Writing RESTful Web Services with Generic Controllers

Pecan simplifies RESTful web services by providing a way to overload URLs based on the request method. For most API’s, the use of generic controller definitions give you everything you need to build out robust RESTful interfaces (and is the recommended approach to writing RESTful web services in pecan):

from pecan import abort, expose

# Note: this is *not* thread-safe.  In real life, use a persistent data store.
BOOKS = {
    '0': 'The Last of the Mohicans',
    '1': 'Catch-22'
}


class BookController(object):

    def __init__(self, id_):
        self.id_ = id_
        assert self.book

    @property
    def book(self):
        if self.id_ in BOOKS:
            return dict(id=self.id_, name=BOOKS[self.id_])
        abort(404)

    # HTTP GET /<id>/
    @expose(generic=True, template='json')
    def index(self):
        return self.book

    # HTTP PUT /<id>/
    @index.when(method='PUT', template='json')
    def index_PUT(self, **kw):
        BOOKS[self.id_] = kw['name']
        return self.book

    # HTTP DELETE /<id>/
    @index.when(method='DELETE', template='json')
    def index_DELETE(self):
        del BOOKS[self.id_]
        return dict()


class RootController(object):

    @expose()
    def _lookup(self, id_, *remainder):
        return BookController(id_), remainder

    # HTTP GET /
    @expose(generic=True, template='json')
    def index(self):
        return [dict(id=k, name=v) for k, v in BOOKS.items()]

    # HTTP POST /
    @index.when(method='POST', template='json')
    def index_POST(self, **kw):
        id_ = str(len(BOOKS))
        BOOKS[id_] = kw['name']
        return dict(id=id_, name=kw['name'])

Writing RESTful Web Services with RestController

For compatability with the TurboGears2 library, Pecan also provides a class-based solution to RESTful routing, RestController:

from pecan import expose
from pecan.rest import RestController

from mymodel import Book

class BooksController(RestController):

    @expose()
    def get(self, id):
        book = Book.get(id)
        if not book:
            abort(404)
        return book.title

URL Mapping

By default, RestController routes as follows:

Method	Description	Example Method(s) / URL(s)
get_one	Display one record.	GET /books/1
get_all	Display all records in a resource.	GET /books/
get	A combo of get_one and get_all.	GET /books/
		GET /books/1
new	Display a page to create a new resource.	GET /books/new
edit	Display a page to edit an existing resource.	GET /books/1/edit
post	Create a new record.	POST /books/
put	Update an existing record.	POST /books/1?_method=put
		PUT /books/1
get_delete	Display a delete confirmation page.	GET /books/1/delete
delete	Delete an existing record.	POST /books/1?_method=delete
		DELETE /books/1

Pecan’s RestController uses the ?_method= query string to work around the lack of support for the PUT and DELETE verbs when submitting forms in most current browsers.

In addition to handling REST, the RestController also supports the index(), _default(), and _lookup() routing overrides.

Warning

If you need to override _route(), make sure to call RestController._route() at the end of your custom method so that the REST routing described above still occurs.

Nesting RestController

RestController instances can be nested so that child resources receive the parameters necessary to look up parent resources.

For example:

from pecan import expose
from pecan.rest import RestController

from mymodel import Author, Book

class BooksController(RestController):

    @expose()
    def get(self, author_id, id):
        author = Author.get(author_id)
        if not author_id:
            abort(404)
        book = author.get_book(id)
        if not book:
            abort(404)
        return book.title

class AuthorsController(RestController):

    books = BooksController()

    @expose()
    def get(self, id):
        author = Author.get(id)
        if not author:
            abort(404)
        return author.name

class RootController(object):

    authors = AuthorsController()

Accessing /authors/1/books/2 invokes BooksController.get() with author_id set to 1 and id set to 2.

To determine which arguments are associated with the parent resource, Pecan looks at the get_one() then get() method signatures, in that order, in the parent controller. If the parent resource takes a variable number of arguments, Pecan will pass it everything up to the child resource controller name (e.g., books in the above example).

Defining Custom Actions

In addition to the default methods defined above, you can add additional behaviors to a RestController by defining a special _custom_actions dictionary.

For example:

from pecan import expose
from pecan.rest import RestController

from mymodel import Book

class BooksController(RestController):

    _custom_actions = {
        'checkout': ['POST']
    }

    @expose()
    def checkout(self, id):
        book = Book.get(id)
        if not book:
            abort(404)
        book.checkout()

_custom_actions maps method names to the list of valid HTTP verbs for those custom actions. In this case checkout() supports POST.