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Project Description
© 2009-2016 Alice Bevan-McGregor and contributors.


Dispatch is the process of taking some starting point and a path, then resolving the object that path refers to. This process is common to almost every web application framework (transforming URLs into controllers), RPC system, and even filesystem shell. Other terms for this process include: “traversal”, “routing”, or “lookup”.

Route-based dispatch is the variant of dispatch that uses handlers for explicitly registered paths, optionally with regular expression (regex)-based path elements. This implementation exposes an API that particularly benefits from the use of mix-ins as traits. This gives a clean flexability to routes that are difficult to beat.

Most implementations of regex-based routing do so in a naïve way, often iterating lists of all routes at O(n) worst-case. Others allow you to manually partition the space with sub-routers, or optimize by declaration or manual lexicographical order. Some produce monolithic regular expressions that can cause instability when an application grows beyond a certain size. Some even iterate the whole list even after finding an endpoint.

This dispatcher does not. It builds a tree, and descends the tree preferring static elements to dynamic ones, with a controllalbe presedence at declaration. It optionally handles binding matched dynamic elements to arguments on the resulting endpoint. Performance is O(depth) worst-case.

This package speaks a standardized dispatch protocol and is not entirely intended for direct use by most developers. The target audience is instead the authors of frameworks that may require such modular dispatch for use by their own users.


Installing web.dispatch.route is easy, just execute the following in a terminal:

pip install web.dispatch.route

Note: We strongly recommend always using a container, virtualization, or sandboxing environment of some kind when developing using Python; installing things system-wide is yucky (for a variety of reasons) nine times out of ten. We prefer light-weight virtualenv, others prefer solutions as robust as Vagrant.

If you add web.dispatch.route to the install_requires argument of the call to setup() in your application’s file, this dispatcher will be automatically installed and made available when your own application or library is installed. We recommend using “less than” version numbers to ensure there are no unintentional side-effects when updating. Use web.dispatch.route<1.1 to get all bugfixes for the current release, and web.dispatch.route<2.0 to get bugfixes and feature updates while ensuring that large breaking changes are not installed.

Development Version

Development takes place on GitHub in the web.dispatch.route project. Issue tracking, documentation, and downloads are provided there.

Installing the current development version requires Git, a distributed source code management system. If you have Git you can run the following to download and link the development version into your Python runtime:

git clone
(cd web.dispatch.route; python develop)

You can then upgrade to the latest version at any time:

(cd web.dispatch.route; git pull; python develop)

If you would like to make changes and contribute them back to the project, fork the GitHub project, make your changes, and submit a pull request. This process is beyond the scope of this documentation; for more information see GitHub’s documentation.


This section is split between framework authors who will be integrating the overall protocol into their systems, and the “producers” using the system to register routes according to the API.

Framework Use

To begin resolving paths against routes registered in objects, first instantiate the dispatcher:

from web.dispatch.route import RouteDispatch

dispatch = RouteDispatch()

Currently the route dispatcher has no configuration options. With a prepared dispatcher, and supposing you have some object to dispatch against, you’ll need to prepare the path according to the protocol:

path = "/foo/bar/baz"  # Initial path, i.e. an HTTP request's PATH_INFO.
path = path.split('/')  # Find the path components.
path = path[1:]  # Skip the singular leading slash; see the API specification.
path = deque(path)  # Provide the path as a deque instance, allowing popleft.

Of course, the above is rarely split apart like that. We split apart the invidiual steps of path processing here to more clearly illustrate. In a web framework the above would happen once per request that uses dispatch. This, of course, frees your framework to use whatever internal or public representation of path you want: choices of separators, and the ability for deque to consume arbitrary iterables. An RPC system might split on a period and simply not have the possibility of leading separators. Etc.

You can now call the dispatcher and iterate the dispatch events:

for segment, handler, endpoint, *meta in dispatch(None, some_object, path):
    print(segment, handler, endpoint)  # Do something with this information.

The initial None value there represents the “context” to pass along to initializers of classes encountered during dispatch. If the value None is provided, classes won’t be instantiated with any arguments. If a context is provided it will be passed as the first positional argument to instantiation.

After completing iteration, check the final endpoint. If it is True then the path was successfully mapped to the object referenced by the handler variable. If dispatch is unsuccessful, a LookupError is raised with an explanation referencing the path element that caused the erorr.

You can always just skip straight to the answer if you so choose:

    segment, handler, endpoint, *meta = list(dispatch(None, some_object, path))[-1]
except LookupError:
    ... # Dispatch failed.

However, providing some mechanism for callbacks or notifications of dispatch is often far more generally useful.

Note: It is entirely permissable for dispatchers to return None as a processed path segment. Route-based dispatch will do this to announce the starting point of dispatch. This is especially useful if you need to know if the initial object was a class that was instantiated. (In that event handler will be an instance of some_object during the first iteration instead of being literally some_object.) Other dispatchers may return None at other times, such as to indicate multiple steps of intermediate processing.

Python 2 & 3 Compatibility

The dispatch protocol is designed to be extendable in the future by using namedtuple subclasses, however this has an impact on usage as you may have noticed the *meta in there. This syntax, introduced in Python 3, will gather any extraneous tuple elements into a separate list. If you actually care about the metadata do not unpack the tuple this way. Instead:

for meta in dispatch(None, some_object, path):
    segment, handler, endpoint = step[:3]  # Unpack, but preserve.
    print(segment, handler, endpoint, meta)  # Do something with this information.

This document is written from the perspective of modern Python 3, and throwing away the metadata within the for statement itself provides more compact examples. The above method of unpacking the first three values is the truly portable way to do this across versions.

Basic Routable Objects

The simplest routable object is one that has some attribute with a __route__ attribute of its own:

class Root:
    def hello(self, name):
        return "Hello " + name

    hello.__route__ = '/{name}'

This defines a method capable of handling any single path element. Because this is a common pattern, and having such annotations after the method body, divorced from the method’s definition, is ugly, a decorator is provided:

from web.dispatch.route import route

class Root:
    def hello(self, name):
        return "Hello " + name

Now an attempt to access a path such as /world will result in version of the method with that argument already bound to it. The syntax allows for customization of the default expression, which is simply “any single path element”. To do so, after the name add a colon (:) followed by the custom expression. Be careful not to use any forward slashes within your expression:

class Root:
    @route('/{name:[a-zA-Z ]+}/{age:[1-9][0-9]*}')
    def hello(self, name, age):
        return name + " is " + age + " years old"

Now access to /dad/27 is valid, returning a callable that when executed will return dad is 27 years old, but /42/dad is invalid, and won’t match any routes. When using the route decorator declaration order is preserved via the __index__ annotation.

Version History

Version 1.0

  • Initial extract from WebCore 2.


web.dispatch.route has been released under the MIT Open Source license.

The MIT License

Copyright © 2009-2016 Alice Bevan-McGregor and contributors.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


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Release History


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