The view ZCML directive has many possible attributes. Some of the attributes are descriptive or influence rendering. Other attributes are :term:`predicate` attributes, meaning that they imply an evaluation to true or false when view lookup is performed.
All predicates named in a view configuration must evaluate to true in order for the view callable it names to be considered "invokable" for a given request. See :ref:`view_lookup` for a description of how a view configuration matches (or doesn't match) during a request.
The possible attributes of the view ZCML directive are described below. They are divided into predicate and non-predicate categories.
- The :term:`dotted Python name` to a :term:`view callable`. This attribute is required unless a renderer attribute also exists. If a renderer attribute exists on the directive, this attribute defaults to a view that returns an empty dictionary (see :ref:`views_which_use_a_renderer`).
- The name of a permission that the user must possess in order to call the view. See :ref:`view_security_section` for more information about view security and permissions.
- The view machinery defaults to using the __call__ method of the view callable (or the function itself, if the view callable is a function) to obtain a response dictionary. 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.
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). If the renderer value is a single term (does not contain a dot .), the specified term 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 term 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. The renderer implementation will be used to construct a response from the view return value.
Note that if the view itself returns a response (see :ref:`the_response`), the specified renderer implementation is never called.
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 in which the ZCML file is defined), 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 BFG machinery unmolested).
- The :term:`view name` (not an object dotted name) of another view declared elsewhere in ZCML (or via the @view_config decorator) which will receive the response body of this view as the request.wrapped_body attribute of its own request, and the 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 this attribute is unspecified, no view wrapping is done.
- The view name. Read the :ref:`traversal_chapter` to understand the concept of a view name.
- A :term:`dotted Python name` representing the Python class that the :term:`context` must be an instance of, or the :term:`interface` that the :term:`context` must provide in order for this view to be found and called. This predicate is true when the :term:`context` is an instance of the represented class or if the :term:`context` provides the represented interface; it is otherwise false. An alternate name for this attribute is for (this is an older spelling).
- This attribute services an advanced feature that isn't often used unless you want to perform traversal after a route has matched. This value must match the name of a <route> declaration (see :ref:`urldispatch_chapter`) that must match before this view will be called. Note that the route configuration referred to by route_name usually has a *traverse token in the value of its path, representing a part of the path that will be used by traversal against the result of the route's :term:`root factory`. See :ref:`hybrid_chapter` for more information on using this advanced feature.
- This value should be a :term:`dotted Python name` string representing the :term:`interface` that the :term:`request` must have in order for this view to be found and called. The presence of this attribute is largely for backwards compatibility with older iterations of this framework.
- 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 attribute ensures that the view will only be called when the request's method (aka REQUEST_METHOD) string matches the supplied value.
- This value can be any string. A view declaration with this attribute ensures that the view will only be called when the 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 to the attribute 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.
- This value should be a :term:`dotted Python name` string representing the class that a graph traversal parent object of the :term:`context` must be an instance of (or :term:`interface` that a parent object must provide) in order for this view to be found and called. Your resources must be "location-aware" to use this feature. See :ref:`location_aware` for more information about location-awareness.
- The value of this attribute 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.
- The value of this attribute represents an HTTP header name or a header name/value pair. If the value contains a : (colon), it will be considered a name/value pair (e.g. User-Agent:Mozilla/.* or Host:localhost). The value of an attribute that represent a name/value pair should be a regular expression. If the value does not contain a colon, the entire value will be considered to be the header name (e.g. If-Modified-Since). If the value evaluates to a header name only without a value, the header specified by the name must be present in the request for this predicate to be true. If the value evaluates to a header name/value pair, the header specified by the name must be present in the request and the regular expression specified as the value must match the header value. Whether or not the value represents a header name or a header name/value pair, the case of the header name is not significant.
- The value of this attribute represents a regular expression pattern that will be tested against the PATH_INFO WSGI environment variable. If the regex matches, this predicate will be true.
- This value should be a sequence of references to custom predicate callables (e.g. dotted.name.one dotted.name.two, if used in ZCML; a :term:`dotted Python name` to each callable separated by a space). 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 and/or the request. If all callables return True, the associated view callable will be considered viable for a given request.
- A :term:`dotted Python name` to a function that 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).
- A :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.
Registering A Default View for a Class
Registering A View With a Predicate
You can also add a :term:`view configuration` via:
- Using the :class:`pyramid.view.view_config` class as a decorator.
- Using the :meth:`pyramid.config.Configurator.add_view` method.
See also :ref:`views_chapter`.