deb-python-pecan/docs/source/routing.rst

Controllers and Routing

When a user requests a certain URL in your app, how does Pecan know which controller to route to? Pecan uses a routing strategy known as object-dispatch to map an HTTP request to a controller.

Object-dispatch begins by splitting the path into a list of components and then walking an object path, starting at the root controller. You can imagine your application's controllers as a tree of objects (branches of the object tree map directly to URL paths). Let's look at a simple bookstore application:

from pecan import expose

class BooksController(object):
    @expose()
    def index(self):
        return "Welcome to book section."

    @expose()
    def bestsellers(self):
        return "We have 5 books in the top 10."

class CatalogController(object):
    @expose()
    def index(self):
        return "Welcome to the catalog."

    books = BooksController()

class RootController(object):
    @expose()
    def index(self):
        return "Welcome to store.example.com!"

    @expose()
    def hours(self):
        return "Open 24/7 on the web."

    catalog = CatalogController()

A request for /catalog/books/bestsellers from the online store would begin with Pecan breaking the request up into catalog, books, and bestsellers. Next, Pecan would lookup catalog on the root controller. Using the catalog object, Pecan would then lookup books, followed by bestsellers. What if the URL ends in a slash? Pecan will check for an index method on the current object.

To illustrate further, the following paths...

   └── /
       ├── /hours
       └── /catalog
            └── /catalog/books
               └── /catalog/books/bestsellers

Would route to the following controller methods...

   └── RootController.index
       ├── RootController.hours
       └── CatalogController.index
            └── BooksController.index
               └── BooksController.bestsellers

Exposing Controllers

At its core, @expose is how you tell Pecan which methods in a class are publically-visible controllers. If a method is not decorated with @expose, it will never be routed to. @expose accepts three optional parameters, some of which can impact routing and the content type of the response body.

from pecan import expose

class RootController(object):
    @expose(
        template        = None,
        content_type    = 'text/html',
        generic         = False
    )
    def hello(self):
        return 'Hello World' 

Let's look at an example using template and content_type:

from pecan import expose

class RootController(object):
    @expose('json')
    @expose('text_template.mako', content_type='text/plain')
    @expose('html_template.mako')
    def hello(self):
        return {'msg': 'Hello!'}

You'll notice that we used three expose decorators.

The first tells Pecan to serialize the response namespace using JSON serialization when the client requests /hello.json.

The second tells Pecan to use the text_template.mako template file when the client requests /hello.txt.

The third tells Pecan to use the html_template.mako template file when the client requests /hello.html. If the client requests /hello, Pecan will use the text/html content type by default.

Please see pecan_decorators for more information on @expose.

Pecan's Routing Algorithm

Sometimes, the standard object-dispatch routing isn't adequate to properly route a URL to a controller. Pecan provides several ways to short-circuit the object-dispatch system to process URLs with more control, including the special _lookup, _default, and _route methods. Defining these methods on your controller objects provides additional flexibility for processing all or part of a URL.

Setting a Return Status Code

Setting a specific HTTP response code (such as 201 Created) is simple:

from pecan import expose, response

class RootController(object):

    @expose('json')
    def hello(self):
        response.status = 201
        return {'foo': 'bar'}

Pecan also comes with abort, a utility function for raising HTTP errors:

from pecan import expose, abort

class RootController(object):

    @expose('json')
    def hello(self):
        abort(404)

Under the hood, abort raises an instance of webob.exc.WSGIHTTPException which is used by pecan to render default response bodies for HTTP errors. This exception is stored in the WSGI request environ at pecan.original_exception, where it can be accessed later in the request cycle (by, for example, other middleware or errors).

Routing to Subcontrollers with _lookup

The _lookup special method provides a way to process a portion of a URL, and then return a new controller object to route to for the remainder.

A _lookup method will accept one or more arguments, representing chunks of the URL to be processed, split on /, and then provide a *remainder list which will be processed by the returned controller via object-dispatch.

Additionally, the _lookup method on a controller is called as a last resort, when no other controller matches the URL via standard object-dispatch.

from pecan import expose, abort
from somelib import get_student_by_name

class StudentController(object):
    def __init__(self, student):
        self.student = student

    @expose()
    def name(self):
        return self.student.name

class RootController(object):
    @expose()
    def _lookup(self, primary_key, *remainder):
        student = get_student_by_primary_key(primary_key)
        if student:
            return StudentController(student), remainder
        else:
            abort(404)

An HTTP GET request to /8/name would return the name of the student where primary_key == 8.

Falling Back with _default

The _default controller is called as a last resort when no other controller methods match the URL via standard object-dispatch.

from pecan import expose

class RootController(object):
    @expose()
    def english(self):
        return 'hello'

    @expose()
    def french(self):
        return 'bonjour'

    @expose()
    def _default(self):
        return 'I cannot say hello in that language'

...so in the example above, a request to /spanish would route to RootController._default.

Defining Customized Routing with _route

The _route method allows a controller to completely override the routing mechanism of Pecan. Pecan itself uses the _route method to implement its RestController. If you want to design an alternative routing system on top of Pecan, defining a base controller class that defines a _route method will enable you to have total control.

Mapping Controller Arguments

In Pecan, HTTP GET and POST variables that are not consumed during the routing process can be passed onto the controller as arguments.

Depending on the signature of your controller, these arguments can be mapped explicitly to method arguments:

from pecan import expose

class RootController(object):
    @expose()
    def index(self, arg):
        return arg

    @expose()
    def kwargs(self, **kwargs):
        return str(kwargs)

$ curl http://localhost:8080/?arg=foo
foo
$ curl http://localhost:8080/kwargs?a=1&b=2&c=3
{u'a': u'1', u'c': u'3', u'b': u'2'}

...or can be consumed positionally:

from pecan import expose

class RootController(object):
    @expose()
    def args(self, *args):
        return ','.join(args)

$ curl http://localhost:8080/args/one/two/three
one,two,three

The same effect can be achieved with HTTP POST body variables:

from pecan import expose

class RootController(object):
    @expose()
    def index(self, arg):
        return arg

$ curl -X POST "http://localhost:8080/" -H "Content-Type: application/x-www-form-urlencoded" -d "arg=foo"
foo

Helper Functions

Pecan also provides several useful helper functions for moving between different routes. The redirect function allows you to issue internal or HTTP 302 redirects. The redirect utility, along with several other useful helpers, are documented in pecan_core.