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 :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.
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:
__init__method that accepts 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 classes documented in the :mod:`pyramid.httpexceptions` module documented as inheriting from the :class:`pryamid.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 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
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
raise exception_response(401) is functionally
raise HTTPUnauthorized(). Documentation which maps each
HTTP response code to its purpose and its associated HTTP exception object is
provided within :mod:`pyramid.httpexceptions`.
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 are raised when it cannot find a view to service a request. Pyramid will raise the :exc:`~pyramid.httpexceptions.Forbidden` exception or 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 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
You can wire a view callable to be called whenever any of your other code
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
home, meaning that
it will only be called when the route matched has a name of
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
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.POST APIs (see :ref:`request_module` for details about these
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:
myview view code in the :app:`Pyramid` application must expect that
the values returned by
request.params will be of type
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
unicode) values obtained from
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,
meta http-equiv tag that implies that the charset is UTF-8 within
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
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
If you return a
Content-Type header without an explicit charset, a
request will add a
;charset=utf-8 trailer to the
value for you, for response content types that are textual
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.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
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:
- Classes that have an
__init__method that accepts
context, requestand 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
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.