATH_INFO in that case is /foo/bar/baz/biz/buz.txt. Let's further pretend that when this request comes in that we're traversing the following resource tree:

/--
   |
   |-- foo
        |
        ----bar

Here's what happens:

The fact that it does not find "baz" at this point does not signify an error condition. It signifies that:

At this point, traversal has ended, and :term:`view lookup` begins.

Because it's the "context" resource, the view lookup machinery examines "bar" to find out what "type" it is. Let's say it finds that the context is a Bar type (because "bar" happens to be an instance of the class Bar). Using the :term:`view name` (baz) and the type, view lookup asks the :term:`application registry` this question:

Let's say that view lookup finds no matching view type. In this circumstance, the :app:`Pyramid` :term:`router` returns the result of the :term:`not found view` and the request ends.

However, for this tree:

/--
   |
   |-- foo
        |
        ----bar
             |
             ----baz
                    |
                    biz

The user asks for http://example.com/foo/bar/baz/biz/buz.txt

  • :mod:`traversal` traverses "foo", and attempts to find "bar", which it finds.
  • :mod:`traversal` traverses "bar", and attempts to find "baz", which it finds.
  • :mod:`traversal` traverses "baz", and attempts to find "biz", which it finds.
  • :mod:`traversal` traverses "biz", and attempts to find "buz.txt" which it does not find.

The fact that it does not find a resource related to "buz.txt" at this point does not signify an error condition. It signifies that:

At this point, traversal has ended, and :term:`view lookup` begins.

Because it's the "context" resource, the view lookup machinery examines the "biz" resource to find out what "type" it is. Let's say it finds that the resource is a Biz type (because "biz" is an instance of the Python class Biz). Using the :term:`view name` (buz.txt) and the type, view lookup asks the :term:`application registry` this question:

Let's say that question is answered by the application registry; in such a situation, the application registry returns a :term:`view callable`. The view callable is then called with the current :term:`WebOb` :term:`request` as the sole argument: request; it is expected to return a response.

The Example View Callables Accept Only a Request; How Do I Access the Context Resource?

Most of the examples in this book assume that a view callable is typically passed only a :term:`request` object. Sometimes your view callables need access to the :term:`context` resource, especially when you use :term:`traversal`. You might use a supported alternate view callable argument list in your view callables such as the (context, request) calling convention described in :ref:`request_and_context_view_definitions`. But you don't need to if you don't want to. In view callables that accept only a request, the :term:`context` resource found by traversal is available as the context attribute of the request object, e.g. request.context. The :term:`view name` is available as the view_name attribute of the request object, e.g. request.view_name. Other :app:`Pyramid` -specific request attributes are also available as described in :ref:`special_request_attributes`.

Using Resource Interfaces In View Configuration

Instead of registering your views with a context that names a Python resource class, you can optionally register a view callable with a context which is an :term:`interface`. An interface can be attached arbitrarily to any resource object. View lookup treats context interfaces specially, and therefore the identity of a resource can be divorced from that of the class which implements it. As a result, associating a view with an interface can provide more flexibility for sharing a single view between two or more different implementations of a resource type. For example, if two resource objects of different Python class types share the same interface, you can use the same view configuration to specify both of them as a context.

In order to make use of interfaces in your application during view dispatch, you must create an interface and mark up your resource classes or instances with interface declarations that refer to this interface.

To attach an interface to a resource class, you define the interface and use the :func:`zope.interface.implements` function to associate the interface with the class.

To attach an interface to a resource instance, you define the interface and use the :func:`zope.interface.alsoProvides` function to associate the interface with the instance. This function mutates the instance in such a way that the interface is attached to it.

Regardless of how you associate an interface, with a resource instance, or a resource class, the resulting code to associate that interface with a view callable is the same. Assuming the above code that defines an IHello interface lives in the root of your application, and its module is named "resources.py", the interface declaration below will associate the mypackage.views.hello_world view with resources that implement, or provide, this interface.

Any time a resource that is determined to be the :term:`context` provides this interface, and a view named hello.html is looked up against it as per the URL, the mypackage.views.hello_world view callable will be invoked.

Note, in cases where a view is registered against a resource class, and a view is also registered against an interface that the resource class implements, an ambiguity arises. Views registered for the resource class take precedence over any views registered for any interface the resource class implements. Thus, if one view configuration names a context of both the class type of a resource, and another view configuration names a context of interface implemented by the resource's class, and both view configurations are otherwise identical, the view registered for the context's class will "win".

For more information about defining resources with interfaces for use within view configuration, see :ref:`resources_which_implement_interfaces`.

References

A tutorial showing how :term:`traversal` can be used within a :app:`Pyramid` application exists in :ref:`bfg_wiki_tutorial`.

See the :ref:`view_config_chapter` chapter for detailed information about :term:`view lookup`.

The :mod:`pyramid.traversal` module contains API functions that deal with traversal, such as traversal invocation from within application code.

The :meth:`pyramid.request.Request.resource_url` method generates a URL when given a resource retrieved from a resource tree.

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