.. _views_chapter: Views ===== The primary job of any :mod:`pyramid` application is is to find and invoke a :term:`view callable` when a :term:`request` reaches the application. View callables are bits of code written by you -- the application developer -- which do something interesting in response to a request made to your application. .. note:: A :mod:`pyramid` :term:`view callable` is often referred to in conversational shorthand as a :term:`view`. In this documentation, however, we need to use less ambiguous terminology because there are significant differences between view *configuration*, the code that implements a view *callable*, and the process of view *lookup*. The chapter named :ref:`contextfinding_chapter` describes how, using information from the :term:`request`, a :term:`context` and a :term:`view name` are computed. But neither the context nor the view name found are very useful unless those elements can eventually be mapped to a :term:`view callable`. The job of actually locating and invoking the "best" :term:`view callable` is the job of the :term:`view lookup` subsystem. The view lookup subsystem compares information supplied by :term:`context finding` against :term:`view configuration` statements made by the developer stored in the :term:`application registry` to choose the most appropriate view callable for a specific request. This chapter provides documentation detailing the process of creating view callables, documentation about performing view configuration, and a detailed explanation of view lookup. View Callables -------------- No matter how a view callable is eventually found, all view callables used by :mod:`pyramid` must be constructed in the same way, and must return the same kind of return value. Most view callables accept a single argument named ``request``. This argument represents a :mod:`pyramid` :term:`Request` object. A request object encapsulates a WSGI environment as represented to :mod:`pyramid` by the upstream :term:`WSGI` server. A view callable may always return a :mod:`Pyramid` :term:`Response` object directly. It may optionally return another arbitrary non-Response value: if a view callable returns a non-Response result, the result must be converted into a response by the :term:`renderer` associated with the :term:`view configuration` for the view. View callables can be functions, instances, or classes. View callables can optionally be defined with an alternate calling convention. .. index:: single: view calling convention single: view function .. _function_as_view: Defining a View Callable as a Function ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The easiest way to define a view callable is to create a function that accepts a single argument named ``request`` and which returns a :term:`Response` object. For example, this is a "hello world" view callable implemented as a function: .. code-block:: python :linenos: from pyramid.response import Response def hello_world(request): return Response('Hello world!') .. index:: single: view calling convention single: view class .. _class_as_view: Defining a View Callable as a Class ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A view callable may also be a class instead of a function. When a view callable is a class, the calling semantics are slightly different than when it is a function or another non-class callable. When a view callable is a class, the class' ``__init__`` is called with a ``request`` parameter. As a result, an instance of the class is created. Subsequently, that instance's ``__call__`` method is invoked with no parameters. Views defined as classes must have the following traits: - an ``__init__`` method that accepts a ``request`` as its sole positional argument or an ``__init__`` method that accepts two arguments: ``request`` and ``context`` as per :ref:`request_and_context_view_definitions`. - a ``__call__`` method that accepts no parameters and which returns a response. For example: .. code-block:: python :linenos: from pyramid.response import Response class MyView(object): def __init__(self, request): self.request = request def __call__(self): return Response('hello') The request object passed to ``__init__`` is the same type of request object described in :ref:`function_as_view`. If you'd like to use a different attribute than ``__call__`` to represent the method expected to return a response, you can use an ``attr`` value as part of view configuration. See :ref:`view_configuration_parameters`. .. index:: single: view calling convention .. _request_and_context_view_definitions: Context-And-Request View Callable Definitions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Usually, view callables are defined to accept only a single argument: ``request``. However, view callables may alternately be defined as classes or functions (or any callable) that accept *two* positional arguments: a :term:`context` as the first argument and a :term:`request` as the second argument. The :term:`context` and :term:`request` arguments passed to a view function defined in this style can be defined as follows: context An instance of a :term:`context` found via graph :term:`traversal` or :term:`URL dispatch`. If the context is found via traversal, it will be a :term:`model` object. request A :mod:`pyramid` Request object representing the current WSGI request. The following types work as view callables in this style: #. Functions that accept two arguments: ``context``, and ``request``, e.g.: .. code-block:: python :linenos: from pyramid.response import Response def view(context, request): return Response('OK') #. Classes that have an ``__init__`` method that accepts ``context, request`` and a ``__call__`` which accepts no arguments, e.g.: .. code-block:: python :linenos: from pyramid.response import Response class view(object): def __init__(self, context, request): self.context = context self.request = request def __call__(self): return Response('OK') #. Arbitrary callables that have a ``__call__`` method that accepts ``context, request``, e.g.: .. code-block:: python :linenos: from pyramid.response import Response class View(object): def __call__(self, context, request): return Response('OK') view = View() # this is the view callable This style of calling convention is most useful for :term:`traversal` based applications, where the context object is frequently used within the view callable code itself. No matter which view calling convention is used, the view code always has access to the context via ``request.context``. .. index:: single: view response single: response .. _the_response: View Callable Responses ~~~~~~~~~~~~~~~~~~~~~~~ A view callable may always return an object that implements the :mod:`pyramid` :term:`Response` interface. The easiest way to return something that implements the :term:`Response` interface is to return a :class:`pyramid.response.Response` object instance directly. For example: .. code-block:: python :linenos: from pyramid.response import Response def view(request): return Response('OK') You don't need to always use :class:`pyramid.response.Response` to represent a response. :mod:`pyramid` provides a range of different "exception" classes which can act as response objects too. For example, an instance of the class :class:`pyramid.httpexceptions.HTTPFound` is also a valid response object (see :ref:`http_redirect`). A view can actually return any object that has the following attributes (these attributes form the notional "Pyramid Response interface"): status The HTTP status code (including the name) for the response as a string. E.g. ``200 OK`` or ``401 Unauthorized``. headerlist A sequence of tuples representing the list of headers that should be set in the response. E.g. ``[('Content-Type', 'text/html'), ('Content-Length', '412')]`` app_iter An iterable representing the body of the response. This can be a list, e.g. ``['Hello world!']`` or it can be a file-like object, or any other sort of iterable. Furthermore, a view needn't *always* return a Response object. If a view happens to return something which does not implement the Pyramid Response interface, :mod:`pyramid` will attempt to use a :term:`renderer` to construct a response. For example: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config(renderer='json') def hello_world(request): return {'content':'Hello!'} The above example returns a *dictionary* from the view callable. A dictionary does not implement the Pyramid response interface, so you might believe that this example would fail. However, since a ``renderer`` is associated with the view callable through its :term:`view configuration` (in this case, using a ``renderer`` argument passed to :func:`pyramid.view.view_config`), if the view does *not* return a Response object, the renderer will attempt to convert the result of the view to a response on the developer's behalf. Of course, if no renderer is associated with a view's configuration, returning anything except an object which implements the Response interface will result in an error. And, if a renderer *is* used, whatever is returned by the view must be compatible with the particular kind of renderer used, or an error may occur during view invocation. One exception exists: it is *always* OK to return a Response object, even when a ``renderer`` is configured. If a view callable returns a response object from a view that is configured with a renderer, the renderer is bypassed entirely. Various types of renderers exist, including serialization renderers and renderers which use templating systems. See also :ref:`views_which_use_a_renderer`. .. index:: single: view http redirect single: http redirect (from a view) .. _http_redirect: Using a View Callable to Do A HTTP Redirect ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can issue an HTTP redirect from within a view by returning a particular kind of response. .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPFound def myview(request): return HTTPFound(location='http://example.com') All exception types from the :mod:`pyramid.httpexceptions` module implement the :term:`Response` interface; any can be returned as the response from a view. See :mod:`pyramid.httpexceptions` for the documentation for the ``HTTPFound`` exception; it also includes other response types that imply other HTTP response codes, such as ``HTTPUnauthorized`` for ``401 Unauthorized``. .. index:: single: renderer single: view renderer .. _views_which_use_a_renderer: Writing View Callables Which Use a Renderer ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ View callables needn't always return a Response object. Instead, they may return an arbitrary Python object, with the expectation that a :term:`renderer` will convert that object into a response instance on behalf of the developer. Some renderers use a templating system; other renderers use object serialization techniques. If you do not define a ``renderer`` attribute in :term:`view configuration` for an associated :term:`view callable`, no renderer is associated with the view. In such a configuration, an error is raised when a view callable does not return an object which implements the :term:`Response` interface, documented within :ref:`the_response`. View configuration can vary the renderer associated with a view callable via the ``renderer`` attribute. For example, this call to :meth:`pyramid.configuration.Configurator.add_view` associates the ``json`` renderer with a view callable: .. code-block:: python :linenos: config.add_view('myproject.views.my_view', renderer='json') When this configuration is added to an application, the ``myproject.views.my_view`` view callable will now use a ``json`` renderer, which renders view return values to a :term:`JSON` serialization. Other built-in renderers include renderers which use the :term:`Chameleon` templating language to render a dictionary to a response. If the :term:`view callable` associated with a :term:`view configuration` returns a Response object directly (an object with the attributes ``status``, ``headerlist`` and ``app_iter``), any renderer associated with the view configuration is ignored, and the response is passed back to :mod:`pyramid` unmolested. For example, if your view callable returns an instance of the :class:`pyramid.httpexceptions.HTTPFound` class as a response, no renderer will be employed. .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPFound def view(request): return HTTPFound(location='http://example.com') # renderer avoided Views which use a renderer can vary non-body response attributes (such as headers and the HTTP status code) by attaching properties to the request. See :ref:`response_request_attrs`. Additional renderers can be added to the system as necessary (see :ref:`adding_and_overriding_renderers`). .. index:: single: renderers (built-in) single: built-in renderers .. _built_in_renderers: Built-In Renderers ~~~~~~~~~~~~~~~~~~ Several built-in "renderers" exist in :mod:`pyramid`. These renderers can be used in the ``renderer`` attribute of view configurations. .. index:: pair: renderer; string ``string``: String Renderer +++++++++++++++++++++++++++ The ``string`` renderer is a renderer which renders a view callable result to a string. If a view callable returns a non-Response object, and the ``string`` renderer is associated in that view's configuration, the result will be to run the object through the Python ``str`` function to generate a string. Note that if a Unicode object is returned by the view callable, it is not ``str()`` -ified. Here's an example of a view that returns a dictionary. If the ``string`` renderer is specified in the configuration for this view, the view will render the returned dictionary to the ``str()`` representation of the dictionary: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config(renderer='string') def hello_world(request): return {'content':'Hello!'} The body of the response returned by such a view will be a string representing the ``str()`` serialization of the return value: .. code-block:: python :linenos: {'content': 'Hello!'} Views which use the string renderer can vary non-body response attributes by attaching properties to the request. See :ref:`response_request_attrs`. .. index:: pair: renderer; JSON ``json``: JSON Renderer +++++++++++++++++++++++ The ``json`` renderer is a renderer which renders view callable results to :term:`JSON`. If a view callable returns a non-Response object it is called. It passes the return value through the ``json.dumps`` standard library function, and wraps the result in a response object. It also sets the response content-type to ``application/json``. Here's an example of a view that returns a dictionary. If the ``json`` renderer is specified in the configuration for this view, the view will render the returned dictionary to a JSON serialization: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config(renderer='json') def hello_world(request): return {'content':'Hello!'} The body of the response returned by such a view will be a string representing the JSON serialization of the return value: .. code-block:: python :linenos: '{"content": "Hello!"}' The return value needn't be a dictionary, but the return value must contain values serializable by :func:`json.dumps`. You can configure a view to use the JSON renderer by naming ``json`` as the ``renderer`` argument of a view configuration, e.g. by using :meth:`pyramid.configuration.Configurator.add_view`: .. code-block:: python :linenos: config.add_view('myproject.views.hello_world', name='hello' context='myproject.models.Hello', renderer='json') Views which use the JSON renderer can vary non-body response attributes by attaching properties to the request. See :ref:`response_request_attrs`. .. index:: pair: renderer; chameleon .. _chameleon_template_renderers: ``*.pt`` or ``*.txt``: Chameleon Template Renderers +++++++++++++++++++++++++++++++++++++++++++++++++++ Two built-in renderers exist for :term:`Chameleon` templates. If the ``renderer`` attribute of a view configuration is an absolute path, a relative path or :term:`resource specification` which has a final path element with a filename extension of ``.pt``, the Chameleon ZPT renderer is used. See :ref:`chameleon_zpt_templates` for more information about ZPT templates. If the ``renderer`` attribute of a view configuration is an absolute path or a :term:`resource specification` which has a final path element with a filename extension of ``.txt``, the :term:`Chameleon` text renderer is used. See :ref:`chameleon_zpt_templates` for more information about Chameleon text templates. The behavior of these renderers is the same, except for the engine used to render the template. When a ``renderer`` attribute that names a template path or :term:`resource specification` (e.g. ``myproject:templates/foo.pt`` or ``myproject:templates/foo.txt``) is used, the view must return a :term:`Response` object or a Python *dictionary*. If the view callable with an associated template returns a Python dictionary, the named template will be passed the dictionary as its keyword arguments, and the template renderer implementation will return the resulting rendered template in a response to the user. If the view callable returns anything but a Response object or a dictionary, an error will be raised. Before passing keywords to the template, the keyword arguments derived from the dictionary returned by the view are augmented. The callable object -- whatever object was used to define the ``view`` -- will be automatically inserted into the set of keyword arguments passed to the template as the ``view`` keyword. If the view callable was a class, the ``view`` keyword will be an instance of that class. Also inserted into the keywords passed to the template are ``renderer_name`` (the string used in the ``renderer`` attribute of the directive), ``renderer_info`` (an object containing renderer-related information), ``context`` (the context of the view used to render the template), and ``request`` (the request passed to the view used to render the template). Here's an example view configuration which uses a Chameleon ZPT renderer: .. code-block:: python :linenos: # config is an instance of pyramid.configuration.Configurator config.add_view('myproject.views.hello_world', name='hello', context='myproject.models.Hello', renderer='myproject:templates/foo.pt') Here's an example view configuration which uses a Chameleon text renderer: .. code-block:: python :linenos: config.add_view('myproject.views.hello_world', name='hello', context='myproject.models.Hello', renderer='myproject:templates/foo.txt') Views which use a Chameleon renderer can vary response attributes by attaching properties to the request. See :ref:`response_request_attrs`. .. index:: pair: renderer; mako .. _mako_template_renderers: ``*.mak`` or ``*.mako``: Mako Template Renderer +++++++++++++++++++++++++++++++++++++++++++++++ The ``Mako`` template renderer is a renderer which renders a Mako template. When used, the view must return a Response object or a Python *dictionary*. The dictionary items will then be used in the global template space. If the view callable returns anything but a Response object or a dictionary, an error will be raised. When using the ``renderer`` attribute to specify a Mako template, the template can be specified in two ways. First, a relative path can be used to name a Mako template relative to the configured Mako template directories. Second, a :term:`resource specification` can be used to locate a template to render. These two styles of naming a template to render also carry through to Mako templates, so that Mako template's can inherit using a :term:`resource specification` if desired. Here's an example view configuration which uses a relative path: .. code-block:: python :linenos: # config is an instance of pyramid.configuration.Configurator config.add_view('myproject.views.hello_world', name='hello', context='myproject.models.Hello', renderer='foo.mak') It's important to note that in Mako's case, the 'relative' path name ``foo.mak`` above is not relative to the package, but is relative to the directory (or directories) configured for Mako via the ``mako.directories`` configuration file setting. The renderer can also be provided in :term:`resource specification` format. Here's an example view configuration which uses a :term:`resource specification`: .. code-block:: python :linenos: config.add_view('myproject.views.hello_world', name='hello', context='myproject.models.Hello', renderer='mypackage:templates/foo.mak') The above configuration will use the file named ``foo.mak`` in the ``templates`` directory of the ``mypackage`` package. The ``Mako`` template renderer can take additional arguments beyond the standard ``reload_templates`` setting, see the :ref:`environment_chapter` for additional :ref:`mako_template_renderer_settings`. .. index:: single: response headers (from a renderer) single: renderer response headers .. _response_request_attrs: Varying Attributes of Rendered Responses ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Before a response that is constructed as the result of the use of a :term:`renderer` is returned to :mod:`pyramid`, several attributes of the request are examined which have the potential to influence response behavior. View callables that don't directly return a response should set these values on the ``request`` object via ``setattr`` within the view callable to influence associated response attributes. ``response_content_type`` Defines the content-type of the resulting response, e.g. ``text/xml``. ``response_headerlist`` A sequence of tuples describing cookie values that should be set in the response, e.g. ``[('Set-Cookie', 'abc=123'), ('X-My-Header', 'foo')]``. ``response_status`` A WSGI-style status code (e.g. ``200 OK``) describing the status of the response. ``response_charset`` The character set (e.g. ``UTF-8``) of the response. ``response_cache_for`` A value in seconds which will influence ``Cache-Control`` and ``Expires`` headers in the returned response. The same can also be achieved by returning various values in the ``response_headerlist``, this is purely a convenience. For example, if you need to change the response status from within a view callable that uses a renderer, assign the ``response_status`` attribute to the request before returning a result: .. code-block:: python :linenos: from pyramid.view import view_config @view_config(name='gone', renderer='templates/gone.pt') def myview(request): request.response_status = '404 Not Found' return {'URL':request.URL} For more information on attributes of the request, see the API documentation in :ref:`request_module`. .. index:: single: renderer (adding) .. _adding_and_overriding_renderers: Adding and Overriding Renderers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ New templating systems and serializers can be associated with :mod:`pyramid` renderer names. To this end, configuration declarations can be made which override an existing :term:`renderer factory` and which add a new renderer factory. Renderers can be registered imperatively using the :meth:`pyramid.configuration.Configurator.add_renderer` API. .. note:: The tasks described in this section can also be performed via :term:`declarative configuration`. See :ref:`zcml_adding_and_overriding_renderers`. For example, to add a renderer which renders views which have a ``renderer`` attribute that is a path that ends in ``.jinja2``: .. code-block:: python :linenos: config.add_renderer('.jinja2', 'mypackage.MyJinja2Renderer') The first argument is the renderer name. The second argument is a reference to an implementation of a :term:`renderer factory` or a :term:`dotted Python name` referring to such an object. .. _adding_a_renderer: Adding a New Renderer +++++++++++++++++++++ You may a new renderer by creating and registering a :term:`renderer factory`. A renderer factory implementation is usually a class which has the following interface: .. code-block:: python :linenos: class RendererFactory: def __init__(self, info): """ Constructor: ``info`` will be an object having the the following attributes: ``name`` (the renderer name), ``package`` (the package that was 'current' at the time the renderer was registered), ``type`` (the renderer type name), ``registry`` (the current application registry) and ``settings`` (the deployment settings dictionary). """ def __call__(self, value, system): """ Call a the renderer implementation with the value and the system value passed in as arguments and return the result (a string or unicode object). The value is the return value of a view. The system value is a dictionary containing available system values (e.g. ``view``, ``context``, and ``request``). """ The formal interface definition of the ``info`` object passed to a renderer factory constructor is available as :class:`pyramid.interfaces.IRendererInfo`. There are essentially two different kinds of renderer factories: - A renderer factory which expects to accept a :term:`resource specification` or an absolute path as the ``name`` attribute of the ``info`` object fed to its constructor. These renderer factories are registered with a ``name`` value that begins with a dot (``.``). These types of renderer factories usually relate to a file on the filesystem, such as a template. - A renderer factory which expects to accept a token that does not represent a filesystem path or a resource specification in the ``name`` attribute of the ``info`` object fed to its constructor. These renderer factories are registered with a ``name`` value that does not begin with a dot. These renderer factories are typically object serializers. .. sidebar:: Resource Specifications A resource specification is a colon-delimited identifier for a :term:`resource`. The colon separates a Python :term:`package` name from a package subpath. For example, the resource specification ``my.package:static/baz.css`` identifies the file named ``baz.css`` in the ``static`` subdirectory of the ``my.package`` Python :term:`package`. Here's an example of the registration of a simple renderer factory via :meth:`pyramid.configuration.Configurator.add_renderer`: .. code-block:: python :linenos: # config is an instance of pyramid.configuration.Configurator config.add_renderer(name='amf', factory='my.package.MyAMFRenderer') Adding the above code to your application startup configuration will allow you to use the ``my.package.MyAMFRenderer`` renderer factory implementation in view configurations by referring to it as ``amf`` in the ``renderer`` attribute of a :term:`view configuration`: .. code-block:: python :linenos: from pyramid.view import view_config @view_config(renderer='amf') def myview(request): return {'Hello':'world'} At startup time, when a :term:`view configuration` is encountered which has a ``name`` argument that does not contain a dot, such as the above ``amf`` is encountered, the full value of the ``name`` attribute is used to construct a renderer from the associated renderer factory. In this case, the view configuration will create an instance of an ``AMFRenderer`` for each view configuration which includes ``amf`` as its renderer value. The ``name`` passed to the ``AMFRenderer`` constructor will always be ``amf``. Here's an example of the registration of a more complicated renderer factory, which expects to be passed a filesystem path: .. code-block:: python :linenos: config.add_renderer(name='.jinja2', factory='my.package.MyJinja2Renderer') Adding the above code to your application startup will allow you to use the ``my.package.MyJinja2Renderer`` renderer factory implementation in view configurations by referring to any ``renderer`` which *ends in* ``.jinja`` in the ``renderer`` attribute of a :term:`view configuration`: .. code-block:: python :linenos: from pyramid.view import view_config @view_config(renderer='templates/mytemplate.jinja2') def myview(request): return {'Hello':'world'} When a :term:`view configuration` which has a ``name`` attribute that does contain a dot, such as ``templates/mytemplate.jinja2`` above is encountered at startup time, the value of the name attribute is split on its final dot. The second element of the split is typically the filename extension. This extension is used to look up a renderer factory for the configured view. Then the value of ``renderer`` is passed to the factory to create a renderer for the view. In this case, the view configuration will create an instance of a ``Jinja2Renderer`` for each view configuration which includes anything ending with ``.jinja2`` as its ``renderer`` value. The ``name`` passed to the ``Jinja2Renderer`` constructor will be whatever the user passed as ``renderer=`` to the view configuration. See also :ref:`renderer_directive` and :meth:`pyramid.configuration.Configurator.add_renderer`. Overriding an Existing Renderer +++++++++++++++++++++++++++++++ You can associate more than one filename extension with the same existing renderer implementation as necessary if you need to use a different file extension for the same kinds of templates. For example, to associate the ``.zpt`` extension with the Chameleon ZPT renderer factory, use the :meth:`pyramid.configuration.Configurator.add_renderer` method: .. code-block:: python :linenos: config.add_renderer('.zpt', 'pyramid.chameleon_zpt.renderer_factory') After you do this, :mod:`pyramid` will treat templates ending in both the ``.pt`` and ``.zpt`` filename extensions as Chameleon ZPT templates. To override the default mapping in which files with a ``.pt`` extension are rendered via a Chameleon ZPT page template renderer, use a variation on the following in your application's startup code: .. code-block:: python :linenos: config.add_renderer('.pt', 'mypackage.pt_renderer') After you do this, the :term:`renderer factory` in ``mypackage.pt_renderer`` will be used to render templates which end in ``.pt``, replacing the default Chameleon ZPT renderer. To associate a *default* renderer with *all* view configurations (even ones which do not possess a ``renderer`` attribute), use a variation on the following (ie. pass ``None`` as the ``name`` attribute to the renderer tag): .. code-block:: python :linenos: config.add_renderer(None, 'mypackage.json_renderer_factory') .. index:: single: view exceptions .. _special_exceptions_in_callables: Using Special Exceptions In View Callables ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Usually when a Python exception is raised within a view callable, :mod:`pyramid` allows the exception to propagate all the way out to the :term:`WSGI` server which invoked the application. However, for convenience, two special exceptions exist which are always handled by :mod:`pyramid` itself. These are :exc:`pyramid.exceptions.NotFound` and :exc:`pyramid.exceptions.Forbidden`. Both are exception classes which accept a single positional constructor argument: a ``message``. If :exc:`pyramid.exceptions.NotFound` is raised within view code, the result of the :term:`Not Found View` will be returned to the user agent which performed the request. If :exc:`pyramid.exceptions.Forbidden` is raised within view code, the result of the :term:`Forbidden View` will be returned to the user agent which performed the request. In all cases, the message provided to the exception constructor is made available to the view which :mod:`pyramid` invokes as ``request.exception.args[0]``. .. index:: single: exception views .. _exception_views: Exception Views ~~~~~~~~~~~~~~~~ The machinery which allows the special :exc:`pyramid.exceptions.NotFound` and :exc:`pyramid.exceptions.Forbidden` exceptions to be caught by specialized views as described in :ref:`special_exceptions_in_callables` can also be used by application developers to convert arbitrary exceptions to responses. To register a view that should be called whenever a particular exception is raised from with :mod:`pyramid` view code, use the exception class or one of its superclasses as the ``context`` of a view configuration which points at a view callable you'd like to generate a response. For example, given the following exception class in a module named ``helloworld.exceptions``: .. code-block:: python :linenos: class ValidationFailure(Exception): def __init__(self, msg): self.msg = msg You can wire a view callable to be called whenever any of your *other* code raises a ``hellworld.exceptions.ValidationFailure`` exception: .. code-block:: python :linenos: from helloworld.exceptions import ValidationFailure @view_config(context=ValidationFailure) def failed_validation(exc, request): response = Response('Failed validation: %s' % exc.msg) response.status_int = 500 return response Assuming that a :term:`scan` was run to pick up this view registration, this view callable will be invoked whenever a ``helloworld.exceptions.ValidationError`` is raised by your application's view code. The same exception raised by a custom root factory or a custom traverser is also caught and hooked. Other normal view predicates can also be used in combination with an exception view registration: .. code-block:: python :linenos: from pyramid.view import view_config from pyramid.exceptions import NotFound from pyramid.httpexceptions import HTTPNotFound @view_config(context=NotFound, route_name='home') def notfound_view(request): return HTTPNotFound() The above exception view names the ``route_name`` of ``home``, meaning that it will only be called when the route matched has a name of ``home``. You can therefore have more than one exception view for any given exception in the system: the "most specific" one will be called when the set of request circumstances which match the view registration. The only view predicate that cannot be not be used successfully when creating an exception view configuration is ``name``. The name used to look up an exception view is always the empty string. Views registered as exception views which have a name will be ignored. .. note:: Normal (non-exception) views registered against a context which inherits from :exc:`Exception` will work normally. When an exception view configuraton is processed, *two* views are registered. One as a "normal" view, the other as an "exception" view. This means that you can use an exception as ``context`` for a normal view. The feature can be used with any view registration mechanism (``@view_config`` decorator, ZCML, or imperative ``add_view`` styles). .. index:: single: unicode, views, and forms single: forms, views, and unicode single: views, forms, and unicode Handling Form Submissions in View Callables (Unicode and Character Set Issues) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Most web applications need to accept form submissions from web browsers and various other clients. In :mod:`pyramid`, form submission handling logic is always part of a :term:`view`. For a general overview of how to handle form submission data using the :term:`WebOb` API, see :ref:`webob_chapter` and `"Query and POST variables" within the WebOb documentation `_. :mod:`pyramid` defers to WebOb for its request and response implementations, and handling form submission data is a property of the request implementation. Understanding WebOb's request API is the key to understanding how to process form submission data. There are some defaults that you need to be aware of when trying to handle form submission data in a :mod:`pyramid` view. Because having high-order (non-ASCII) characters in data contained within form submissions is exceedingly common, and because the UTF-8 encoding is the most common encoding used on the web for non-ASCII character data, and because working and storing Unicode values is much saner than working with and storing bytestrings, :mod:`pyramid` configures the :term:`WebOb` request machinery to attempt to decode form submission values into Unicode from the UTF-8 character set implicitly. This implicit decoding happens when view code obtains form field values via the ``request.params``, ``request.GET``, or ``request.POST`` APIs (see :ref:`request_module` for details about these APIs). For example, let's assume that the following form page is served up to a browser client, and its ``action`` points at some :mod:`pyramid` view code: .. code-block:: xml :linenos:
The ``myview`` view code in the :mod:`pyramid` application *must* expect that the values returned by ``request.params`` will be of type ``unicode``, as opposed to type ``str``. The following will work to accept a form post from the above form: .. code-block:: python :linenos: def myview(request): firstname = request.params['firstname'] lastname = request.params['lastname'] But the following ``myview`` view code *may not* work, as it tries to decode already-decoded (``unicode``) values obtained from ``request.params``: .. code-block:: python :linenos: def myview(request): # the .decode('utf-8') will break below if there are any high-order # characters in the firstname or lastname firstname = request.params['firstname'].decode('utf-8') lastname = request.params['lastname'].decode('utf-8') For implicit decoding to work reliably, you should ensure that every form you render that posts to a :mod:`pyramid` view is rendered via a response that has a ``;charset=UTF-8`` in its ``Content-Type`` header; or, as in the form above, with a ``meta http-equiv`` tag that implies that the charset is UTF-8 within the HTML ``head`` of the page containing the form. This must be done explicitly because all known browser clients assume that they should encode form data in the character set implied by ``Content-Type`` value of the response containing the form when subsequently submitting that form; there is no other generally accepted way to tell browser clients which charset to use to encode form data. If you do not specify an encoding explicitly, the browser client will choose to encode form data in its default character set before submitting it. The browser client may have a non-UTF-8 default encoding. If such a request is handled by your view code, when the form submission data is encoded in a non-UTF8 charset, eventually the request code accessed within your view will throw an error when it can't decode some high-order character encoded in another character set within form data e.g. when ``request.params['somename']`` is accessed. If you are using the :class:`pyramid.response.Response` class to generate a response, or if you use the ``render_template_*`` templating APIs, the UTF-8 charset is set automatically as the default via the ``Content-Type`` header. If you return a ``Content-Type`` header without an explicit charset, a request will add a ``;charset=utf-8`` trailer to the ``Content-Type`` header value for you for response content types that are textual (e.g. ``text/html``, ``application/xml``, etc) as it is rendered. If you are using your own response object, you will need to ensure you do this yourself. .. note:: Only the *values* of request params obtained via ``request.params``, ``request.GET`` or ``request.POST`` are decoded to Unicode objects implicitly in the :mod:`pyramid` default configuration. The keys are still strings. .. index:: single: view configuration .. _view_configuration: View Configuration: Mapping a Context to a View ----------------------------------------------- A developer makes a :term:`view callable` available for use within a :mod:`pyramid` application via :term:`view configuration`. A view configuration associates a view callable with a set of statements about 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` and in the :term:`request`, as well as the :term:`view name`. These three pieces of information are known, collectively, as a :term:`triad`. View configuration is performed in one of these ways: - by running a :term:`scan` against application source code which has a :class:`pyramid.view.view_config` decorator attached to a Python object as per :class:`pyramid.view.view_config` and :ref:`mapping_views_using_a_decorator_section`. - by using the :meth:`pyramid.configuration.Configurator.add_view` method as per :meth:`pyramid.configuration.Configurator.add_view` and :ref:`mapping_views_using_imperative_config_section`. Both of these mechanisms is completely equivalent to the other. .. note:: You can also add view configuration by adding a ```` declaration to :term:`ZCML` used by your application as per :ref:`mapping_views_using_zcml_section` and :ref:`view_directive`. A view configuration might also be performed by virtue of :term:`route configuration`. View configuration via route configuration is performed by using the :meth:`pyramid.configuration.Configurator.add_route` method to create a route with a ``view`` argument. .. note:: ZCML users can use :ref:`route_directive` to perform the same task. See also :ref:`zcml_route_configuration`. .. _view_configuration_parameters: 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 :mod:`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. 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`` This is either a single string term (e.g. ``json``) or a string implying a path or :term:`resource 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. The renderer implementation will be used to construct a :term:`response` from the view return value. 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:`resource specification` in the form ``some.dotted.package_name:relative/path``, making it possible to address template resources 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 :mod:`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. Predicate Arguments +++++++++++++++++++ ``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 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. If ``context`` is not supplied, the value ``None``, which matches any model, 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`` usually has 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 for when the :term:`triad` includes a request object that does not indicate it matched a 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 :term:`lineage` must provide in order for this view to be found and called. The nodes in your object graph 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 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. .. index:: single: view_config decorator .. _mapping_views_using_a_decorator_section: 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 :term:`ZCML` or 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. Additionally, if you use decorators, it means that other people will not be able to override your view declarations externally using ZCML: this is a common requirement if you're developing an extensible application (e.g. a framework). See :ref:`extending_chapter` for more information about building extensible applications. Usage of the ``view_config`` decorator is a form of :term:`declarative configuration`, like ZCML, but in decorator form. :class:`pyramid.view.view_config` can be used to associate :term:`view configuration` information -- as done via the equivalent imperative code or ZCML -- with a function that acts as a :mod:`pyramid` view callable. All arguments to the :meth:`pyramid.configuration.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 :mod:`pyramid` application module ``views.py``: .. ignore-next-block .. code-block:: python :linenos: from models import MyModel from pyramid.view import view_config from pyramid.chameleon_zpt import render_template_to_response @view_config(name='my_view', request_method='POST', context=MyModel, permission='read', renderer='templates/my.pt') def my_view(request): return {'a':1} Using this decorator as above replaces the need to add this imperative configuration stanza: .. ignore-next-block .. code-block:: python config.add_view('.views.my_view', name='my_view', request_method='POST', context=MyModel, permission='read') All arguments to ``view_config`` may be omitted. For example: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config() def my_view(request): """ My view """ return Response() 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 model type, using no permission, registered against requests with any request method / request type / request param / route name / containment. The mere existence of a ``@view_config`` decorator doesn't suffice to perform view configuration. To make :mod:`pyramid` process your :class:`pyramid.view.view_config` declarations, you *must* do use the ``scan`` method of a :class:`pyramid.configuration.Configurator`: .. code-block:: python # config is assumed to be an instance of the # pyramid.configuration.Configurator class config.scan() .. note:: See :ref:`zcml_scanning` for information about how to invoke a scan via ZCML (if you're not using imperative configuration). 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.configuration.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: .. code-block:: python :linenos: from pyramid.view import view_config from pyramid.response import Response @view_config(name='edit') def edit(request): return Response('edited!') 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: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config() class MyView(object): def __init__(self, request): self.request = request def __call__(self): return Response('hello') 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: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config class MyView(object): def __init__(self, request): self.request = request def __call__(self): return Response('hello') my_view = view_config()(MyView) 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: .. code-block:: python :linenos: from pyramid.view import view_config from pyramid.response import Response @view_config(name='edit') @view_config(name='change') def edit(request): return Response('edited!') This registers the same view under two different names. The decorator can also be used against class methods: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config class MyView(object): def __init__(self, request): self.request = request @view_config(name='hello') def amethod(self): return Response('hello') 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`` as per :ref:`request_and_context_view_definitions`. The method which is decorated must return a :term:`response` or it must rely on a :term:`renderer` to generate one. 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: .. code-block:: python :linenos: from pyramid.response import Response from pyramid.view import view_config @view_config(attr='amethod', name='hello') class MyView(object): def __init__(self, request): self.request = request def amethod(self): return Response('hello') .. index:: single: add_view .. _mapping_views_using_imperative_config_section: View Configuration Using the ``add_view`` Method of a Configurator ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The :meth:`pyramid.configuration.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: .. code-block:: python :linenos: from pyramid.response import Response def hello_world(request): return Response('hello!') # config is assumed to be an instance of the # pyramid.configuration.Configurator class config.add_view(hello_world, name='hello.html') 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.configuration.Configurator.add_view` for more information. .. index:: single: model interfaces .. _using_model_interfaces: Using Model Interfaces In View Configuration ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Instead of registering your views with a ``context`` that names a Python model *class*, you can optionally register a view callable with a ``context`` which is an :term:`interface`. An interface can be attached arbitrarily to any model instance. View lookup treats context interfaces specially, and therefore the identity of a model 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 model type. For example, if two model object instances of different Python class types share the same interface, you can use the same view against each of them. In order to make use of interfaces in your application during view dispatch, you must create an interface and mark up your model classes or instances with interface declarations that refer to this interface. To attach an interface to a model *class*, you define the interface and use the :func:`zope.interface.implements` function to associate the interface with the class. .. code-block:: python :linenos: from zope.interface import Interface from zope.interface import implements class IHello(Interface): """ A marker interface """ class Hello(object): implements(IHello) To attach an interface to a model *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. .. code-block:: python :linenos: from zope.interface import Interface from zope.interface import alsoProvides class IHello(Interface): """ A marker interface """ class Hello(object): pass def make_hello(): hello = Hello() alsoProvides(hello, IHello) return hello Regardless of how you associate an interface with a model instance or a model 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 "models.py", the below interface declaration will associate the ``mypackage.views.hello_world`` view with models that implement (aka provide) this interface. .. code-block:: python :linenos: # config is an instance of pyramid.configuration.Configurator config.add_view('mypackage.views.hello_world', name='hello.html', context='mypackage.models.IHello') Any time a model 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 that views registered against a model class take precedence over views registered for any interface the model class implements when an ambiguity arises. If a view is registered for both the class type of the context and an interface implemented by the context's class, the view registered for the context's class will "win". For more information about defining models with interfaces for use within view configuration, see :ref:`models_which_implement_interfaces`. .. index:: single: view security pair: security; view .. _view_security_section: Configuring View Security ~~~~~~~~~~~~~~~~~~~~~~~~~ If a :term:`authorization policy` is active, any :term:`permission` attached to a :term:`view configuration` found during view lookup will be consulted to ensure that the currently authenticated user possesses that permission against the :term:`context` before the view function is actually called. Here's an example of specifying a permission in a view configuration using :meth:`pyramid.configuration.Configurator.add_view`: .. code-block:: python :linenos: # config is an instance of pyramid.configuration.Configurator config.add_view('myproject.views.add_entry', name='add.html', context='myproject.models.IBlog', permission='add') When an authentication 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` and an authorization policy is enabled. Instead the :term:`forbidden view` result will be returned to the client as per :ref:`protecting_views`. .. index:: single: view lookup .. _view_lookup: View Lookup and Invocation -------------------------- :term:`View lookup` is the :mod:`pyramid` subsystem responsible for finding an invoking a :term:`view callable`. The view lookup subsystem is passed a :term:`context`, a :term:`view name`, and the :term:`request` object. These three bits of information are referred to within this chapter as a :term:`triad`. :term:`View configuration` information stored within in the :term:`application registry` is compared against a triad by the view lookup subsystem in order to find the "best" view callable for the set of circumstances implied by the triad. Predicate attributes of view configuration 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. For any given request, 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 :mod:`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) view 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 which has predicates which allow it to be matched up with the request, :mod:`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. .. index:: single: debugging not found errors single: not found error (debugging) .. _debug_notfound_section: :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 ``BFG_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.