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.
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.
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 :app:`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.
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:
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.
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.
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:
: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`.
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`.
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.
All :mod:`pyramid.httpexceptions` classes which are 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:
An HTTP exception, instead of being raised, can alternately be returned (HTTP exceptions are also valid response objects):
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.
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`.
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` gets raised when a view to service a request is not found.
- :exc:`~pyramid.httpexceptions.HTTPForbidden` gets raised 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.
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 within :app:`Pyramid` view code, use the exception class (or one of its superclasses) as the :term:`context` of a view configuration which points at a view callable you'd like to generate a response for.
For example, given the following exception class in a module named helloworld.exceptions:
You can wire a view callable to be called whenever any of your other code raises a helloworld.exceptions.ValidationFailure exception:
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:
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.
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.
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.
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.
Alternately, you can raise an HTTPFound exception instead of returning one.
When the instance is raised, it is caught by the default :term:`exception response` handler and turned into a response.
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).
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:
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:
But the following myview view code may not work, as it tries to decode already-decoded (unicode) values obtained from request.params:
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.
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.
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:`resource` object found via tree :term:`traversal` or :term:`URL dispatch`.
- 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.:
- Classes that have an __init__ method that accepts context, request and a __call__ method which accepts no arguments, e.g.:
- Arbitrary callables that have a __call__ method that accepts context, request, e.g.:
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.
Passing Configuration Variables to a View
For information on passing a variable from the configuration .ini files to a view, see :ref:`deployment_settings`.
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.