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View Configuration

:term:`View configuration` controls how :term:`view lookup` operates in your application. In earlier chapters, you have been exposed to a few simple view configuration declarations without much explanation. In this chapter we will explore the subject in detail.

View Lookup and Invocation

:term:`View lookup` is the :app:`Pyramid` subsystem responsible for finding an invoking a :term:`view callable`. The view lookup subsystem is passed a :term:`context` and a :term:`request` object.

:term:`View configuration` information stored within in the :term:`application registry` is compared against the context and request by the view lookup subsystem in order to find the "best" view callable for the set of circumstances implied by the context and request.

:term:`View predicate` attributes are an important part of view configuration that enables the :term:`View lookup` subsystem to find and invoke the appropriate view. Predicate attributes can be thought of like "narrowers". In general, the greater number of predicate attributes possessed by a view's configuration, the more specific the circumstances need to be before the registered view callable will be invoked.

Mapping a Resource or URL Pattern to a View Callable

A developer makes a :term:`view callable` available for use within a :app:`Pyramid` application via :term:`view configuration`. A view configuration associates a view callable with a set of statements that determine the set of circumstances which must be true for the view callable to be invoked.

A view configuration statement is made about information present in the :term:`context` resource and the :term:`request`.

View configuration is performed in one of these ways:

Note

A package named pyramid_handlers (available from PyPI) provides an analogue of :term:`Pylons` -style "controllers", which are a special kind of view class which provides more automation when your application uses :term:`URL dispatch` solely.

View Configuration Parameters

All forms of view configuration accept the same general types of arguments.

Many arguments supplied during view configuration are :term:`view predicate` arguments. View predicate arguments used during view configuration are used to narrow the set of circumstances in which :term:`view lookup` will find a particular view callable.

In general, the fewer number of predicates which are supplied to a particular view configuration, the more likely it is that the associated view callable will be invoked. The greater the number supplied, the less likely. A view with five predicates will always be found and evaluated before a view with two, for example. All predicates must match for the associated view to be called.

This does not mean however, that :app:`Pyramid` "stops looking" when it finds a view registration with predicates that don't match. If one set of view predicates does not match, the "next most specific" view (if any) is consulted for predicates, and so on, until a view is found, or no view can be matched up with the request. The first view with a set of predicates all of which match the request environment will be invoked.

If no view can be found with predicates which allow it to be matched up with the request, :app:`Pyramid` will return an error to the user's browser, representing a "not found" (404) page. See :ref:`changing_the_notfound_view` for more information about changing the default notfound view.

Some view configuration arguments are non-predicate arguments. These tend to modify the response of the view callable or prevent the view callable from being invoked due to an authorization policy. The presence of non-predicate arguments in a view configuration does not narrow the circumstances in which the view callable will be invoked.

Non-Predicate Arguments

permission

The name of a :term:`permission` that the user must possess in order to invoke the :term:`view callable`. See :ref:`view_security_section` for more information about view security and permissions.

If permission is not supplied, no permission is registered for this view (it's accessible by any caller).

attr

The view machinery defaults to using the __call__ method of the :term:`view callable` (or the function itself, if the view callable is a function) to obtain a response. The attr value allows you to vary the method attribute used to obtain the response. For example, if your view was a class, and the class has a method named index and you wanted to use this method instead of the class' __call__ method to return the response, you'd say attr="index" in the view configuration for the view. This is most useful when the view definition is a class.

If attr is not supplied, None is used (implying the function itself if the view is a function, or the __call__ callable attribute if the view is a class).

renderer

Denotes the :term:`renderer` implementation which will be used to construct a :term:`response` from the associated view callable's return value. (see also :ref:`renderers_chapter`).

This is either a single string term (e.g. json) or a string implying a path or :term:`asset specification` (e.g. templates/views.pt) naming a :term:`renderer` implementation. If the renderer value does not contain a dot (.), the specified string will be used to look up a renderer implementation, and that renderer implementation will be used to construct a response from the view return value. If the renderer value contains a dot (.), the specified term will be treated as a path, and the filename extension of the last element in the path will be used to look up the renderer implementation, which will be passed the full path.

When the renderer is a path, although a path is usually just a simple relative pathname (e.g. templates/foo.pt, implying that a template named "foo.pt" is in the "templates" directory relative to the directory of the current :term:`package`), a path can be absolute, starting with a slash on UNIX or a drive letter prefix on Windows. The path can alternately be a :term:`asset specification` in the form some.dotted.package_name:relative/path, making it possible to address template assets which live in a separate package.

The renderer attribute is optional. If it is not defined, the "null" renderer is assumed (no rendering is performed and the value is passed back to the upstream :app:`Pyramid` machinery unmolested). Note that if the view callable itself returns a :term:`response` (see :ref:`the_response`), the specified renderer implementation is never called.

wrapper

The :term:`view name` of a different :term:`view configuration` which will receive the response body of this view as the request.wrapped_body attribute of its own :term:`request`, and the :term:`response` returned by this view as the request.wrapped_response attribute of its own request. Using a wrapper makes it possible to "chain" views together to form a composite response. The response of the outermost wrapper view will be returned to the user. The wrapper view will be found as any view is found: see :ref:`view_lookup`. The "best" wrapper view will be found based on the lookup ordering: "under the hood" this wrapper view is looked up via pyramid.view.render_view_to_response(context, request, 'wrapper_viewname'). The context and request of a wrapper view is the same context and request of the inner view.

If wrapper is not supplied, no wrapper view is used.

decorator
A :term:`dotted Python name` to function (or the function itself) which will be used to decorate the registered :term:`view callable`. The decorator function will be called with the view callable as a single argument. The view callable it is passed will accept (context, request). The decorator must return a replacement view callable which also accepts (context, request).
mapper
A Python object or :term:`dotted Python name` which refers to a :term:`view mapper`, or None. By default it is None, which indicates that the view should use the default view mapper. This plug-point is useful for Pyramid extension developers, but it's not very useful for 'civilians' who are just developing stock Pyramid applications. Pay no attention to the man behind the curtain.

Predicate Arguments

These arguments modify view lookup behavior. In general, the more predicate arguments that are supplied, the more specific, and narrower the usage of the configured view.

name

The :term:`view name` required to match this view callable. Read :ref:`traversal_chapter` to understand the concept of a view name.

If name is not supplied, the empty string is used (implying the default view).

context

An object representing a Python class that the :term:`context` resource must be an instance of or the :term:`interface` that the :term:`context` resource must provide in order for this view to be found and called. This predicate is true when the :term:`context` resource is an instance of the represented class or if the :term:`context` resource provides the represented interface; it is otherwise false.

If context is not supplied, the value None, which matches any resource, is used.

route_name

If route_name is supplied, the view callable will be invoked only when the named route has matched.

This value must match the name of a :term:`route configuration` declaration (see :ref:`urldispatch_chapter`) that must match before this view will be called. Note that the route configuration referred to by route_name will usually have a *traverse token in the value of its pattern, representing a part of the path that will be used by :term:`traversal` against the result of the route's :term:`root factory`.

If route_name is not supplied, the view callable will be have a chance of being invoked if no other route was matched. This is when the request/context pair found via :term:`resource location` does not indicate it matched any configured route.

request_type

This value should be an :term:`interface` that the :term:`request` must provide in order for this view to be found and called.

If request_type is not supplied, the value None is used, implying any request type.

This is an advanced feature, not often used by "civilians".

request_method

This value can either be one of the strings GET, POST, PUT, DELETE, or HEAD representing an HTTP REQUEST_METHOD. A view declaration with this argument ensures that the view will only be called when the request's method attribute (aka the REQUEST_METHOD of the WSGI environment) string matches the supplied value.

If request_method is not supplied, the view will be invoked regardless of the REQUEST_METHOD of the :term:`WSGI` environment.

request_param

This value can be any string. A view declaration with this argument ensures that the view will only be called when the :term:`request` has a key in the request.params dictionary (an HTTP GET or POST variable) that has a name which matches the supplied value.

If the value supplied has a = sign in it, e.g. request_params="foo=123", then the key (foo) must both exist in the request.params dictionary, and the value must match the right hand side of the expression (123) for the view to "match" the current request.

If request_param is not supplied, the view will be invoked without consideration of keys and values in the request.params dictionary.

containment

This value should be a reference to a Python class or :term:`interface` that a parent object in the context resource's :term:`lineage` must provide in order for this view to be found and called. The resources in your resource tree must be "location-aware" to use this feature.

If containment is not supplied, the interfaces and classes in the lineage are not considered when deciding whether or not to invoke the view callable.

See :ref:`location_aware` for more information about location-awareness.

xhr

This value should be either True or False. If this value is specified and is True, the :term:`WSGI` environment must possess an HTTP_X_REQUESTED_WITH (aka X-Requested-With) header that has the value XMLHttpRequest for the associated view callable to be found and called. This is useful for detecting AJAX requests issued from jQuery, Prototype and other Javascript libraries.

If xhr is not specified, the HTTP_X_REQUESTED_WITH HTTP header is not taken into consideration when deciding whether or not to invoke the associated view callable.

accept

The value of this argument represents a match query for one or more mimetypes in the Accept HTTP request header. If this value is specified, it must be in one of the following forms: a mimetype match token in the form text/plain, a wildcard mimetype match token in the form text/* or a match-all wildcard mimetype match token in the form */*. If any of the forms matches the Accept header of the request, this predicate will be true.

If accept is not specified, the HTTP_ACCEPT HTTP header is not taken into consideration when deciding whether or not to invoke the associated view callable.

header

This value represents an HTTP header name or a header name/value pair.

If header is specified, it must be a header name or a headername:headervalue pair.

If header is specified without a value (a bare header name only, e.g. If-Modified-Since), the view will only be invoked if the HTTP header exists with any value in the request.

If header is specified, and possesses a name/value pair (e.g. User-Agent:Mozilla/.*), the view will only be invoked if the HTTP header exists and the HTTP header matches the value requested. When the headervalue contains a : (colon), it will be considered a name/value pair (e.g. User-Agent:Mozilla/.* or Host:localhost). The value portion should be a regular expression.

Whether or not the value represents a header name or a header name/value pair, the case of the header name is not significant.

If header is not specified, the composition, presence or absence of HTTP headers is not taken into consideration when deciding whether or not to invoke the associated view callable.

path_info

This value represents a regular expression pattern that will be tested against the PATH_INFO WSGI environment variable to decide whether or not to call the associated view callable. If the regex matches, this predicate will be True.

If path_info is not specified, the WSGI PATH_INFO is not taken into consideration when deciding whether or not to invoke the associated view callable.

custom_predicates

If custom_predicates is specified, it must be a sequence of references to custom predicate callables. Use custom predicates when no set of predefined predicates do what you need. Custom predicates can be combined with predefined predicates as necessary. Each custom predicate callable should accept two arguments: context and request and should return either True or False after doing arbitrary evaluation of the context resource and/or the request. If all callables return True, the associated view callable will be considered viable for a given request.

If custom_predicates is not specified, no custom predicates are used.

View Configuration Using the @view_config Decorator

For better locality of reference, you may use the :class:`pyramid.view.view_config` decorator to associate your view functions with URLs instead of using imperative configuration for the same purpose.

Warning

Using this feature tends to slows down application startup slightly, as more work is performed at application startup to scan for view declarations.

Usage of the view_config decorator is a form of :term:`declarative configuration` in decorator form. :class:`~pyramid.view.view_config` can be used to associate :term:`view configuration` information -- as done via the equivalent imperative code -- with a function that acts as a :app:`Pyramid` view callable. All arguments to the :meth:`pyramid.config.Configurator.add_view` method (save for the view argument) are available in decorator form and mean precisely the same thing.

An example of the :class:`~pyramid.view.view_config` decorator might reside in a :app:`Pyramid` application module views.py:

Using this decorator as above replaces the need to add this imperative configuration stanza:

All arguments to view_config may be omitted. For example:

Such a registration as the one directly above implies that the view name will be my_view, registered with a context argument that matches any resource type, using no permission, registered against requests with any request method, request type, request param, route name, or containment.

The mere existence of a @view_config decorator doesn't suffice to perform view configuration. All that the decorator does is "annotate" the function with your configuration declarations, it doesn't process them. To make :app:`Pyramid` process your :class:`~pyramid.view.view_config` declarations, you must do use the scan method of a :class:`~pyramid.config.Configurator`:

Please see :ref:`decorations_and_code_scanning` for detailed information about what happens when code is scanned for configuration declarations resulting from use of decorators like :class:`~pyramid.view.view_config`.

See :ref:`configuration_module` for additional API arguments to the :meth:`~pyramid.config.Configurator.scan` method. For example, the method allows you to supply a package argument to better control exactly which code will be scanned.

@view_config Placement

A :class:`~pyramid.view.view_config` decorator can be placed in various points in your application.

If your view callable is a function, it may be used as a function decorator:

If your view callable is a class, the decorator can also be used as a class decorator in Python 2.6 and better (Python 2.5 and below do not support class decorators). All the arguments to the decorator are the same when applied against a class as when they are applied against a function. For example:

You can use the :class:`~pyramid.view.view_config` decorator as a simple callable to manually decorate classes in Python 2.5 and below without the decorator syntactic sugar, if you wish:

More than one :class:`~pyramid.view.view_config` decorator can be stacked on top of any number of others. Each decorator creates a separate view registration. For example:

This registers the same view under two different names.

The decorator can also be used against class methods:

When the decorator is used against a class method, a view is registered for the class, so the class constructor must accept an argument list in one of two forms: either it must accept a single argument request or it must accept two arguments, context, request.

The method which is decorated must return a :term:`response`.

Using the decorator against a particular method of a class is equivalent to using the attr parameter in a decorator attached to the class itself. For example, the above registration implied by the decorator being used against the amethod method could be spelled equivalently as the below:

View Registration Using :meth:`~pyramid.config.Configurator.add_view`

The :meth:`pyramid.config.Configurator.add_view` method within :ref:`configuration_module` is used to configure a view imperatively. The arguments to this method are very similar to the arguments that you provide to the @view_config decorator. For example:

The first argument, view, is required. It must either be a Python object which is the view itself or a :term:`dotted Python name` to such an object. All other arguments are optional. See :meth:`pyramid.config.Configurator.add_view` for more information.

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`.

Configuring View Security

If an :term:`authorization policy` is active, any :term:`permission` attached to a :term:`view configuration` found during view lookup will be verified. This will ensure that the currently authenticated user possesses that permission against the :term:`context` resource before the view function is actually called. Here's an example of specifying a permission in a view configuration using :meth:`~pyramid.config.Configurator.add_view`:

When an :term:`authorization policy` is enabled, this view will be protected with the add permission. The view will not be called if the user does not possess the add permission relative to the current :term:`context`. Instead the :term:`forbidden view` result will be returned to the client as per :ref:`protecting_views`.

:exc:`NotFound` Errors

It's useful to be able to debug :exc:`NotFound` error responses when they occur unexpectedly due to an application registry misconfiguration. To debug these errors, use the PYRAMID_DEBUG_NOTFOUND environment variable or the debug_notfound configuration file setting. Details of why a view was not found will be printed to stderr, and the browser representation of the error will include the same information. See :ref:`environment_chapter` for more information about how, and where to set these values.

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