One of the primary jobs of Pyramid is to find and invoke a view callable when a 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 Pyramid view callable is often referred to in conversational shorthand as a 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 View Configuration) to find out how we actually tell 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
__call__ method (making the instance callable).
View callables must, at a minimum, accept a single argument named
This argument represents a Pyramid Request object. A request
object represents a WSGI environment provided to 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
A view callable's ultimate responsibility is to create a Pyramid Response object. This can be done by creating a 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 ways to define a view callable is to create a function that
accepts a single argument named
request, and which returns a
Response object. For example, this is a "hello world" view callable
implemented as a function:
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from pyramid.response import Response def hello_world(request): return Response('Hello world!')
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's
__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
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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 Defining a View Callable as a Function.
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 View Configuration Parameters. The
same view callable class can be used in different view configuration statements
attr values, each pointing at a different method of the
class if you'd like the class to represent a collection of related view
View Callable Responses¶
A view callable may return an object that implements the Pyramid
Response interface. The easiest way to return something that
implements the Response interface is to return a
pyramid.response.Response object instance directly. For example:
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from pyramid.response import Response def view(request): return Response('OK')
Pyramid provides a range of different "exception" classes which inherit
pyramid.response.Response. For example, an instance of the class
pyramid.httpexceptions.HTTPFound is also a valid response object
because it inherits from
Response. For examples,
see HTTP Exceptions and Using a View Callable to do an HTTP Redirect.
You can also return objects from view callables that aren't instances of
pyramid.response.Response in various circumstances. This can be
helpful when writing tests and when attempting to share code between view
callables. See Renderers 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 Changing How Pyramid Treats View Responses.
Using Special Exceptions in View Callables¶
Usually when a Python exception is raised within a view callable, Pyramid allows the exception to propagate all the way out to the 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 Pyramid to generate a response. These are known as HTTP exception objects.
pyramid.httpexceptions classes which are documented as inheriting
pyramid.httpexceptions.HTTPException are 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
pyramid.httpexceptions.HTTPUnauthorized exception can
be raised. This will cause a response to be generated with a
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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):
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from pyramid.httpexceptions import HTTPUnauthorized def aview(request): return HTTPUnauthorized()
A shortcut for creating an HTTP exception is the
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
HTTPUnauthorized response object, you can use
exception_response() function to construct
and return the same object.
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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
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
New in version 1.1: The
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.
HTTPNotFoundgets raised when a view to service a request is not found.
HTTPForbiddengets raised when authorization was forbidden by a security policy.
Custom Exception Views¶
The machinery which allows HTTP exceptions to be raised and caught by specialized views as described in Using Special Exceptions in View Callables can also be used by application developers to convert arbitrary exceptions to responses.
To register an exception view that should be called whenever a
particular exception is raised from within Pyramid view code, use
pyramid.config.Configurator.add_exception_view() to register a view
configuration which matches the exception (or a subclass of the exception) and
points at a view callable for which you'd like to generate a response. The
exception will be passed as the
context argument to any
view predicate registered with the view, as well as to the view itself.
For convenience a new decorator exists,
pyramid.views.exception_view_config, which may be used to easily
register exception views.
For example, given the following exception class in a module named
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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
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from pyramid.view import exception_view_config from helloworld.exceptions import ValidationFailure @exception_view_config(ValidationFailure) def failed_validation(exc, request): response = Response('Failed validation: %s' % exc.msg) response.status_int = 500 return response
Assuming that a 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:
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from pyramid.view import view_config from helloworld.exceptions import ValidationFailure @exception_view_config(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
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.
In most cases, you should register an exception view by using
pyramid.config.Configurator.add_exception_view(). However, it is
possible to register "normal" (i.e., non-exception) views against a context
resource type which inherits from
config.add_view(context=Exception)). When the 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.
The view derivers that wrap these two views may behave differently. See Exception Views and View Derivers for more information about this.
Exception views can be configured with any view registration mechanism:
@exception_view_config decorator or imperative
Pyramid's exception view handling logic is implemented as a tween
pyramid.tweens.excview_tween_factory(). If Pyramid
exception view handling is desired, and tween factories are specified via
pyramid.tweens configuration setting, the
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
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
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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.
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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 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 Pyramid, form submission handling logic is always part of a view. For a general overview of how to handle form submission data using the WebOb API, see Request and Response Objects and "Query and POST variables" within the WebOb documentation. 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 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, Pyramid configures the 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
pyramid.request 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 Pyramid view code:
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<html xmlns="http://www.w3.org/1999/xhtml"> <form method="POST" action="myview" accept-charset="UTF-8"> <div> <input type="text" name="firstname"/> </div> <div> <input type="text" name="lastname"/> </div> <input type="submit" value="Submit"/> </form> </html>
myview view code in the Pyramid application must expect that
the values returned by
request.params will be of type
str, as opposed
bytes. The following will work to accept a form post from the
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def myview(request): firstname = request.params['firstname'] lastname = request.params['lastname']
For implicit decoding to work reliably, you should ensure that every form you
render that posts to a 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,
accept-charset attribute, informing the browser that the
server expects the form content to be encoded using UTF-8. This must be done
explicitly because all known browser clients assume that they should encode
form data in the same character set implied by the
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-UTF-8
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
Response class to generate a
response, or if you use the
pyramid.renderers.render_* templating APIs,
charset is set automatically as the default via the
Content-Type header. If you return a
Content-Type header without an
charset, a request will add a
;charset=utf-8 trailer to the
Content-Type header value for you for response content types that are
application/xml) as it is rendered. If you
are using your own response object, you will need to ensure you do this
Alternate View Callable Argument/Calling Conventions¶
Usually view callables are defined to accept only a single argument:
request. However, a view callable may alternately be defined as any class, function, or callable that accepts two positional arguments: a
context resource as the first argument and a request as the
A Pyramid Request object representing the current WSGI request.
The following types work as view callables in this style:
Functions that accept two arguments:
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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.:
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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.:
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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 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
Passing Configuration Variables to a View¶
For information on passing a variable from the configuration .ini files to a view, see Deployment Settings.