A :term:`resource` is an object that represents a "place" in a tree related to your application. Every :app:`Pyramid` application has at least one resource object: the :term:`root` resource. Even if you don't define a root resource manually, a default one is created for you. The root resource is the root of a :term:`resource tree`. A resource tree is a set of nested dictionary-like objects which you can use to represent your website's structure.
In an application which uses :term:`traversal` to map URLs to code, the resource tree structure is used heavily to map each URL to a :term:`view callable`. When :term:`traversal` is used, :app:`Pyramid` will walk through the resource tree by traversing through its nested dictionary structure in order to find a :term:`context` resource. Once a context resource is found, the context resource and data in the request will be used to find a :term:`view callable`.
In an application which uses :term:`URL dispatch`, the resource tree is only used indirectly, and is often "invisible" to the developer. In URL dispatch applications, the resource "tree" is often composed of only the root resource by itself. This root resource sometimes has security declarations attached to it, but is not required to have any. In general, the resource tree is much less important in applications that use URL dispatch than applications that use traversal.
In "Zope-like" :app:`Pyramid` applications, resource objects also often store data persistently, and offer methods related to mutating that persistent data. In these kinds of applications, resources not only represent the site structure of your website, but they become the :term:`domain model` of the application.
- The context and containment predicate arguments to :meth:`~pyramid.config.Configurator.add_view` (or a :func:`~pyramid.view.view_config` decorator) reference a resource class or resource :term:`interface`.
- A :term:`root factory` returns a resource.
- A resource is exposed to :term:`view` code as the :term:`context` of a view.
- Various helpful :app:`Pyramid` API methods expect a resource as an argument (e.g. :meth:`~pyramid.request.Request.resource_url` and others).
Defining a Resource Tree
When :term:`traversal` is used (as opposed to a purely :term:`url dispatch` based application), :app:`Pyramid` expects to be able to traverse a tree composed of resources (the :term:`resource tree`). Traversal begins at a root resource, and descends into the tree recursively, trying each resource's __getitem__ method to resolve a path segment to another resource object. :app:`Pyramid` imposes the following policy on resource instances in the tree:
- A container resource (a resource which contains other resources) must supply a __getitem__ method which is willing to resolve a unicode name to a sub-resource. If a sub-resource by a particular name does not exist in a container resource, __getitem__ method of the container resource must raise a :exc:`KeyError`. If a sub-resource by that name does exist, the container's __getitem__ should return the sub-resource.
- Leaf resources, which do not contain other resources, must not implement a __getitem__, or if they do, their __getitem__ method must always raise a :exc:`KeyError`.
See :ref:`traversal_chapter` for more information about how traversal works against resource instances.
Here's a sample resource tree, represented by a variable named root:
The resource tree we've created above is represented by a dictionary-like root object which has a single child named 'a'. 'a' has a single child named 'b', and 'b' has a single child named 'c', which has no children. It is therefore possible to access the 'c' leaf resource like so:
In this example, each of the resources in the tree is of the same class. This is not a requirement. Resource elements in the tree can be of any type. We used a single class to represent all resources in the tree for the sake of simplicity, but in a "real" app, the resources in the tree can be arbitrary.
Although the example tree above can service a traversal, the resource instances in the above example are not aware of :term:`location`, so their utility in a "real" application is limited. To make best use of built-in :app:`Pyramid` API facilities, your resources should be "location-aware". The next section details how to make resources location-aware.
In order for certain :app:`Pyramid` location, security, URL-generation, and traversal APIs to work properly against the resources in a resource tree, all resources in the tree must be :term:`location` -aware. This means they must have two attributes: __parent__ and __name__.
The __parent__ attribute of a location-aware resource should be a reference to the resource's parent resource instance in the tree. The __name__ attribute should be the name with which a resource's parent refers to the resource via __getitem__.
The __parent__ of the root resource should be None and its __name__ should be the empty string. For instance:
A resource returned from the root resource's __getitem__ method should have a __parent__ attribute that is a reference to the root resource, and its __name__ attribute should match the name by which it is reachable via the root resource's __getitem__. A container resource within the root resource should have a __getitem__ that returns resources with a __parent__ attribute that points at the container, and these subobjects should have a __name__ attribute that matches the name by which they are retrieved from the container via __getitem__. This pattern continues recursively "up" the tree from the root.
The __parent__ attributes of each resource form a linked list that points "downwards" toward the root. This is analogous to the .. entry in filesystem directories. If you follow the __parent__ values from any resource in the resource tree, you will eventually come to the root resource, just like if you keep executing the cd .. filesystem command, eventually you will reach the filesystem root directory.
If your root resource has a __name__ argument that is not None or the empty string, URLs returned by the :func:`~pyramid.request.Request.resource_url` function and paths generated by the :func:`~pyramid.traversal.resource_path` and :func:`~pyramid.traversal.resource_path_tuple` APIs will be generated improperly. The value of __name__ will be prepended to every path and URL generated (as opposed to a single leading slash or empty tuple element).
If you'd rather not manage the __name__ and __parent__ attributes of your resources "by hand", an add-on package named :mod:`pyramid_traversalwrapper` can help.
In order to use this helper feature, you must first install the :mod:`pyramid_traversalwrapper` package (available via PyPI), then register its ModelGraphTraverser as the traversal policy, rather than the default :app:`Pyramid` traverser. The package contains instructions for doing so.
Once :app:`Pyramid` is configured with this feature, you will no longer need to manage the __parent__ and __name__ attributes on resource objects "by hand". Instead, as necessary, during traversal :app:`Pyramid` will wrap each resource (even the root resource) in a LocationProxy which will dynamically assign a __name__ and a __parent__ to the traversed resource (based on the last traversed resource and the name supplied to __getitem__). The root resource will have a __name__ attribute of None and a __parent__ attribute of None.
Applications which use tree-walking :app:`Pyramid` APIs require location-aware resources. These APIs include (but are not limited to) :meth:`~pyramid.request.Request.resource_url`, :func:`~pyramid.traversal.find_resource`, :func:`~pyramid.traversal.find_root`, :func:`~pyramid.traversal.find_interface`, :func:`~pyramid.traversal.resource_path`, :func:`~pyramid.traversal.resource_path_tuple`, or :func:`~pyramid.traversal.traverse`, :func:`~pyramid.traversal.virtual_root`, and (usually) :func:`~pyramid.security.has_permission` and :func:`~pyramid.security.principals_allowed_by_permission`.
In general, since so much :app:`Pyramid` infrastructure depends on location-aware resources, it's a good idea to make each resource in your tree location-aware.
Generating The URL Of A Resource
If your resources are :term:`location` aware, you can use the :meth:`pyramid.request.Request.resource_url` API to generate a URL for the resource. This URL will use the resource's position in the parent tree to create a resource path, and it will prefix the path with the current application URL to form a fully-qualified URL with the scheme, host, port, and path. You can also pass extra arguments to :meth:`~pyramid.request.Request.resource_url` to influence the generated URL.
The simplest call to :meth:`~pyramid.request.Request.resource_url` looks like this:
If the resource referred to as resource in the above example was the root resource, and the host that was used to contact the server was example.com, the URL generated would be http://example.com/. However, if the resource was a child of the root resource named a, the generated URL would be http://example.com/a/.
A slash is appended to all resource URLs when :meth:`~pyramid.request.Request.resource_url` is used to generate them in this simple manner, because resources are "places" in the hierarchy, and URLs are meant to be clicked on to be visited. Relative URLs that you include on HTML pages rendered as the result of the default view of a resource are more apt to be relative to these resources than relative to their parent.
You can also pass extra elements to :meth:`~pyramid.request.Request.resource_url`:
If the resource referred to as resource in the above example was the root resource, and the host that was used to contact the server was example.com, the URL generated would be http://example.com/foo/bar. Any number of extra elements can be passed to :meth:`~pyramid.request.Request.resource_url` as extra positional arguments. When extra elements are passed, they are appended to the resource's URL. A slash is not appended to the final segment when elements are passed.
You can also pass a query string:
If the resource referred to as resource in the above example was the root resource, and the host that was used to contact the server was example.com, the URL generated would be http://example.com/?a=1.
When a :term:`virtual root` is active, the URL generated by :meth:`~pyramid.request.Request.resource_url` for a resource may be "shorter" than its physical tree path. See :ref:`virtual_root_support` for more information about virtually rooting a resource.
For more information about generating resource URLs, see the documentation for :meth:`pyramid.request.Request.resource_url`.
Overriding Resource URL Generation
If a resource object implements a __resource_url__ method, this method will be called when :meth:`~pyramid.request.Request.resource_url` is called to generate a URL for the resource, overriding the default URL returned for the resource by :meth:`~pyramid.request.Request.resource_url`.
- The "physical path" computed for the resource, as defined by pyramid.traversal.resource_path(resource).
- The "virtual path" computed for the resource, as defined by :ref:`virtual_root_support`. This will be identical to the physical path if virtual rooting is not enabled.
The __resource_url__ method of a resource should return a string representing a URL. If it cannot override the default, it should return None. If it returns None, the default URL will be returned.
Here's an example __resource_url__ method.
The above example actually just generates and returns the default URL, which would have been what was returned anyway, but your code can perform arbitrary logic as necessary. For example, your code may wish to override the hostname or port number of the generated URL.
Note that the URL generated by __resource_url__ should be fully qualified, should end in a slash, and should not contain any query string or anchor elements (only path elements) to work best with :meth:`~pyramid.request.Request.resource_url`.
Generating the Path To a Resource
:func:`pyramid.traversal.resource_path` returns a string object representing the absolute physical path of the resource object based on its position in the resource tree. Each segment of the path is separated with a slash character.
If resource in the example above was accessible in the tree as root['a']['b'], the above example would generate the string /a/b.
Any positional arguments passed in to :func:`~pyramid.traversal.resource_path` will be appended as path segments to the end of the resource path.
If resource in the example above was accessible in the tree as root['a']['b'], the above example would generate the string /a/b/foo/bar.
The resource passed in must be :term:`location`-aware.
Finding a Resource by Path
If you have a string path to a resource, you can grab the resource from that place in the application's resource tree using :func:`pyramid.traversal.find_resource`.
You can resolve an absolute path by passing a string prefixed with a / as the path argument:
Or you can resolve a path relative to the resource you pass in by passing a string that isn't prefixed by /:
See the :func:`pyramid.traversal.find_resource` documentation for more information about resolving a path to a resource.
Obtaining the Lineage of a Resource
The :func:`~pyramid.location.lineage` function returns the resource it is passed, then each parent of the resource, in order. For example, if the resource tree is composed like so:
Calling lineage(thing2) will return a generator. When we turn it into a list, we will get:
The generator returned by :func:`~pyramid.location.lineage` first returns the resource it was passed unconditionally. Then, if the resource supplied a __parent__ attribute, it returns the resource represented by resource.__parent__. If that resource has a __parent__ attribute, return that resource's parent, and so on, until the resource being inspected either has no __parent__ attribute or has a __parent__ attribute of None.
See the documentation for :func:`pyramid.location.lineage` for more information.
Determining if a Resource is In The Lineage of Another Resource
For example, if the resource tree is:
Calling inside(b, a) will return True, because b has a lineage that includes a. However, calling inside(a, b) will return False because a does not have a lineage that includes b.
The argument list for :func:`~pyramid.location.inside` is (resource1, resource2). resource1 is 'inside' resource2 if resource2 is a :term:`lineage` ancestor of resource1. It is a lineage ancestor if its parent (or one of its parent's parents, etc.) is an ancestor.
See :func:`pyramid.location.inside` for more information.
Finding the Root Resource
Use the :func:`pyramid.traversal.find_root` API to find the :term:`root` resource. The root resource is the root resource of the :term:`resource tree`. The API accepts a single argument: resource. This is a resource that is :term:`location` aware. It can be any resource in the tree for which you want to find the root.
For example, if the resource tree is:
Calling find_root(b) will return a.
The root resource is also available as request.root within :term:`view callable` code.
Resources Which Implement Interfaces
Resources can optionally be made to implement an :term:`interface`. An interface is used to tag a resource object with a "type" that can later be referred to within :term:`view configuration` and by :func:`pyramid.traversal.find_interface`.
Specifying an interface instead of a class as the context or containment predicate arguments within :term:`view configuration` statements makes it possible to use a single view callable for more than one class of resource object. If your application is simple enough that you see no reason to want to do this, you can skip reading this section of the chapter.
For example, here's some code which describes a blog entry which also declares that the blog entry implements an :term:`interface`.
This resource consists of two things: the class which defines the resource constructor as the class BlogEntry, and an :term:`interface` attached to the class via an implements statement at class scope using the IBlogEntry interface as its sole argument.
The interface object used must be an instance of a class that inherits from :class:`zope.interface.Interface`.
A resource class may implement zero or more interfaces. You specify that a resource implements an interface by using the :func:`zope.interface.implements` function at class scope. The above BlogEntry resource implements the IBlogEntry interface.
You can also specify that a particular resource instance provides an interface, as opposed to its class. When you declare that a class implements an interface, all instances of that class will also provide that interface. However, you can also just say that a single object provides the interface. To do so, use the :func:`zope.interface.directlyProvides` function:
:func:`zope.interface.directlyProvides` will replace any existing interface that was previously provided by an instance. If a resource object already has instance-level interface declarations that you don't want to replace, use the :func:`zope.interface.alsoProvides` function:
For more information about how resource interfaces can be used by view configuration, see :ref:`using_resource_interfaces`.
Finding a Resource With a Class or Interface in Lineage
For example, if your resource tree is composed as follows:
Calling find_interface(a, Thing1) will return the a resource because a is of class Thing1 (the resource passed as the first argument is considered first, and is returned if the class or interface spec matches).
Calling find_interface(b, Thing1) will return the a resource because a is of class Thing1 and a is the first resource in b's lineage of this class.
Calling find_interface(b, Thing2) will return the b resource.
The second argument to find_interface may also be a :term:`interface` instead of a class. If it is an interface, each resource in the lineage is checked to see if the resource implements the specificed interface (instead of seeing if the resource is of a class). See also :ref:`resources_which_implement_interfaces`.
:app:`Pyramid` API Functions That Act Against Resources
The APIs provided by :ref:`traversal_module` are used against resource objects. These functions can be used to find the "path" of a resource, the root resource in a resource tree, or to generate a URL for a resource.
The APIs provided by :ref:`location_module` are used against resources. These can be used to walk down a resource tree, or conveniently locate one resource "inside" another.
Some APIs in :ref:`security_module` accept a resource object as a parameter. For example, the :func:`~pyramid.security.has_permission` API accepts a resource object as one of its arguments; the ACL is obtained from this resource or one of its ancestors. Other APIs in the :mod:`pyramid.security` module also accept :term:`context` as an argument, and a context is always a resource.