Falcon

Perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away.

- Antoine de Saint-Exupéry

Falcon is a high-performance Python framework for building cloud APIs. It encourages the REST architectural style, and tries to do as little as possible while remaining highly effective.

Quick Links

Design Goals

Fast. Cloud APIs need to turn around requests quickly, and make efficient use of hardware. This is particularly important when serving many concurrent requests. Falcon is among the fastest WSGI frameworks available, processing requests several times faster than other Python web frameworks.

Light. Only the essentials are included, with six and mimeparse being the only dependencies outside the standard library. We work hard to keep the code lean, making Falcon easier to test, secure, optimize, and deploy.

Flexible. Falcon is not opinionated when it comes to talking to databases, rendering content, authorizing requests, etc. You are free to mix and match your own favorite libraries. Falcon apps work with any WSGI server, and run great under CPython 2.6-2.7, PyPy, Jython 2.7, and CPython 3.3-3.5.

Features

Highly-optimized, extensible code base
Intuitive routing via URI templates and REST-inspired resource classes
Easy access to headers and bodies through request and response classes
DRY request processing via middleware components and hooks
Idiomatic HTTP error responses
Straightforward exception handling
Snappy unit testing through WSGI helpers and mocks
CPython 2.6-2.7, PyPy, Jython 2.7, and CPython 3.3-3.5 support
~20% speed boost when Cython is available

Install

PyPy

PyPy is the fastest way to run your Falcon app. However, note that only the PyPy 2.7 compatible release is currently supported.

$ pip install falcon

CPython

Falcon also fully supports CPython 2.6-3.5.

A universal wheel is available on PyPI for the the Falcon framework. Installing it is as simple as:

$ pip install falcon

Installing the wheel is a great way to get up and running with Falcon quickly in a development environment, but for an extra speed boost when deploying your application in production, Falcon can compile itself with Cython.

The following commands tell pip to install Cython, and then to invoke Falcon's setup.py, which will in turn detect the presence of Cython and then compile (AKA cythonize) the Falcon framework with the system's default C compiler.

$ pip install cython
$ pip install --no-binary :all: falcon

Installing on OS X

Xcode Command Line Tools are required to compile Cython. Install them with this command:

$ xcode-select --install

The Clang compiler treats unrecognized command-line options as errors; this can cause problems under Python 2.6, for example:

clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]

You might also see warnings about unused functions. You can work around these issues by setting additional Clang C compiler flags as follows:

$ export CFLAGS="-Qunused-arguments -Wno-unused-function"

Test

$ pip install -r tools/test-requires
$ pip install nose && nosetests

To run the default set of tests:

$ pip install tox && tox

Read the docs

The docstrings in the Falcon code base are quite extensive, and we recommend keeping a REPL running while learning the framework so that you can query the various modules and classes as you have questions.

Online docs are available at: http://falcon.readthedocs.org

You can build the same docs locally as follows:

$ pip install -r tools/doc-requires
$ cd doc
$ make html

$ # open _build/html/index.html

Getting started

Here is a simple, contrived example showing how to create a Falcon-based API.

# things.py

# Let's get this party started!
import falcon


# Falcon follows the REST architectural style, meaning (among
# other things) that you think in terms of resources and state
# transitions, which map to HTTP verbs.
class ThingsResource(object):
    def on_get(self, req, resp):
        """Handles GET requests"""
        resp.status = falcon.HTTP_200  # This is the default status
        resp.body = ('\nTwo things awe me most, the starry sky '
                     'above me and the moral law within me.\n'
                     '\n'
                     '    ~ Immanuel Kant\n\n')

# falcon.API instances are callable WSGI apps
app = falcon.API()

# Resources are represented by long-lived class instances
things = ThingsResource()

# things will handle all requests to the '/things' URL path
app.add_route('/things', things)

You can run the above example using any WSGI server, such as uWSGI or Gunicorn. For example:

$ pip install gunicorn
$ gunicorn things:app

Then, in another terminal:

$ curl localhost:8000/things

A more complex example

Here is a more involved example that demonstrates reading headers and query parameters, handling errors, and working with request and response bodies.

import json
import logging
import uuid
from wsgiref import simple_server

import falcon
import requests


class StorageEngine(object):

    def get_things(self, marker, limit):
        return [{'id': str(uuid.uuid4()), 'color': 'green'}]

    def add_thing(self, thing):
        thing['id'] = str(uuid.uuid4())
        return thing


class StorageError(Exception):

    @staticmethod
    def handle(ex, req, resp, params):
        description = ('Sorry, couldn\'t write your thing to the '
                       'database. It worked on my box.')

        raise falcon.HTTPError(falcon.HTTP_725,
                               'Database Error',
                               description)


class SinkAdapter(object):

    engines = {
        'ddg': 'https://duckduckgo.com',
        'y': 'https://search.yahoo.com/search',
    }

    def __call__(self, req, resp, engine):
        url = self.engines[engine]
        params = {'q': req.get_param('q', True)}
        result = requests.get(url, params=params)

        resp.status = str(result.status_code) + ' ' + result.reason
        resp.content_type = result.headers['content-type']
        resp.body = result.text


class AuthMiddleware(object):

    def process_request(self, req, resp):
        token = req.get_header('Authorization')
        account_id = req.get_header('Account-ID')

        challenges = ['Token type="Fernet"']

        if token is None:
            description = ('Please provide an auth token '
                           'as part of the request.')

            raise falcon.HTTPUnauthorized('Auth token required',
                                          description,
                                          challenges,
                                          href='http://docs.example.com/auth')

        if not self._token_is_valid(token, account_id):
            description = ('The provided auth token is not valid. '
                           'Please request a new token and try again.')

            raise falcon.HTTPUnauthorized('Authentication required',
                                          description,
                                          challenges,
                                          href='http://docs.example.com/auth')

    def _token_is_valid(self, token, account_id):
        return True  # Suuuuuure it's valid...


class RequireJSON(object):

    def process_request(self, req, resp):
        if not req.client_accepts_json:
            raise falcon.HTTPNotAcceptable(
                'This API only supports responses encoded as JSON.',
                href='http://docs.examples.com/api/json')

        if req.method in ('POST', 'PUT'):
            if 'application/json' not in req.content_type:
                raise falcon.HTTPUnsupportedMediaType(
                    'This API only supports requests encoded as JSON.',
                    href='http://docs.examples.com/api/json')


class JSONTranslator(object):

    def process_request(self, req, resp):
        # req.stream corresponds to the WSGI wsgi.input environ variable,
        # and allows you to read bytes from the request body.
        #
        # See also: PEP 3333
        if req.content_length in (None, 0):
            # Nothing to do
            return

        body = req.stream.read()
        if not body:
            raise falcon.HTTPBadRequest('Empty request body',
                                        'A valid JSON document is required.')

        try:
            req.context['doc'] = json.loads(body.decode('utf-8'))

        except (ValueError, UnicodeDecodeError):
            raise falcon.HTTPError(falcon.HTTP_753,
                                   'Malformed JSON',
                                   'Could not decode the request body. The '
                                   'JSON was incorrect or not encoded as '
                                   'UTF-8.')

    def process_response(self, req, resp, resource):
        if 'result' not in req.context:
            return

        resp.body = json.dumps(req.context['result'])


def max_body(limit):

    def hook(req, resp, resource, params):
        length = req.content_length
        if length is not None and length > limit:
            msg = ('The size of the request is too large. The body must not '
                   'exceed ' + str(limit) + ' bytes in length.')

            raise falcon.HTTPRequestEntityTooLarge(
                'Request body is too large', msg)

    return hook


class ThingsResource(object):

    def __init__(self, db):
        self.db = db
        self.logger = logging.getLogger('thingsapp.' + __name__)

    def on_get(self, req, resp, user_id):
        marker = req.get_param('marker') or ''
        limit = req.get_param_as_int('limit') or 50

        try:
            result = self.db.get_things(marker, limit)
        except Exception as ex:
            self.logger.error(ex)

            description = ('Aliens have attacked our base! We will '
                           'be back as soon as we fight them off. '
                           'We appreciate your patience.')

            raise falcon.HTTPServiceUnavailable(
                'Service Outage',
                description,
                30)

        # An alternative way of doing DRY serialization would be to
        # create a custom class that inherits from falcon.Request. This
        # class could, for example, have an additional 'doc' property
        # that would serialize to JSON under the covers.
        req.context['result'] = result

        resp.set_header('Powered-By', 'Falcon')
        resp.status = falcon.HTTP_200

    @falcon.before(max_body(64 * 1024))
    def on_post(self, req, resp, user_id):
        try:
            doc = req.context['doc']
        except KeyError:
            raise falcon.HTTPBadRequest(
                'Missing thing',
                'A thing must be submitted in the request body.')

        proper_thing = self.db.add_thing(doc)

        resp.status = falcon.HTTP_201
        resp.location = '/%s/things/%s' % (user_id, proper_thing['id'])


# Configure your WSGI server to load "things.app" (app is a WSGI callable)
app = falcon.API(middleware=[
    AuthMiddleware(),
    RequireJSON(),
    JSONTranslator(),
])

db = StorageEngine()
things = ThingsResource(db)
app.add_route('/{user_id}/things', things)

# If a responder ever raised an instance of StorageError, pass control to
# the given handler.
app.add_error_handler(StorageError, StorageError.handle)

# Proxy some things to another service; this example shows how you might
# send parts of an API off to a legacy system that hasn't been upgraded
# yet, or perhaps is a single cluster that all data centers have to share.
sink = SinkAdapter()
app.add_sink(sink, r'/search/(?P<engine>ddg|y)\Z')

# Useful for debugging problems in your API; works with pdb.set_trace(). You
# can also use Gunicorn to host your app. Gunicorn can be configured to
# auto-restart workers when it detects a code change, and it also works
# with pdb.
if __name__ == '__main__':
    httpd = simple_server.make_server('127.0.0.1', 8000, app)
    httpd.serve_forever()

Community

The Falcon community maintains a mailing list that you can use to share your ideas and ask questions about the framework. We use the appropriately minimalistic Librelist to host the discussions.

To join the mailing list, simply send your first email to falcon@librelist.com! This will automatically subscribe you to the mailing list and sends your email along to the rest of the subscribers. For more information about managing your subscription, check out the Librelist help page.

We expect everyone who participates on the mailing list to act professionally, and lead by example in encouraging constructive discussions. Each individual in the community is responsible for creating a positive, constructive, and productive culture. See also the Falcon Code of Conduct

Discussions are archived for posterity.

We also hang out in #falconframework on freenode, where everyone is always welcome to ask questions and share ideas.

Contributing

Kurt Griffiths (kgriffs) is the creator and current maintainer of the Falcon framework, with the generous help of a number of stylish and talented contributors.

Pull requests are always welcome. We use the GitHub issue tracker to organize our work, put you do not need to open a new issue before submitting a PR.

Before submitting a pull request, please ensure you have added/updated the appropriate tests (and that all existing tests still pass with your changes), and that your coding style follows PEP 8 and doesn't cause pyflakes to complain.

Commit messages should be formatted using AngularJS conventions.

Comments follow Google's style guide, with the additional requirement of prefixing inline comments using your GitHub nick and an appropriate prefix:

TODO(riker): Damage report!
NOTE(riker): Well, that's certainly good to know.
PERF(riker): Travel time to the nearest starbase?
APPSEC(riker): In all trust, there is the possibility for betrayal.

Legal

Falcon image courtesy of John O'Neill.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use any portion of the Falcon framework except in compliance with the License. Contributors agree to license their work under the same License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

TheRealFalcon/falcon