.. _views_chapter: Views ===== One of the primary jobs of :app:`Pyramid` is to find and invoke a :term:`view callable` when a :term:`request` reaches your application. View callables are bits of code which do something interesting in response to a request made to your application. They are the "meat" of any interesting web application. .. note:: A :app:`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*. This chapter describes how view callables should be defined. We'll have to wait until a following chapter (entitled :ref:`view_config_chapter`) to find out how we actually tell :app:`Pyramid` to wire up view callables to particular URL patterns and other request circumstances. .. index:: single: view callables View Callables -------------- View callables are, at the risk of sounding obvious, callable Python objects. Specifically, view callables can be functions, classes, or instances that implement a ``__call__`` method (making the instance callable). View callables must, at a minimum, accept a single argument named ``request``. This argument represents a :app:`Pyramid` :term:`Request` object. A request object represents a :term:`WSGI` environment provided to :app:`Pyramid` by the upstream WSGI server. As you might expect, the request object contains everything your application needs to know about the specific HTTP request being made. A view callable's ultimate responsibility is to create a :mod:`Pyramid` :term:`Response` object. This can be done by creating a :term:`Response` object in the view callable code and returning it directly or by raising special kinds of exceptions from within the body of a view callable. .. index:: single: view calling convention single: view function .. _function_as_view: Defining a View Callable as a Function -------------------------------------- One of 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 represented by a Python 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__`` method 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`` argument. - a ``__call__`` (or other) 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 the configuration for the view. See :ref:`view_configuration_parameters`. The same view callable class can be used in different view configuration statements with different ``attr`` values, each pointing at a different method of the class if you'd like the class to represent a collection of related view callables. .. index:: single: view response single: response .. _the_response: View Callable Responses ----------------------- A view callable may return an object that implements the :app:`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') :app:`Pyramid` provides a range of different "exception" classes which inherit from :class:`pyramid.response.Response`. For example, an instance of the class :class:`pyramid.httpexceptions.HTTPFound` is also a valid response object because it inherits from :class:`~pyramid.response.Response`. For examples, see :ref:`http_exceptions` and :ref:`http_redirect`. .. note:: You can also return objects from view callables that aren't instances of :class:`pyramid.response.Response` in various circumstances. This can be helpful when writing tests and when attempting to share code between view callables. See :ref:`renderers_chapter` for the common way to allow for this. A much less common way to allow for view callables to return non-Response objects is documented in :ref:`using_iresponse`. .. 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, :app:`Pyramid` allows the exception to propagate all the way out to the :term:`WSGI` server which invoked the application. It is usually caught and logged there. However, for convenience, a special set of exceptions exists. When one of these exceptions is raised within a view callable, it will always cause :app:`Pyramid` to generate a response. These are known as :term:`HTTP exception` objects. .. index:: single: HTTP exceptions .. _http_exceptions: HTTP Exceptions ~~~~~~~~~~~~~~~ All classes documented in the :mod:`pyramid.httpexceptions` module documented as inheriting from the :class:`pyramid.httpexceptions.HTTPException` are :term:`http exception` objects. Instances of an HTTP exception object may either be *returned* or *raised* from within view code. In either case (return or raise) the instance will be used as the view's response. For example, the :class:`pyramid.httpexceptions.HTTPUnauthorized` exception can be raised. This will cause a response to be generated with a ``401 Unauthorized`` status: .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPUnauthorized def aview(request): raise HTTPUnauthorized() An HTTP exception, instead of being raised, can alternately be *returned* (HTTP exceptions are also valid response objects): .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPUnauthorized def aview(request): return HTTPUnauthorized() A shortcut for creating an HTTP exception is the :func:`pyramid.httpexceptions.exception_response` function. This function accepts an HTTP status code and returns the corresponding HTTP exception. For example, instead of importing and constructing a :class:`~pyramid.httpexceptions.HTTPUnauthorized` response object, you can use the :func:`~pyramid.httpexceptions.exception_response` function to construct and return the same object. .. code-block:: python :linenos: from pyramid.httpexceptions import exception_response def aview(request): raise exception_response(401) This is the case because ``401`` is the HTTP status code for "HTTP Unauthorized". Therefore, ``raise exception_response(401)`` is functionally equivalent to ``raise HTTPUnauthorized()``. Documentation which maps each HTTP response code to its purpose and its associated HTTP exception object is provided within :mod:`pyramid.httpexceptions`. .. note:: The :func:`~pyramid.httpexceptions.exception_response` function is new as of Pyramid 1.1. How Pyramid Uses HTTP Exceptions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ HTTP exceptions are meant to be used directly by application developers. However, Pyramid itself will raise two HTTP exceptions at various points during normal operations: :exc:`pyramid.httpexceptions.HTTPNotFound` and :exc:`pyramid.httpexceptions.HTTPForbidden`. Pyramid will raise the :exc:`~pyramid.httpexceptions.HTTPNotFound` exception when it cannot find a view to service a request. Pyramid will raise the :exc:`~pyramid.httpexceptions.HTTPForbidden` exception when authorization was forbidden by a security policy. If :exc:`~pyramid.httpexceptions.HTTPNotFound` is raised by Pyramid itself or 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.httpexceptions.HTTPForbidden` is raised by Pyramid itself within view code, the result of the :term:`Forbidden View` will be returned to the user agent which performed the request. .. index:: single: exception views .. _exception_views: Custom Exception Views ---------------------- The machinery which allows HTTP exceptions to be raised and 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 :app:`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 ``helloworld.exceptions.ValidationFailure`` exception: .. code-block:: python :linenos: from pyramid.view import view_config 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.ValidationFailure`` is raised by your application's view code. The same exception raised by a custom root factory, a custom traverser, or a custom view or route predicate 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 helloworld.exceptions import ValidationFailure @view_config(context=ValidationFailure, route_name='home') def failed_validation(exc, request): response = Response('Failed validation: %s' % exc.msg) response.status_int = 500 return response 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 match the view registration. The only view predicate that cannot 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 (i.e., non-exception) views registered against a context resource type which inherits from :exc:`Exception` will work normally. When an exception view configuration 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. Exception views can be configured with any view registration mechanism: ``@view_config`` decorator or imperative ``add_view`` styles. .. note:: Pyramid's :term:`exception view` handling logic is implemented as a tween factory function: :func:`pyramid.tweens.excview_tween_factory`. If Pyramid exception view handling is desired, and tween factories are specified via the ``pyramid.tweens`` configuration setting, the :func:`pyramid.tweens.excview_tween_factory` function must be added to the ``pyramid.tweens`` configuration setting list explicitly. If it is not present, Pyramid will not perform exception view handling. .. index:: single: view http redirect single: http redirect (from a view) .. _http_redirect: Using a View Callable to Do an HTTP Redirect -------------------------------------------- You can issue an HTTP redirect by using the :class:`pyramid.httpexceptions.HTTPFound` class. Raising or returning an instance of this class will cause the client to receive a "302 Found" response. To do so, you can *return* a :class:`pyramid.httpexceptions.HTTPFound` instance. .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPFound def myview(request): return HTTPFound(location='http://example.com') Alternately, you can *raise* an HTTPFound exception instead of returning one. .. code-block:: python :linenos: from pyramid.httpexceptions import HTTPFound def myview(request): raise HTTPFound(location='http://example.com') When the instance is raised, it is caught by the default :term:`exception response` handler and turned into a response. .. 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 :app:`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 `_. :app:`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 :app:`Pyramid` view. Having high-order (i.e., non-ASCII) characters in data contained within form submissions is exceedingly common, and the UTF-8 encoding is the most common encoding used on the web for character data. Since Unicode values are much saner than working with and storing bytestrings, :app:`Pyramid` configures the :term:`WebOb` request machinery to attempt to decode form submission values into Unicode from UTF-8 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). .. note:: Many people find the difference between Unicode and UTF-8 confusing. Unicode is a standard for representing text that supports most of the world's writing systems. However, there are many ways that Unicode data can be encoded into bytes for transit and storage. UTF-8 is a specific encoding for Unicode, that is backwards-compatible with ASCII. This makes UTF-8 very convenient for encoding data where a large subset of that data is ASCII characters, which is largely true on the web. UTF-8 is also the standard character encoding for URLs. As an example, let's assume that the following form page is served up to a browser client, and its ``action`` points at some :app:`Pyramid` view code: .. code-block:: xml :linenos:
The ``myview`` view code in the :app:`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 :app:`Pyramid` view explicitly defines a charset encoding of UTF-8. This can be done 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 same 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, which may not be UTF-8 as the server expects. If a request containing form data encoded in a non-UTF8 charset is handled by your view code, 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 :app:`Pyramid` default configuration. The keys are still (byte) strings. .. index:: single: view calling convention .. _request_and_context_view_definitions: Alternate View Callable Argument/Calling Conventions ---------------------------------------------------- Usually, view callables are defined to accept only a single argument: ``request``. However, view callables may alternately be defined as classes, functions, or any callable that accept *two* positional arguments: a :term:`context` resource 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 The :term:`resource` object found via tree :term:`traversal` or :term:`URL dispatch`. request A :app:`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__`` method 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: Passing in configuration variables .. _passing_in_config_variables: Passing Configuration Variables to a View ----------------------------------------- For information on passing a variable from the configuration .ini files to a view, see :ref:`deployment_settings`. .. index:: single: Pylons-style controller dispatch Pylons-1.0-Style "Controller" Dispatch -------------------------------------- A package named :term:`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.