URI routing is the process of taking the requested URI and deciding which
application handler will handle the current request. For this, we initialize
WSGIApplication defining a list of routes: each route
analyses the current request and, if it matches certain criteria, returns
the handler and optional variables extracted from the URI.
webapp2 offers a powerful and extensible system to match and build URIs, which is explained in details in this section.
The simplest form of URI route in webapp2 is a tuple
where regex is a regular expression to match the requested URI path and
handler is a callable to handle the request. This routing mechanism is
fully compatible with App Engine’s webapp framework.
This is how it works: a list of routes is registered in the WSGI application. When the application receives a request, it tries to match each one in order until one matches, and then call the corresponding handler. Here, for example, we define three handlers and register three routes that point to those handlers:
class HomeHandler(webapp2.RequestHandler): def get(self): self.response.write('This is the HomeHandler.') class ProductListHandler(webapp2.RequestHandler): def get(self): self.response.write('This is the ProductListHandler.') class ProductHandler(webapp2.RequestHandler): def get(self, product_id): self.response.write('This is the ProductHandler. ' 'The product id is %s' % product_id) app = webapp2.WSGIApplication([ (r'/', HomeHandler), (r'/products', ProductListHandler), (r'/products/(\d+)', ProductHandler), ])
When a request comes in, the application will match the request path to find
the corresponding handler. If no route matches, an
HTTPException is raised
with status code 404, and the WSGI application can handle it accordingly (see
The regex part is an ordinary regular expression (see the
module) that can define groups inside parentheses. The matched group values are
passed to the handler as positional arguments. In the example above, the last
route defines a group, so the handler will receive the matched value when the
route matches (one or more digits in this case).
Important note: If the route includes a regex group, all handler methods that receive requests from that route must include a parameter for the positional arguments; otherwise the application will attempt to pass an argument to the handler with no matching parameter and an exception will occur.
The handler part is a callable as explained in Request handlers, and can also be a string in dotted notation to be lazily imported when needed (see explanation below in Lazy Handlers).
Simple routes are easy to use and enough for a lot of cases but don’t support keyword arguments, URI building, domain and subdomain matching, automatic redirection and other useful features. For this, webapp2 offers the extended routing mechanism that we’ll see next.
webapp2 introduces a routing mechanism that extends the webapp model to provide additional features:
And several other features and benefits.
The concept is similar to the simple routes we saw before, but instead of a
(regex, handler), we define each route using the class
webapp2.Route. Let’s remake our previous routes using it:
app = webapp2.WSGIApplication([ webapp2.Route(r'/', handler=HomeHandler, name='home'), webapp2.Route(r'/products', handler=ProductListHandler, name='product-list'), webapp2.Route(r'/products/<product_id:\d+>', handler=ProductHandler, name='product'), ])
The first argument in the routes above is a URL template, the handler argument is the request handler to be used, and the name argument third is a name used to build a URI for that route.
webapp2.Route.__init__() in the API reference for the parameters
accepted by the
Route constructor. We will explain some of them in details
The URL template defines the URL path to be matched. It can have regular
expressions for variables using the syntax
<> is not interpreted as a regular expression to be matched.
Both name and regex are optional, like in the examples below:
The same template can mix parts with name, regular expression or both.
The name, if defined, is used to build URLs for the route. When it is set, the value of the matched regular expression is passed as keyword argument to the handler. Otherwise it is passed as positional argument.
If only the name is set, it will match anything except a slash. So these routes are equivalent:
Route('/<user_id>/settings', handler=SettingsHandler, name='user-settings') Route('/<user_id:[^/]+>/settings', handler=SettingsHandler, name='user-settings')
The handler only receives
*args if no named variables are
set. Otherwise, the handler only receives
allows you to set regular expressions that are not captured:
just mix named and unnamed variables and the handler will
only receive the named ones.
One additional feature compared to webapp is that the handler can also be defined as a string in dotted notation to be lazily imported when needed.
This is useful to avoid loading all modules when the app is initialized: we can define handlers in different modules without needing to import all of them to initialize the app. This is not only convenient but also speeds up the application startup.
The string must contain the package or module name and the name of the handler
(a class or function name). Our previous example could be rewritten using
strings instead of handler classes and splitting our handlers in two files,
app = webapp2.WSGIApplication([ (r'/', 'handlers.HomeHandler'), (r'/products', 'products.ProductListHandler'), (r'/products/(\d+)', 'products.ProductHandler'), ])
In the first time that one of these routes matches, the handlers will be automatically imported by the routing system.
handler_method can define the method of the handler that will
be called, if handler is a class. If not defined, the default behavior is to
translate the HTTP method to a handler method, as explained in
Request handlers. For example:
webapp2.Route(r'/products', handler='handlers.ProductsHandler', name='products-list', handler_method='list_products')
Alternatively, the handler method can be defined in the handler string, separated by a colon. This is equivalent to the previous example:
webapp2.Route(r'/products', handler='handlers.ProductsHandler:list_products', name='products-list')
If needed, the route can define a sequence of allowed HTTP methods. Only if the
request method is in that list or tuple the route will match. If the method is
not allowed, an
HTTPMethodNotAllowed exception is raised with status code
405. For example:
webapp2.Route(r'/products', handler='handlers.ProductsHandler', name='products-list', methods=['GET'])
This is useful when using functions as handlers, or alternative handlers that
don’t translate the HTTP method to the handler method like the default
Like with HTTP methods, you can specify the URI schemes allowed for a route,
if needed. This is useful if some URIs must be accessed using ‘http’ or ‘https’
only. For this, set the
schemes parameter when defining a route:
webapp2.Route(r'/products', handler='handlers.ProductsHandler', name='products-list', schemes=['https'])
The above route will only match if the URI scheme is ‘https’.
The routing system can also handle domain and subdomain matching. This is done
using a special route class provided in the
webapp2_extras.routes.DomainRoute. It is initialized with
a pattern to match the current server name and a list of nested
webapp2.Route instances that will only be tested if the domain or
For example, to restrict routes to a subdomain of the appspot domain:
import webapp2 from webapp2_extras import routes app = webapp2.WSGIApplication([ routes.DomainRoute('<subdomain>.app-id.appspot.com', [ webapp2.Route('/', handler=SubdomainHomeHandler, name='subdomain-home'), ]), webapp2.Route('/', handler=HomeHandler, name='home'), ])
In the example above, we define a template
for the domain matching. When a request comes in, only if the request server
name matches that pattern, the nested route will be tested. Otherwise the
routing system will test the next route until one matches. So the first route
/ will only match when a subdomain of the
domain is accessed. Otherwise the second route with path
/ will be used.
The template follows the same syntax used by
must define named groups if any value must be added to the match results.
In the example above, an extra subdomain keyword is passed to the handler,
but if the regex didn’t define any named groups, nothing would be added.
A common need is to set some routes for the main subdomain (
different routes for other subdomains. The webapp2 routing system can handle
To match only the
www subdomain, simple set the domain template to a fixed
routes.DomainRoute('www.mydomain.com', [ webapp2.Route('/', handler=HomeHandler, name='home'), ])
To match any subdomain except the
www subdomain, set a regular expression
routes.DomainRoute(r'<subdomain:(?!www\.)[^.]+>.mydomain.com', [ webapp2.Route('/', handler=HomeHandler, name='home'), ])
Any subdomain that matches and is not
www will be passed as a parameter
subdomain to the handler.
Similarly, you can restrict matches to the main
appspot domain or
www domain from a custom domain:
routes.DomainRoute(r'<:(app-id\.appspot\.com|www\.mydomain\.com)>', [ webapp2.Route('/', handler=HomeHandler, name='home'), ])
And then have a route that matches subdomains of the main
or from a custom domain, except
routes.DomainRoute(r'<subdomain:(?!www)[^.]+>.<:(app-id\.appspot\.com|mydomain\.com)>', [ webapp2.Route('/', handler=HomeHandler, name='home'), ])
webapp2_extras.routes provides a class to wrap routes that start
with a common path: the
The intention is to avoid repetition when defining routes.
For example, imagine we have these routes:
app = WSGIApplication([ Route('/users/<user:\w+>/', UserOverviewHandler, 'user-overview'), Route('/users/<user:\w+>/profile', UserProfileHandler, 'user-profile'), Route('/users/<user:\w+>/projects', UserProjectsHandler, 'user-projects'), ])
We could refactor them to reuse the common path prefix:
import webapp2 from webapp2_extras import routes app = WSGIApplication([ routes.PathPrefixRoute('/users/<user:\w+>', [ webapp2.Route('/', UserOverviewHandler, 'user-overview'), webapp2.Route('/profile', UserProfileHandler, 'user-profile'), webapp2.Route('/projects', UserProjectsHandler, 'user-projects'), ]), ])
This is not only convenient, but also performs better: the nested routes will only be tested if the path prefix matches.
webapp2_extras.routes has other convenience classes that accept
nested routes with a common attribute prefix:
webapp2_extras.routes.HandlerPrefixRoute: receives a handler module prefix in dotted notation and a list of routes that use that module.
webapp2_extras.routes.NamePrefixRoute: receives a handler name prefix and a list of routes that start with that name.
Because our routes have a
name, we can use the routing system to build
URIs whenever we need to reference those resources inside the application.
This is done using the function
webapp2.uri_for() or the method
webapp2.RequestHandler.uri_for() inside a handler, or calling
webapp2.Router.build() directly (a
Router instance is set as an
router in the WSGI application).
For example, if you have these routes defined for the application:
app = webapp2.WSGIApplication([ webapp2.Route('/', handler='handlers.HomeHandler', name='home'), webapp2.Route('/wiki', handler=WikiHandler, name='wiki'), webapp2.Route('/wiki/<page>', handler=WikiHandler, name='wiki-page'), ])
Here are some examples of how to generate URIs for them:
# / uri = uri_for('home') # http://localhost:8080/ uri = uri_for('home', _full=True) # /wiki uri = uri_for('wiki') # http://localhost:8080/wiki uri = uri_for('wiki', _full=True) # http://localhost:8080/wiki#my-heading uri = uri_for('wiki', _full=True, _fragment='my-heading') # /wiki/my-first-page uri = uri_for('wiki-page', page='my-first-page') # /wiki/my-first-page?format=atom uri = uri_for('wiki-page', page='my-first-page', format='atom')
Variables are passed as positional or keyword arguments and are required if the route defines them. Keyword arguments that are not present in the route are added to the URI as a query string.
Also, when calling
uri_for(), a few keywords have special meaning:
webapp2.Router.build() in the API reference for a complete
explanation of the parameters used to build URIs.
The parameters from the matched route are set as attributes of the request
object when a route matches. They are
request.route_args, for positional
request.route_kwargs, for keyword arguments.
The matched route object is also available as