Using Hooks

“Hooks” can be used to influence the behavior of the Pyramid framework in various ways.

Changing the Not Found View

When Pyramid can’t map a URL to view code, it invokes a not found view, which is a view callable. A default notfound view exists. The default not found view can be overridden through application configuration.

The not found view callable is a view callable like any other. The view configuration which causes it to be a “not found” view consists only of naming the pyramid.exceptions.NotFound class as the context of the view configuration.

If your application uses imperative configuration, you can replace the Not Found view by using the pyramid.config.Configurator.add_view() method to register an “exception view”:

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from pyramid.exceptions import NotFound
from helloworld.views import notfound_view
config.add_view(notfound_view, context=NotFound)

Replace helloworld.views.notfound_view with a reference to the view callable you want to use to represent the Not Found view.

Like any other view, the notfound view must accept at least a request parameter, or both context and request. The request is the current request representing the denied action. The context (if used in the call signature) will be the instance of the NotFound exception that caused the view to be called.

Here’s some sample code that implements a minimal NotFound view callable:

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from pyramid.httpexceptions import HTTPNotFound

def notfound_view(request):
    return HTTPNotFound()

Note

When a NotFound view callable is invoked, it is passed a request. The exception attribute of the request will be an instance of the NotFound exception that caused the not found view to be called. The value of request.exception.args[0] will be a value explaining why the not found error was raised. This message will be different when the debug_notfound environment setting is true than it is when it is false.

Warning

When a NotFound view callable accepts an argument list as described in Alternate View Callable Argument/Calling Conventions, the context passed as the first argument to the view callable will be the NotFound exception instance. If available, the resource context will still be available as request.context.

Changing the Forbidden View

When Pyramid can’t authorize execution of a view based on the authorization policy in use, it invokes a forbidden view. The default forbidden response has a 403 status code and is very plain, but the view which generates it can be overridden as necessary.

The forbidden view callable is a view callable like any other. The view configuration which causes it to be a “not found” view consists only of naming the pyramid.exceptions.Forbidden class as the context of the view configuration.

You can replace the forbidden view by using the pyramid.config.Configurator.add_view() method to register an “exception view”:

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from helloworld.views import forbidden_view
from pyramid.exceptions import Forbidden
config.add_view(forbidden_view, context=Forbidden)

Replace helloworld.views.forbidden_view with a reference to the Python view callable you want to use to represent the Forbidden view.

Like any other view, the forbidden view must accept at least a request parameter, or both context and request. The context (available as request.context if you’re using the request-only view argument pattern) is the context found by the router when the view invocation was denied. The request is the current request representing the denied action.

Here’s some sample code that implements a minimal forbidden view:

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from pyramid.views import view_config
from pyramid.response import Response

def forbidden_view(request):
    return Response('forbidden')

Note

When a forbidden view callable is invoked, it is passed a request. The exception attribute of the request will be an instance of the Forbidden exception that caused the forbidden view to be called. The value of request.exception.args[0] will be a value explaining why the forbidden was raised. This message will be different when the debug_authorization environment setting is true than it is when it is false.

Changing the Request Factory

Whenever Pyramid handles a WSGI request, it creates a request object based on the WSGI environment it has been passed. By default, an instance of the pyramid.request.Request class is created to represent the request object.

The class (aka “factory”) that Pyramid uses to create a request object instance can be changed by passing a request_factory argument to the constructor of the configurator. This argument can be either a callable or a dotted Python name representing a callable.

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from pyramid.request import Request

class MyRequest(Request):
    pass

config = Configurator(request_factory=MyRequest)

If you’re doing imperative configuration, and you’d rather do it after you’ve already constructed a configurator it can also be registered via the pyramid.config.Configurator.set_request_factory() method:

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from pyramid.config import Configurator
from pyramid.request import Request

class MyRequest(Request):
    pass

config = Configurator()
config.set_request_factory(MyRequest)

Adding Renderer Globals

Whenever Pyramid handles a request to perform a rendering (after a view with a renderer= configuration attribute is invoked, or when the any of the methods beginning with render within the pyramid.renderers module are called), renderer globals can be injected into the system values sent to the renderer. By default, no renderer globals are injected, and the “bare” system values (such as request, context, and renderer_name) are the only values present in the system dictionary passed to every renderer.

A callback that Pyramid will call every time a renderer is invoked can be added by passing a renderer_globals_factory argument to the constructor of the configurator. This callback can either be a callable object or a dotted Python name representing such a callable.

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def renderer_globals_factory(system):
    return {'a':1}

config = Configurator(
         renderer_globals_factory=renderer_globals_factory)

Such a callback must accept a single positional argument (notionally named system) which will contain the original system values. It must return a dictionary of values that will be merged into the system dictionary. See System Values Used During Rendering for discription of the values present in the system dictionary.

If you’re doing imperative configuration, and you’d rather do it after you’ve already constructed a configurator it can also be registered via the pyramid.config.Configurator.set_renderer_globals_factory() method:

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from pyramid.config import Configurator

def renderer_globals_factory(system):
    return {'a':1}

config = Configurator()
config.set_renderer_globals_factory(renderer_globals_factory)

Another mechanism which allows event subscribers to add renderer global values exists in Using The Before Render Event.

Using The Before Render Event

Subscribers to the pyramid.events.BeforeRender event may introspect the and modify the set of renderer globals before they are passed to a renderer. This event object iself has a dictionary-like interface that can be used for this purpose. For example:

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 from pyramid.events import subscriber
 from pyramid.events import BeforeRender

 @subscriber(BeforeRender)
 def add_global(event):
     event['mykey'] = 'foo'

An object of this type is sent as an event just before a renderer is invoked (but after the application-level renderer globals factory added via set_renderer_globals_factory, if any, has injected its own keys into the renderer globals dictionary).

If a subscriber attempts to add a key that already exist in the renderer globals dictionary, a KeyError is raised. This limitation is enforced because event subscribers do not possess any relative ordering. The set of keys added to the renderer globals dictionary by all pyramid.events.BeforeRender subscribers and renderer globals factories must be unique.

See the API documentation for the BeforeRender event interface at pyramid.interfaces.IBeforeRender.

Another mechanism which allows event subscribers more control when adding renderer global values exists in Adding Renderer Globals.

Using Response Callbacks

Unlike many other web frameworks, Pyramid does not eagerly create a global response object. Adding a response callback allows an application to register an action to be performed against a response object once it is created, usually in order to mutate it.

The pyramid.request.Request.add_response_callback() method is used to register a response callback.

A response callback is a callable which accepts two positional parameters: request and response. For example:

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def cache_callback(request, response):
    """Set the cache_control max_age for the response"""
    if request.exception is not None:
        response.cache_control.max_age = 360
request.add_response_callback(cache_callback)

No response callback is called if an unhandled exception happens in application code, or if the response object returned by a view callable is invalid. Response callbacks are, however, invoked when a exception view is rendered successfully: in such a case, the request.exception attribute of the request when it enters a response callback will be an exception object instead of its default value of None.

Response callbacks are called in the order they’re added (first-to-most-recently-added). All response callbacks are called after the NewResponse event is sent. Errors raised by response callbacks are not handled specially. They will be propagated to the caller of the Pyramid router application.

A response callback has a lifetime of a single request. If you want a response callback to happen as the result of every request, you must re-register the callback into every new request (perhaps within a subscriber of a NewRequest event).

Using Finished Callbacks

A finished callback is a function that will be called unconditionally by the Pyramid router at the very end of request processing. A finished callback can be used to perform an action at the end of a request unconditionally.

The pyramid.request.Request.add_finished_callback() method is used to register a finished callback.

A finished callback is a callable which accepts a single positional parameter: request. For example:

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import transaction

def commit_callback(request):
    '''commit or abort the transaction associated with request'''
    if request.exception is not None:
        transaction.abort()
    else:
        transaction.commit()
request.add_finished_callback(commit_callback)

Finished callbacks are called in the order they’re added (first-to-most-recently-added). Finished callbacks (unlike a response callback) are always called, even if an exception happens in application code that prevents a response from being generated.

The set of finished callbacks associated with a request are called very late in the processing of that request; they are essentially the very last thing called by the router before a request “ends”. They are called after response processing has already occurred in a top-level finally: block within the router request processing code. As a result, mutations performed to the request provided to a finished callback will have no meaningful effect, because response processing will have already occurred, and the request’s scope will expire almost immediately after all finished callbacks have been processed.

It is often necessary to tell whether an exception occurred within view callable code from within a finished callback: in such a case, the request.exception attribute of the request when it enters a response callback will be an exception object instead of its default value of None.

Errors raised by finished callbacks are not handled specially. They will be propagated to the caller of the Pyramid router application.

A finished callback has a lifetime of a single request. If you want a finished callback to happen as the result of every request, you must re-register the callback into every new request (perhaps within a subscriber of a NewRequest event).

Changing the Traverser

The default traversal algorithm that Pyramid uses is explained in The Traversal Algorithm. Though it is rarely necessary, this default algorithm can be swapped out selectively for a different traversal pattern via configuration.

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from pyramid.interfaces import ITraverser
from zope.interface import Interface
from myapp.traversal import Traverser

config.registry.registerAdapter(Traverser, (Interface,), ITraverser)

In the example above, myapp.traversal.Traverser is assumed to be a class that implements the following interface:

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class Traverser(object):
    def __init__(self, root):
        """ Accept the root object returned from the root factory """

    def __call__(self, request):
        """ Return a dictionary with (at least) the keys ``root``,
        ``context``, ``view_name``, ``subpath``, ``traversed``,
        ``virtual_root``, and ``virtual_root_path``.  These values are
        typically the result of a resource tree traversal.  ``root``
        is the physical root object, ``context`` will be a resource
        object, ``view_name`` will be the view name used (a Unicode
        name), ``subpath`` will be a sequence of Unicode names that
        followed the view name but were not traversed, ``traversed``
        will be a sequence of Unicode names that were traversed
        (including the virtual root path, if any) ``virtual_root``
        will be a resource object representing the virtual root (or the
        physical root if traversal was not performed), and
        ``virtual_root_path`` will be a sequence representing the
        virtual root path (a sequence of Unicode names) or None if
        traversal was not performed.

        Extra keys for special purpose functionality can be added as
        necessary.

        All values returned in the dictionary will be made available
        as attributes of the ``request`` object.
        """

More than one traversal algorithm can be active at the same time. For instance, if your root factory returns more than one type of object conditionally, you could claim that an alternate traverser adapter is for only one particular class or interface. When the root factory returned an object that implemented that class or interface, a custom traverser would be used. Otherwise, the default traverser would be used. For example:

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from pyramid.interfaces import ITraverser
from zope.interface import Interface
from myapp.traversal import Traverser
from myapp.resources import MyRoot

config.registry.registerAdapter(Traverser, (MyRoot,), ITraverser)

If the above stanza was added to a Pyramid __init__.py file’s main function, Pyramid would use the myapp.traversal.Traverser only when the application root factory returned an instance of the myapp.resources.MyRoot object. Otherwise it would use the default Pyramid traverser to do traversal.

Changing How pyramid.url.resource_url Generates a URL

When you add a traverser as described in Changing the Traverser, it’s often convenient to continue to use the pyramid.url.resource_url() API. However, since the way traversal is done will have been modified, the URLs it generates by default may be incorrect.

If you’ve added a traverser, you can change how resource_url() generates a URL for a specific type of resource by adding a registerAdapter call for pyramid.interfaces.IContextURL to your application:

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from pyramid.interfaces import ITraverser
from zope.interface import Interface
from myapp.traversal import URLGenerator
from myapp.resources import MyRoot

config.registry.registerAdapter(URLGenerator, (MyRoot, Interface),
                                IContextURL)

In the above example, the myapp.traversal.URLGenerator class will be used to provide services to resource_url() any time the context passed to resource_url is of class myapp.resources.MyRoot. The second argument in the (MyRoot, Interface) tuple represents the type of interface that must be possessed by the request (in this case, any interface, represented by zope.interface.Interface).

The API that must be implemented by a class that provides IContextURL is as follows:

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from zope.interface import Interface

class IContextURL(Interface):
    """ An adapter which deals with URLs related to a context.
    """
    def __init__(self, context, request):
        """ Accept the context and request """

    def virtual_root(self):
        """ Return the virtual root object related to a request and the
        current context"""

    def __call__(self):
        """ Return a URL that points to the context """

The default context URL generator is available for perusal as the class pyramid.traversal.TraversalContextURL in the traversal module of the Pylons GitHub Pyramid repository.

Using a View Mapper

The default calling conventions for view callables are documented in the Views chapter. You can change the way users define view callbles by employing a view mapper.

A view mapper is an object that accepts a set of keyword arguments and which returns a callable. The returned callable is called with the view callable object. The returned callable should itself return another callable which can be called with the “internal calling protocol” (context, request).

You can use a view mapper in a number of ways:

  • by setting a __view_mapper__ attribute (which is the view mapper object) on the view callable itself
  • by passing the mapper object to pyramid.config.Configurator.add_view() (or its declarative/decorator equivalents) as the mapper argument.
  • by registering a default view mapper.

Here’s an example of a view mapper that emulates (somewhat) a Pylons “controller”. The mapper is initialized with some keyword arguments. Its __call__ method accepts the view object (which will be a class). It uses the attr keyword argument it is passed to determine which attribute should be used as an action method. The wrapper method it returns accepts (context, request) and returns the result of calling the action method with keyword arguments implied by the matchdict after popping the action out of it. This somewhat emulates the Pylons style of calling action methods with routing parameters pulled out of the route matching dict as keyword arguments.

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# framework

class PylonsControllerViewMapper(object):
    def __init__(self, **kw):
        self.kw = kw

    def __call__(self, view):
        attr = self.kw['attr']
        def wrapper(context, request):
            matchdict = request.matchdict.copy()
            matchdict.pop('action', None)
            inst = view()
            meth = getattr(inst, attr)
            return meth(**matchdict)
        return wrapper

class BaseController(object):
    __view_mapper__ = PylonsControllerViewMapper

A user might make use of these framework components like so:

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# user application

from webob import Response
from pyramid.config import Configurator
import pyramid_handlers
from paste.httpserver import serve

class MyController(BaseController):
    def index(self, id):
        return Response(id)

if __name__ == '__main__':
    config = Configurator()
    config.include(pyramid_handlers)
    config.add_handler('one', '/{id}', MyController, action='index')
    config.add_handler('two', '/{action}/{id}', MyController)
    serve(config.make_wsgi_app())

The pyramid.config.Configurator.set_default_mapper() method can be used to set a default view mapper (overriding the superdefault view mapper used by Pyramid itself).

A single view registration can use a view mapper by passing the mapper as the mapper argument to add_view().

Registering Configuration Decorators

Decorators such as view_config don’t change the behavior of the functions or classes they’re decorating. Instead, when a scan is performed, a modified version of the function or class is registered with Pyramid.

You may wish to have your own decorators that offer such behaviour. This is possible by using the Venusian package in the same way that it is used by Pyramid.

By way of example, let’s suppose you want to write a decorator that registers the function it wraps with a Zope Component Architecture “utility” within the application registry provided by Pyramid. The application registry and the utility inside the registry is likely only to be available once your application’s configuration is at least partially completed. A normal decorator would fail as it would be executed before the configuration had even begun.

However, using Venusian, the decorator could be written as follows:

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import venusian
from pyramid.threadlocal import get_current_registry
from mypackage.interfaces import IMyUtility

class registerFunction(object):

    def __init__(self, path):
        self.path = path

    def register(self, scanner, name, wrapped):
        registry = scanner.config.registry
        registry.getUtility(IMyUtility).register(
            self.path, wrapped
            )

    def __call__(self, wrapped):
        venusian.attach(wrapped, self.register)
        return wrapped

This decorator could then be used to register functions throughout your code:

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@registerFunction('/some/path')
def my_function():
   do_stuff()

However, the utility would only be looked up when a scan was performed, enabling you to set up the utility in advance:

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from paste.httpserver import serve
from pyramid.config import Configurator

class UtilityImplementation:

    implements(ISomething)

    def __init__(self):
       self.registrations = {}

    def register(self,path,callable_):
       self.registrations[path]=callable_

if __name__ == '__main__':
    config = Configurator()
    config.registry.registerUtility(UtilityImplementation())
    config.scan()
    app = config.make_wsgi_app()
    serve(app, host='0.0.0.0')

For full details, please read the Venusian documentation.