Views¶

A view is a callable which is invoked when a request enters your application. The primary job of any repoze.bfg application is is to find and call a view when a request reaches it. The value returned by a view must implement the WebOb Response object interface.

Defining a View as a Function¶

The easiest way to define a view is to create a function that accepts two arguments: context, and request. For example, this is a hello world view implemented as a function:

from webob import Response

def hello_world(context, request):
    return Response('Hello world!')

The context and request arguments passed to a view function can be defined as follows:

context

An instance of a context found via graph traversal or URL dispatch. If the context is found via traversal, it will be a model object.

request

A WebOb request object representing the current WSGI request.

Defining a View as a Class¶

Note

This feature is new as of repoze.bfg 0.8.1.

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 is a class, the class’ __init__ is called with the context and the request parameters. As a result, an instance of the class is created. Subsequently, that instance’s __call__ method is invoked with no parameters. The class’ __call__ method must return a response. This provides behavior similar to a Zope ‘browser view’ (Zope ‘browser views’ are typically classes instead of simple callables). So the simplest class that can be a view must have:

an __init__ method that accepts a context and a request as positional arguments.
a __call__ method that accepts no parameters and returns a response.

For example:

from webob import Response

class MyView(object):
    def __init__(self, context, request):
        self.context = context
        self.request = request

    def __call__(self):
        return Response('hello from %r!' % self.context)

The context and request objects passed to __init__ are the same types of objects as described in Defining a View as a Function.

Alternate “Request-Only” View Argument Convention¶

Views may alternately be defined as callables that accept only a request object, instead of both a context and a request. The following types work as views in this style:

Functions that accept a single argument request, e.g.:

from webob import Response

def aview(request):
    return Response('OK')

New and old-style classes that have an __init__ method that accepts self, request, e.g.:

from webob import Response

def View(object):
    __init__(self, request):
    return Response('OK')

Arbitrary callables that have a __call__ method that accepts self, request, e.g.:

from webob import Response

def AView(object):
    def __call__(self, request):
        return Response('OK')
view = AView()

This style of calling convention is useful for url dispatch based applications, where the context is seldom used within the view code itself. The view always has access to the context via request.context in any case, so it’s still available even if you use the request-only calling convention.

The Response¶

A view callable must return an object that implements the WebOb Response interface. The easiest way to return something that implements this interface is to return a webob.Response object. But any object that has the following attributes will work:

status

The HTTP status code (including the name) for the response. 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. ['<html><head></head><body>Hello world!</body></html>'] or it can be a file-like object, or any other sort of iterable.

If a view happens to return something to the repoze.bfg router that does not implement this interface, the router will raise an error.

Mapping Views to URLs Using ZCML¶

You may associate a view with a URL by adding information to your application registry via ZCML in your configure.zcml file using a view declaration.

<view
    for=".models.Hello"
    view=".views.hello_world"
    name="hello.html"
    />

The above maps the .views.hello_world view function to context objects which are instances (or subclasses) of the Python class represented by .models.Hello when the view name is hello.html.

Note

Values prefixed with a period (.) for the for and view attributes of a view (such as those above) mean “relative to the Python package directory in which this ZCML file is stored”. So if the above view declaration was made inside a configure.zcml file that lived in the hello package, you could replace the relative .models.Hello with the absolute hello.models.Hello; likewise you could replace the relative .views.hello_world with the absolute hello.views.hello_world. Either the relative or absolute form is functionally equivalent. It’s often useful to use the relative form, in case your package’s name changes. It’s also shorter to type.

You can also declare a default view for a model type:

<view
    for=".models.Hello"
    view=".views.hello_world"
    />

A default view has no name attribute. When a context is traversed and there is no view name in the request, the default view is the view that is used.

You can also declare that a view is good for any model type by using the special * character in the for attribute:

<view
    for="*"
    view=".views.hello_world"
    name="hello.html"
    />

This indicates that when repoze.bfg identifies that the view name is hello.html against any context, this view will be called.

A ZCML view declaration’s view attribute can also name a class. In this case, the rules described in Defining a View as a Class apply for the class which is named.

The `view` ZCML Directive¶

The view ZCML directive has these possible attributes:

view

The Python dotted-path name to the view callable.

for

A Python dotted-path name representing the Python class that the context must be an instance of, or the interface that the context must provide in order for this view to be found and called.

name

The view name. Read and understand Traversal to understand the concept of a view name.

permission

The name of a permission that the user must possess in order to call the view. See View Security for more information about view security and permissions.

request_type

This value can either be one of the strings ‘GET’, ‘POST’, ‘PUT’, ‘DELETE’, or ‘HEAD’ representing an HTTP method, or it may be Python dotted-path string representing the interface that the request must have in order for this view to be found and called. See Standard View Request Types for more information about request types.

route_name

This attribute services an advanced feature that isn’t often used unless you want to perform traversal *after a route has matched.* This value must match the name of a <route> declaration (see URL Dispatch) that must match before this view will be called. The <route> declaration specified by route_name must exist in ZCML before the view that names the route (XML-ordering-wise) . Note that the <route> declaration referred to by route_name usually has a *traverse token in the value of its path attribute, representing a part of the path that will be used by traversal against the result of the route’s root factory. See Combining Traversal and URL Dispatch for more information on using this advanced feature.

Mapping Views to URLs Using a Decorator¶

If you’re allergic to reading and writing ZCML, or you’re just more comfortable defining your view declarations using Python, you may use the repoze.bfg.view.bfg_view decorator to associate your view functions with URLs instead of using ZCML for the same purpose. repoze.bfg.view.bfg_view can be used to associate for, name, permission and request_type information – as done via the equivalent ZCML – with a function that acts as a repoze.bfg view.

To make repoze.bfg process your bfg_view declarations, you must insert the following boilerplate into your application’s configure.zcml:

<scan package="."/>

After you do so, you will not need to use any other ZCML to configure repoze.bfg view declarations. Instead, you will use a decorator to do this work.

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 Extending An Existing repoze.bfg Application for more information about building extensible applications.

The `bfg_view` Decorator¶

repoze.bfg.view.bfg_view is a decorator which allows Python code to make view registrations instead of using ZCML for the same purpose.

An example might reside in a bfg application module views.py:

from models import MyModel
from repoze.bfg.view import bfg_view
from repoze.bfg.chameleon_zpt import render_template_to_response

@bfg_view(name='my_view', request_type='POST', for_=MyModel,
          permission='read')
def my_view(context, request):
    return render_template_to_response('templates/my.pt')

Using this decorator as above replaces the need to add this ZCML to your application registry:

<view
 for=".models.MyModel"
 view=".views.my_view"
 name="my_view"
 permission="read"
 request_type="POST"
 />

All arguments to bfg_view are optional.

If name is not supplied, the empty string is used (implying the default view).

If request_type is not supplied, the interface None is used, implying any request type.

If for_ is not supplied, the interface zope.interface.Interface (which matches any model) is used. for_ can also name a class, like its ZCML brother.

If permission is not supplied, no permission is registered for this view (it’s accessible by any caller).

If route_name is supplied, the view will be invoked only if the named route matches. This is an advanced feature, not often used by “civilians”.

All arguments may be omitted. For example:

from webob import Response

@bfg_view()
def my_view(context, request):
    """ My view """
    return Response()

Such a registration as the one directly above implies that the view name will be my_view, registered for_ any model type, using no permission, registered against requests which implement any request method or interface.

If your view callable is a class, the bfg_view 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:

from webob import Response
from repoze.bfg.view import bfg_view

@bfg_view()
class MyView(object):
    def __init__(self, context, request):
        self.context = context
        self.request = request

    def __call__(self):
        return Response('hello from %s!' % self.context)

You can use the bfg_view decorator as a simple callable to manually decorate classes in Python 2.5 and below (without the decorator syntactic sugar), if you wish:

from webob import Response
from repoze.bfg.view import bfg_view

class MyView(object):
    def __init__(self, context, request):
        self.context = context
        self.request = request

    def __call__(self):
        return Response('hello from %s!' % self.context)

my_view = bfg_view()(MyView)

Using Model Interfaces¶

Instead of registering your views for a Python model class, you can optionally register a view for an interface. Since an interface can be attached arbitrarily to any model instance (as opposed to its identity being implied by only its class), 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 zope.interface.implements function to associate the interface with the class.

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 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.

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 ZCML to associate that interface with a view 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 .views.hello_world view with models that implement (aka provide) this interface.

<view
    for=".models.IHello"
    view=".views.hello_world"
    name="hello.html"
    />

Any time a model that is determined to be the context provides this interface, and a view named hello.html is looked up against it as per the URL, the .views.hello_world view 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”.

See Interface in the glossary to find more information about interfaces.

Standard View Request Types¶

You can optionally add a request_type attribute to your view declaration or bfg_view decorator, which indicates what “kind” of request the view should be used for. If the request type for a request doesn’t match the request type that a view defines as its request_type argument, that view won’t be called.

The request type can be one of the strings ‘GET’, ‘POST’, ‘PUT’, ‘DELETE’, or ‘HEAD’. When the request type is one of these strings, the view will only be called when the HTTP method of a request matches this type.

For example, the following bit of ZCML will match an HTTP POST request:

<view
    for=".models.Hello"
    view=".views.handle_post"
    name="handle_post"
    request_type="POST"
    />

A bfg_view decorator that does the same as the above ZCML view declaration which matches only on HTTP POST might look something like:

from myproject.models import Hello
from webob import Response

@bfg_view(for=Hello, request_type='POST')
def handle_post(context, request):
    return Response('hello'

The above examples register views for the POST request type, so it will only be called if the request’s HTTP method is POST. Even if all the other specifiers match (e.g. the model type is the class .models.Hello, and the view_name is handle_post), if the request verb is not POST, it will not be invoked. This provides a way to ensure that views you write are only called via specific HTTP verbs.

The least specific request type is None. All requests are guaranteed to implement this request type. It is also the default request type for views that omit a request_type argument.

Custom View Request Types¶

You can make use of custom view request types by attaching an interface to the request and specifying this interface in the request_type parameter. For example, you might want to make use of simple “content negotiation”, only invoking a particular view if the request has a content-type of ‘application/json’.

For information about using interface to specify a request type, see Using An Event to Vary the Request Type.

View Security¶

If a authentication policy (and a authorization policy) is active, any permission attached to a view declaration will be consulted to ensure that the currently authenticated user possesses that permission against the context before the view function is actually called. Here’s an example of specifying a permission in a view declaration:

<view
    for=".models.IBlog"
    view=".views.add_entry"
    name="add.html"
    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 context and an authorization policy is enabled. Instead the forbidden view result will be returned to the client (see Changing the Forbidden View).

Note

See the Security chapter to find out how to turn on an authentication policy.

Note

Packages such as repoze.who are capable of intercepting an Unauthorized response and displaying a form that asks a user to authenticate. Use this kind of package to ask the user for authentication credentials.

Using a View to Do A HTTP Redirect¶

You can issue an HTTP redirect from within a view by returning a slightly different response.

from webob.exc import HTTPFound

def myview(context, request):
    return HTTPFound(location='http://example.com')

All exception types from the webob.exc module implement the Webob Response interface; any can be returned as the response from a view. See WebOb for the documentation for this module; it includes other response types for Unauthorized, etc.

Serving Static Resources Using a View¶

Using the repoze.bfg.view static helper class is the preferred way to serve static resources (like JavaScript and CSS files) within repoze.bfg. This class creates a callable that is capable acting as a repoze.bfg view which serves static resources from a directory. For instance, to serve files within a directory located on your filesystem at /path/to/static/dir mounted at the URL path /static in your application, create an instance of repoze.bfg.view ‘s static class inside a static.py file in your application root as below.

from repoze.bfg.view import static
static_view = static('/path/to/static/dir')

Subsequently, wire this view up to be accessible as /static using ZCML in your application’s configure.zcml against either the class or interface that represents your root object.

 <view
   for=".models.Root"
   view=".static.static_view"
   name="static"
 />

In this case, .models.Root refers to the class of which your repoze.bfg application’s root object is an instance.

Note

You can also give a for of * if you want the name static to be accessible as the static view against any model. This will also allow /static/foo.js to work, but it will allow for /anything/static/foo.js too, as long as anything itself is resolveable.

Now put your static files (JS, etc) on your filesystem in the directory represented as /path/to/static/dir. After this is done, you should be able to view the static files in this directory via a browser at URLs prefixed with /static/, for instance /static/foo.js will return the file /path/to/static/dir/foo.js. The static directory may contain subdirectories recursively, and any subdirectories may hold files; these will be resolved by the static view as you would expect.

Note

To ensure that model objects contained in the root don’t “shadow” your static view (model objects take precedence during traversal), or to ensure that your root object’s __getitem__ is never called when a static resource is requested, you can refer to your static resources as registered above in URLs as, e.g. /@@static/foo.js. This is completely equivalent to /static/foo.js. See Traversal for information about “goggles” (@@).

Using Views to Handle Form Submissions (Unicode and Character Set Issues)¶

Most web applications need to accept form submissions from web browsers and various other clients. In repoze.bfg, 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 “Query and POST variables” within the WebOb documentation. repoze.bfg 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 repoze.bfg 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 an storing bytestrings, repoze.bfg configures the 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 WebOb request.params, request.GET, or request.POST APIs.

For example, let’s assume that the following form page is served up to a browser client, and its action points at some repoze.bfg view code:

<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
  </head>
  <form method="POST" action="myview">
    <div>
      <input type="text" name="firstname"/>
    </div>
    <div>
      <input type="text" name="lastname"/>
    </div>
    <input type="submit" value="Submit"/>
  </form>
</html>

The myview view code in the repoze.bfg 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:

def myview(context, 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:

def myview(context, 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 must ensure that every form you render that posts to a repoze.bfg 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 WebOb 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 webob.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 WebOb 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.

To avoid implicit form submission value decoding, so that the values returned from request.params, request.GET and request.POST are returned as bytestrings rather than Unicode, add the following to your application’s configure.zcml:

<subscriber for="repoze.bfg.interfaces.INewRequest"
            handler="repoze.bfg.request.make_request_ascii"/>

You can then control form post data decoding “by hand” as necessary. For example, when this subscriber is active, the second example above will work unconditionally as long as you ensure that your forms are rendered in a request that has a ;charset=utf-8 stanza on its Content-Type header.

Note

The behavior that form values are decoded from UTF-8 to Unicode implicitly was introduced in repoze.bfg 0.7.0. Previous versions of repoze.bfg performed no implicit decoding of form values (the default was to treat values as bytestrings).

Note

Only the values of request params obtained via request.params, request.GET or request.POST are decoded to Unicode objects implicitly in repoze.bfg‘s default configuration. The keys are still strings.

Views¶

Defining a View as a Function¶

Defining a View as a Class¶

Alternate “Request-Only” View Argument Convention¶

The Response¶

Mapping Views to URLs Using ZCML¶

The `view` ZCML Directive¶

Mapping Views to URLs Using a Decorator¶

The `bfg_view` Decorator¶

Using Model Interfaces¶

Standard View Request Types¶

Custom View Request Types¶

View Security¶

Using a View to Do A HTTP Redirect¶

Serving Static Resources Using a View¶

Using Views to Handle Form Submissions (Unicode and Character Set Issues)¶

Table Of Contents

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Views¶

Defining a View as a Function¶

Defining a View as a Class¶

Alternate “Request-Only” View Argument Convention¶

The Response¶

Mapping Views to URLs Using ZCML¶

The view ZCML Directive¶

Mapping Views to URLs Using a Decorator¶

The bfg_view Decorator¶

Using Model Interfaces¶

Standard View Request Types¶

Custom View Request Types¶

View Security¶

Using a View to Do A HTTP Redirect¶

Serving Static Resources Using a View¶

Using Views to Handle Form Submissions (Unicode and Character Set Issues)¶

Table Of Contents

Previous topic

Next topic

This Page

Quick search

Navigation

The `view` ZCML Directive¶

The `bfg_view` Decorator¶