Resources

A resource is an object that represents a “place” in a tree related to your application. Every Pyramid application has at least one resource object: the 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 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 traversal to map URLs to code, the resource tree structure is used heavily to map each URL to a view callable. When traversal is used, Pyramid will walk through the resource tree by traversing through its nested dictionary structure in order to find a context resource. Once a context resource is found, the context resource and data in the request will be used to find a view callable.

In an application which uses 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” 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 domain model of the application.

Also:

• The context and containment predicate arguments to add_view() (or a view_config() decorator) reference a resource class or resource interface.
• A root factory returns a resource.
• A resource is exposed to view code as the context of a view.
• Various helpful Pyramid API methods expect a resource as an argument (e.g. resource_url() and others).

Defining a Resource Tree

When traversal is used (as opposed to a purely url dispatch based application), Pyramid expects to be able to traverse a tree composed of resources (the 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. 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 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 KeyError.

See Traversal for more information about how traversal works against resource instances.

Here’s a sample resource tree, represented by a variable named root:

 class Resource(dict):
     pass

 root = Resource({'a':Resource({'b':Resource({'c':Resource()})})})

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:

root['a']['b']['c']

If you returned the above root object from a root factory, the path /a/b/c would find the 'c' object in the resource tree as the result of traversal.

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 location, so their utility in a “real” application is limited. To make best use of built-in Pyramid API facilities, your resources should be “location-aware”. The next section details how to make resources location-aware.

Location-Aware Resources

In order for certain 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 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:

class MyRootResource(object):
    __name__ = ''
    __parent__ = None

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.

Warning

If your root resource has a __name__ argument that is not None or the empty string, URLs returned by the resource_url() function and paths generated by the resource_path() and 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).

Using pyramid_traversalwrapper

If you’d rather not manage the __name__ and __parent__ attributes of your resources “by hand”, an add-on package named pyramid_traversalwrapper can help.

In order to use this helper feature, you must first install the pyramid_traversalwrapper package (available via PyPI), then register its ModelGraphTraverser as the traversal policy, rather than the default Pyramid traverser. The package contains instructions for doing so.

Once 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 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 Pyramid APIs require location-aware resources. These APIs include (but are not limited to) resource_url(), find_resource(), find_root(), find_interface(), resource_path(), resource_path_tuple(), or traverse(), virtual_root(), and (usually) has_permission() and principals_allowed_by_permission().

In general, since so much 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 location aware, you can use the 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 resource_url() to influence the generated URL.

The simplest call to resource_url() looks like this:

url = request.resource_url(resource)

The request in the above example is an instance of a Pyramid request object.

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 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 resource_url():

url = request.resource_url(resource, 'foo', 'bar')

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 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:

url = request.resource_url(resource, query={'a':'1'})

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 virtual root is active, the URL generated by resource_url() for a resource may be “shorter” than its physical tree path. See Virtual Root Support for more information about virtually rooting a resource.

For more information about generating resource URLs, see the documentation for pyramid.request.Request.resource_url().

Overriding Resource URL Generation

If a resource object implements a __resource_url__ method, this method will be called when resource_url() is called to generate a URL for the resource, overriding the default URL returned for the resource by resource_url().

The __resource_url__ hook is passed two arguments: request and info. request is the request object passed to resource_url(). info is a dictionary with two keys:

physical_path

The “physical path” computed for the resource, as defined by pyramid.traversal.resource_path(resource).

virtual_path

The “virtual path” computed for the resource, as defined by 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.

class Resource(object):
    def __resource_url__(self, request, info):
        return request.application_url + info['virtual_path']

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 resource_url().

Generating the Path To a Resource

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.

from pyramid.traversal import resource_path
url = resource_path(resource)

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 resource_path() will be appended as path segments to the end of the resource path.

from pyramid.traversal import resource_path
url = resource_path(resource, 'foo', 'bar')

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 location-aware.

The presence or absence of a virtual root has no impact on the behavior of resource_path().

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 pyramid.traversal.find_resource().

You can resolve an absolute path by passing a string prefixed with a / as the path argument:

from pyramid.traversal import find_resource
url = find_resource(anyresource, '/path')

Or you can resolve a path relative to the resource you pass in by passing a string that isn’t prefixed by /:

from pyramid.traversal import find_resource
url = find_resource(anyresource, 'path')

Often the paths you pass to find_resource() are generated by the resource_path() API. These APIs are “mirrors” of each other.

If the path cannot be resolved when calling find_resource() (if the respective resource in the tree does not exist), a KeyError will be raised.

See the pyramid.traversal.find_resource() documentation for more information about resolving a path to a resource.

Obtaining the Lineage of a Resource

pyramid.location.lineage() returns a generator representing the lineage of the location aware resource object.

The 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:

class Thing(object): pass

thing1 = Thing()
thing2 = Thing()
thing2.__parent__ = thing1

Calling lineage(thing2) will return a generator. When we turn it into a list, we will get:

list(lineage(thing2))
[ <Thing object at thing2>, <Thing object at thing1> ]

The generator returned by 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 pyramid.location.lineage() for more information.

Determining if a Resource is In The Lineage of Another Resource

Use the pyramid.location.inside() function to determine if one resource is in the lineage of another resource.

For example, if the resource tree is:

class Thing(object): pass

a = Thing()
b = Thing()
b.__parent__ = a

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 inside() is (resource1, resource2). resource1 is ‘inside’ resource2 if resource2 is a lineage ancestor of resource1. It is a lineage ancestor if its parent (or one of its parent’s parents, etc.) is an ancestor.

See pyramid.location.inside() for more information.

Finding the Root Resource

Use the pyramid.traversal.find_root() API to find the root resource. The root resource is the root resource of the resource tree. The API accepts a single argument: resource. This is a resource that is 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:

class Thing(object): pass

a = Thing()
b = Thing()
b.__parent__ = a

Calling find_root(b) will return a.

The root resource is also available as request.root within view callable code.

The presence or absence of a virtual root has no impact on the behavior of find_root(). The root object returned is always the physical root object.

Resources Which Implement Interfaces

Resources can optionally be made to implement an interface. An interface is used to tag a resource object with a “type” that can later be referred to within view configuration and by pyramid.traversal.find_interface().

Specifying an interface instead of a class as the context or containment predicate arguments within 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 interface.

import datetime
from zope.interface import implements
from zope.interface import Interface

class IBlogEntry(Interface):
    pass

class BlogEntry(object):
    implements(IBlogEntry)
    def __init__(self, title, body, author):
        self.title = title
        self.body = body
        self.author = author
        self.created = datetime.datetime.now()

This resource consists of two things: the class which defines the resource constructor as the class BlogEntry, and an 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 zope.interface.Interface.

A resource class may implement zero or more interfaces. You specify that a resource implements an interface by using the 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 zope.interface.directlyProvides() function:

import datetime
from zope.interface import directlyProvides
from zope.interface import Interface

class IBlogEntry(Interface):
    pass

class BlogEntry(object):
    def __init__(self, title, body, author):
        self.title = title
        self.body = body
        self.author = author
        self.created = datetime.datetime.now()

entry = BlogEntry('title', 'body', 'author')
directlyProvides(entry, IBlogEntry)

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 zope.interface.alsoProvides() function:

import datetime
from zope.interface import alsoProvides
from zope.interface import directlyProvides
from zope.interface import Interface

class IBlogEntry1(Interface):
    pass

class IBlogEntry2(Interface):
    pass

class BlogEntry(object):
    def __init__(self, title, body, author):
        self.title = title
        self.body = body
        self.author = author
        self.created = datetime.datetime.now()

entry = BlogEntry('title', 'body', 'author')
directlyProvides(entry, IBlogEntry1)
alsoProvides(entry, IBlogEntry2)

zope.interface.alsoProvides() will augment the set of interfaces directly provided by an instance instead of overwriting them like zope.interface.directlyProvides() does.

For more information about how resource interfaces can be used by view configuration, see Using Resource Interfaces In View Configuration.

Finding a Resource With a Class or Interface in Lineage

Use the find_interface() API to locate a parent that is of a particular Python class, or which implements some interface.

For example, if your resource tree is composed as follows:

class Thing1(object): pass
class Thing2(object): pass

a = Thing1()
b = Thing2()
b.__parent__ = a

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 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 Resources Which Implement Interfaces.

Pyramid API Functions That Act Against Resources

A resource object is used as the context provided to a view. See Traversal and URL Dispatch for more information about how a resource object becomes the context.

The APIs provided by pyramid.traversal 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 pyramid.location are used against resources. These can be used to walk down a resource tree, or conveniently locate one resource “inside” another.

Some APIs in pyramid.security accept a resource object as a parameter. For example, the 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 pyramid.security module also accept context as an argument, and a context is always a resource.