.. _hooks_chapter: Using Hooks =========== "Hooks" can be used to influence the behavior of the :app:`Pyramid` framework in various ways. .. index:: single: not found view .. _changing_the_notfound_view: Changing the Not Found View --------------------------- When :app:`Pyramid` can't map a URL to view code, it invokes a :term:`not found view`, which is a :term:`view callable`. A default notfound view exists. The default not found view can be overridden through application configuration. This override can be done via :term:`imperative configuration` or :term:`ZCML`. The :term:`not found view` callable is a view callable like any other. The :term:`view configuration` which causes it to be a "not found" view consists only of naming the :exc:`pyramid.exceptions.NotFound` class as the ``context`` of the view configuration. .. topic:: Using Imperative Configuration If your application uses :term:`imperative configuration`, you can replace the Not Found view by using the :meth:`pyramid.configuration.Configurator.add_view` method to register an "exception view": .. code-block:: python :linenos: 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 Python :term:`view callable` you want to use to represent the Not Found view. .. topic:: Using ZCML If your application uses :term:`ZCML`, you can replace the Not Found view by placing something like the following ZCML in your ``configure.zcml`` file. .. code-block:: xml :linenos: Replace ``helloworld.views.notfound_view`` with the Python dotted name to the notfound view you want to use. Like any other view, the notfound view must accept at least a ``request`` parameter, or both ``context`` and ``request``. The ``request`` is the current :term:`request` representing the denied action. The ``context`` (if used in the call signature) will be the instance of the :exc:`pyramid.exceptions.NotFound` exception that caused the view to be called. Here's some sample code that implements a minimal NotFound view callable: .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPNotFound def notfound_view(request): return HTTPNotFound() .. note:: When a NotFound view callable is invoked, it is passed a :term:`request`. The ``exception`` attribute of the request will be an instance of the :exc:`pyramid.exceptions.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 :ref:`request_and_context_view_definitions`, the ``context`` passed as the first argument to the view callable will be the :exc:`pyramid.exceptions.NotFound` exception instance. If available, the *model* context will still be available as ``request.context``. .. index:: single: forbidden view .. _changing_the_forbidden_view: Changing the Forbidden View --------------------------- When :app:`Pyramid` can't authorize execution of a view based on the :term:`authorization policy` in use, it invokes a :term:`forbidden view`. The default forbidden response has a 401 status code and is very plain, but the view which generates it can be overridden as necessary using either :term:`imperative configuration` or :term:`ZCML`. The :term:`forbidden view` callable is a view callable like any other. The :term:`view configuration` which causes it to be a "not found" view consists only of naming the :exc:`pyramid.exceptions.Forbidden` class as the ``context`` of the view configuration. .. topic:: Using Imperative Configuration If your application uses :term:`imperative configuration`, you can replace the Forbidden view by using the :meth:`pyramid.configuration.Configurator.add_view` method to register an "exception view": .. code-block:: python :linenos: 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 :term:`view callable` you want to use to represent the Forbidden view. .. topic:: Using ZCML If your application uses :term:`ZCML`, you can replace the Forbidden view by placing something like the following ZCML in your ``configure.zcml`` file. .. code-block:: xml :linenos: Replace ``helloworld.views.forbidden_view`` with the Python dotted name to the forbidden view you want to use. 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 :term:`request` representing the denied action. Here's some sample code that implements a minimal forbidden view: .. code-block:: python :linenos: from pyramid.views import view_config @view_config(renderer='templates/login_form.pt') def forbidden_view(request): return {} .. note:: When a forbidden view callable is invoked, it is passed a :term:`request`. The ``exception`` attribute of the request will be an instance of the :exc:`pyramid.exceptions.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. .. warning:: the default forbidden view sends a response with a ``401 Unauthorized`` status code for backwards compatibility reasons. You can influence the status code of Forbidden responses by using an alternate forbidden view. For example, it would make sense to return a response with a ``403 Forbidden`` status code. .. index:: single: traverser .. _changing_the_traverser: Changing the Traverser ---------------------- The default :term:`traversal` algorithm that :app:`Pyramid` uses is explained in :ref:`traversal_algorithm`. Though it is rarely necessary, this default algorithm can be swapped out selectively for a different traversal pattern via configuration. Use an ``adapter`` stanza in your application's ``configure.zcml`` to change the default traverser: .. code-block:: xml :linenos: In the example above, ``myapp.traversal.Traverser`` is assumed to be a class that implements the following interface: .. code-block:: python :linenos: 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 an object graph traversal. ``root`` is the physical root object, ``context`` will be a model 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 model 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 :term:`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: .. code-block:: xml :linenos: If the above stanza was added to a ``configure.zcml`` file, :app:`Pyramid` would use the ``myapp.traversal.Traverser`` only when the application :term:`root factory` returned an instance of the ``myapp.models.MyRoot`` object. Otherwise it would use the default :app:`Pyramid` traverser to do traversal. .. index:: single: url generator Changing How :mod:`pyramid.url.model_url` Generates a URL ------------------------------------------------------------ When you add a traverser as described in :ref:`changing_the_traverser`, it's often convenient to continue to use the :func:`pyramid.url.model_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 :func:`pyramid.url.model_url` generates a URL for a specific type of :term:`context` by adding an adapter stanza for :class:`pyramid.interfaces.IContextURL` to your application's ``configure.zcml``: .. code-block:: xml :linenos: In the above example, the ``myapp.traversal.URLGenerator`` class will be used to provide services to :func:`pyramid.url.model_url` any time the :term:`context` passed to ``model_url`` is of class ``myapp.models.MyRoot``. The asterisk following represents the type of interface that must be possessed by the :term:`request` (in this case, any interface, represented by asterisk). The API that must be implemented by a class that provides :class:`pyramid.interfaces.IContextURL` is as follows: .. code-block:: python :linenos: 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 :class:`pyramid.traversal.TraversalContextURL` in the `traversal module `_ of the :term:`Pylons` GitHub Pyramid repository. .. _changing_the_request_factory: Changing the Request Factory ---------------------------- Whenever :app:`Pyramid` handles a :term:`WSGI` request, it creates a :term:`request` object based on the WSGI environment it has been passed. By default, an instance of the :class:`pyramid.request.Request` class is created to represent the request object. The class (aka "factory") that :app:`Pyramid` uses to create a request object instance can be changed by passing a ``request_factory`` argument to the constructor of the :term:`configurator`. This argument can be either a callable or a :term:`dotted Python name` representing a callable. .. code-block:: python :linenos: from pyramid.request import Request class MyRequest(Request): pass config = Configurator(request_factory=MyRequest) The same ``MyRequest`` class can alternately be registered via ZCML as a request factory through the use of the ZCML ``utility`` directive. In the below, we assume it lives in a package named ``mypackage.mymodule``. .. code-block:: xml :linenos: Lastly, if you're doing imperative configuration, and you'd rather do it after you've already constructed a :term:`configurator` it can also be registered via the :meth:`pyramid.configuration.Configurator.set_request_factory` method: .. code-block:: python :linenos: from pyramid.configuration import Configurator from pyramid.request import Request class MyRequest(Request): pass config = Configurator() config.set_request_factory(MyRequest) .. _adding_renderer_globals: Adding Renderer Globals ----------------------- Whenever :app:`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 :mod:`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 :app:`Pyramid` will call every time a renderer is invoked can be added by passing a ``renderer_globals_factory`` argument to the constructor of the :term:`configurator`. This callback can either be a callable object or a :term:`dotted Python name` representing such a callable. .. code-block:: python :linenos: 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 :ref:`renderer_system_values` for discription of the values present in the system dictionary. A renderer globals factory can alternately be registered via ZCML as a through the use of the ZCML ``utility`` directive. In the below, we assume a ``renderers_globals_factory`` function lives in a package named ``mypackage.mymodule``. .. code-block:: xml :linenos: Lastly, if you're doing imperative configuration, and you'd rather do it after you've already constructed a :term:`configurator` it can also be registered via the :meth:`pyramid.configuration.Configurator.set_renderer_globals_factory` method: .. code-block:: python :linenos: from pyramid.configuration 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 :ref:`beforerender_event`. .. _beforerender_event: Using The Before Render Event ----------------------------- Subscribers to the :class:`pyramid.events.BeforeRender` event may introspect the and modify the set of :term:`renderer globals` before they are passed to a :term:`renderer`. This event object iself has a dictionary-like interface that can be used for this purpose. For example: .. code-block:: python :linenos: 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 :term:`renderer` is invoked (but *after* the application-level renderer globals factory added via :class:`pyramid.configuration.Configurator.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 :exc:`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 :class:`pyramid.events.BeforeRender` subscribers and renderer globals factories must be unique. See the API documentation for the :class:`pyramid.events.BeforeRender` event interface at :class:`pyramid.interfaces.IBeforeRender`. Another mechanism which allows event subscribers more control when adding renderer global values exists in :ref:`adding_renderer_globals`. .. _using_response_callbacks: Using Response Callbacks ------------------------ Unlike many other web frameworks, :app:`Pyramid` does not eagerly create a global response object. Adding a :term:`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 :meth:`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: .. code-block:: python :linenos: 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 :term:`view callable` is invalid. Response callbacks *are*, however, invoked when a :term:`exception view` is rendered successfully: in such a case, the :attr:`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 :class:`pyramid.events.NewResponse` event is sent. Errors raised by response callbacks are not handled specially. They will be propagated to the caller of the :app:`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 :class:`pyramid.events.NewRequest` event). .. _using_finished_callbacks: Using Finished Callbacks ------------------------ A :term:`finished callback` is a function that will be called unconditionally by the :app:`Pyramid` :term:`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 :meth:`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: .. code-block:: python :linenos: 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 :term:`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 :term:`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 :term:`view callable` code from within a finished callback: in such a case, the :attr:`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 :app:`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 :class:`pyramid.events.NewRequest` event). .. _registering_configuration_decorators: Registering Configuration Decorators ------------------------------------ Decorators such as :class:`pyramid.view.view_config` don't change the behavior of the functions or classes they're decorating. Instead, when a :term:`scan` is performed, a modified version of the function or class is registered with :app:`Pyramid`. You may wish to have your own decorators that offer such behaviour. This is possible by using the :term:`Venusian` package in the same way that it is used by :app:`Pyramid`. By way of example, let's suppose you want to write a decorator that registers the function it wraps with a :term:`Zope Component Architecture` "utility" within the :term:`application registry` provided by :app:`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 :term:`Venusian`, the decorator could be written as follows: .. code-block:: python :linenos: 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 = get_current_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: .. code-block:: python :linenos: @registerFunction('/some/path') def my_function(): do_stuff() However, the utility would only be looked up when a :term:`scan` was performed, enabling you to set up the utility in advance: .. code-block:: python :linenos: from paste.httpserver import serve from pyramid.configuration 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.begin() config.registry.registerUtility(UtilityImplementation()) config.scan() config.end() app = config.make_wsgi_app() serve(app, host='0.0.0.0') For full details, please read the `Venusian documentation `_.