falcon 3.1.1

Quick Links

• Read the docs (FAQ - getting help - reference)

• Falcon add-ons and complementary packages

• Falcon articles, talks and podcasts

• falconry/user for Falcon users @ Gitter

• falconry/dev for Falcon contributors @ Gitter

What People are Saying

“Falcon is rock solid and it’s fast.”

“We have been using Falcon as a replacement for [another framework] and we simply love the performance (three times faster) and code base size (easily half of our [original] code).”

“I’m loving #falconframework! Super clean and simple, I finally have the speed and flexibility I need!”

“Falcon looks great so far. I hacked together a quick test for a tiny server of mine and was ~40% faster with only 20 minutes of work.”

“I feel like I’m just talking HTTP at last, with nothing in the middle. Falcon seems like the requests of backend.”

“The source code for Falcon is so good, I almost prefer it to documentation. It basically can’t be wrong.”

“What other framework has integrated support for 786 TRY IT NOW ?”

Features

Falcon tries to do as little as possible while remaining highly effective.

• ASGI, WSGI, and WebSocket support

• Native asyncio support

• No reliance on magic globals for routing and state management

• Stable interfaces with an emphasis on backwards-compatibility

• Simple API modeling through centralized RESTful routing

• Highly-optimized, extensible code base

• Easy access to headers and bodies through request and response classes

• DRY request processing via middleware components and hooks

• Strict adherence to RFCs

• Idiomatic HTTP error responses

• Straightforward exception handling

• Snappy testing with WSGI/ASGI helpers and mocks

• CPython 3.5+ and PyPy 3.5+ support

Support Falcon Development

Has Falcon helped you make an awesome app? Show your support today with a one-time donation or by becoming a patron. Supporters get cool gear, an opportunity to promote their brand to Python developers, and prioritized support.

• Learn how to support Falcon development

Thanks!

How is Falcon Different?

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

We designed Falcon to support the demanding needs of large-scale microservices and responsive app backends. Falcon complements more general Python web frameworks by providing bare-metal performance, reliability, and flexibility wherever you need it.

Reliable. We go to great lengths to avoid introducing breaking changes, and when we do they are fully documented and only introduced (in the spirit of SemVer) with a major version increment. The code is rigorously tested with numerous inputs and we require 100% coverage at all times. Falcon has no dependencies outside the standard library, helping minimize your app’s attack surface while avoiding transitive bugs and breaking changes.

Debuggable. Falcon eschews magic. It’s easy to tell which inputs lead to which outputs. Unhandled exceptions are never encapsulated or masked. Potentially surprising behaviors, such as automatic request body parsing, are well-documented and disabled by default. Finally, when it comes to the framework itself, we take care to keep logic paths simple and understandable. All this makes it easier to reason about the code and to debug edge cases in large-scale deployments.

Fast. Same hardware, more requests. Falcon turns around requests significantly faster than other popular Python frameworks like Django and Flask. For an extra speed boost, Falcon compiles itself with Cython when available, and also works well with PyPy. Considering a move to another programming language? Benchmark with Falcon+PyPy first!

Flexible. Falcon leaves a lot of decisions and implementation details to you, the API developer. This gives you a lot of freedom to customize and tune your implementation. It also helps you understand your apps at a deeper level, making them easier to tune, debug, and refactor over the long run. Falcon’s minimalist design provides space for Python community members to independently innovate on Falcon add-ons and complementary packages.

Who’s Using Falcon?

Falcon is used around the world by a growing number of organizations, including:

• 7ideas

• Cronitor

• EMC

• Hurricane Electric

• Leadpages

• OpenStack

• Rackspace

• Shiftgig

• tempfil.es

• Opera Software

If you are using the Falcon framework for a community or commercial project, please consider adding your information to our wiki under Who’s Using Falcon?

Community

A number of Falcon add-ons, templates, and complementary packages are available for use in your projects. We’ve listed several of these on the Falcon wiki as a starting point, but you may also wish to search PyPI for additional resources.

The Falconry community on Gitter is a great place to ask questions and share your ideas. You can find us in falconry/user. We also have a falconry/dev room for discussing the design and development of the framework itself.

Per our Code of Conduct, we expect everyone who participates in community discussions 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.

Installation

$ pip install falcon

$ pip install --pre falcon

$ pip install falcon

$ pip install --pre falcon

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

$ FALCON_DISABLE_CYTHON=Y pip install -v --no-binary :all: falcon

$ xcode-select --install

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

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

WSGI Server

Falcon speaks WSGI (or ASGI; see also below). In order to serve a Falcon app, you will need a WSGI server. Gunicorn and uWSGI are some of the more popular ones out there, but anything that can load a WSGI app will do.

$ pip install [gunicorn|uwsgi]

ASGI Server

In order to serve a Falcon ASGI app, you will need an ASGI server. Uvicorn is a popular choice:

$ pip install uvicorn

Source Code

Falcon lives on GitHub, making the code easy to browse, download, fork, etc. Pull requests are always welcome! Also, please remember to star the project if it makes you happy. :)

Once you have cloned the repo or downloaded a tarball from GitHub, you can install Falcon like this:

$ cd falcon
$ pip install .

Or, if you want to edit the code, first fork the main repo, clone the fork to your desktop, and then run the following to install it using symbolic linking, so that when you change your code, the changes will be automagically available to your app without having to reinstall the package:

$ cd falcon
$ pip install -e .

You can manually test changes to the Falcon framework by switching to the directory of the cloned repo and then running pytest:

$ cd falcon
$ pip install -r requirements/tests
$ pytest tests

Or, to run the default set of tests:

$ pip install tox && tox

See also the tox.ini file for a full list of available environments.

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: https://falcon.readthedocs.io

You can build the same docs locally as follows:

$ pip install tox && tox -e docs

Once the docs have been built, you can view them by opening the following index page in your browser. On OS X it’s as simple as:

$ open docs/_build/html/index.html

Or on Linux:

$ xdg-open docs/_build/html/index.html

Getting Started

Here is a simple, contrived example showing how to create a Falcon-based WSGI app (the ASGI version is included further down):

# examples/things.py

# Let's get this party started!
from wsgiref.simple_server import make_server

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:
    def on_get(self, req, resp):
        """Handles GET requests"""
        resp.status = falcon.HTTP_200  # This is the default status
        resp.content_type = falcon.MEDIA_TEXT  # Default is JSON, so override
        resp.text = ('\nTwo things awe me most, the starry sky '
                     'above me and the moral law within me.\n'
                     '\n'
                     '    ~ Immanuel Kant\n\n')


# falcon.App instances are callable WSGI apps...
# in larger applications the app is created in a separate file
app = falcon.App()

# 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)

if __name__ == '__main__':
    with make_server('', 8000, app) as httpd:
        print('Serving on port 8000...')

        # Serve until process is killed
        httpd.serve_forever()

You can run the above example directly using the included wsgiref server:

$ pip install falcon
$ python things.py

Then, in another terminal:

$ curl localhost:8000/things

The ASGI version of the example is similar:

# examples/things_asgi.py

import falcon
import falcon.asgi


# 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:
    async def on_get(self, req, resp):
        """Handles GET requests"""
        resp.status = falcon.HTTP_200  # This is the default status
        resp.content_type = falcon.MEDIA_TEXT  # Default is JSON, so override
        resp.text = ('\nTwo things awe me most, the starry sky '
                     'above me and the moral law within me.\n'
                     '\n'
                     '    ~ Immanuel Kant\n\n')


# falcon.asgi.App instances are callable ASGI apps...
# in larger applications the app is created in a separate file
app = falcon.asgi.App()

# 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 ASGI version with uvicorn or any other ASGI server:

$ pip install falcon uvicorn
$ uvicorn things_asgi:app

A More Complex Example (WSGI)

Here is a more involved example that demonstrates reading headers and query parameters, handling errors, and working with request and response bodies. Note that this example assumes that the requests package has been installed.

(For the equivalent ASGI app, see: A More Complex Example (ASGI)).

# examples/things_advanced.py

import json
import logging
import uuid
from wsgiref import simple_server

import falcon
import requests


class StorageEngine:

    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):
        # TODO: Log the error, clean up, etc. before raising
        raise falcon.HTTPInternalServerError()


class SinkAdapter:

    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.text = result.text


class AuthMiddleware:

    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(title='Auth token required',
                                          description=description,
                                          challenges=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(title='Authentication required',
                                          description=description,
                                          challenges=challenges,
                                          href='http://docs.example.com/auth')

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


class RequireJSON:

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


class JSONTranslator:
    # NOTE: Normally you would simply use req.media and resp.media for
    # this particular use case; this example serves only to illustrate
    # what is possible.

    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(title='Empty request body',
                                        description='A valid JSON document is required.')

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

        except (ValueError, UnicodeDecodeError):
            description = ('Could not decode the request body. The '
                           'JSON was incorrect or not encoded as '
                           'UTF-8.')

            raise falcon.HTTPBadRequest(title='Malformed JSON',
                                        description=description)

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

        resp.text = json.dumps(resp.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.HTTPPayloadTooLarge(
                title='Request body is too large', description=msg)

    return hook


class ThingsResource:

    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(
                title='Service Outage',
                description=description,
                retry_after=30)

        # NOTE: Normally you would use resp.media for this sort of thing;
        # this example serves only to demonstrate how the context can be
        # used to pass arbitrary values between middleware components,
        # hooks, and resources.
        resp.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 AttributeError:
            raise falcon.HTTPBadRequest(
                title='Missing thing',
                description='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.App(middleware=[
    AuthMiddleware(),
    RequireJSON(),
    JSONTranslator(),
])

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

# If a responder ever raises 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()

Again this code uses wsgiref, but you can also run the above example using any WSGI server, such as uWSGI or Gunicorn. For example:

$ pip install requests gunicorn
$ gunicorn things:app

On Windows you can run Gunicorn and uWSGI via WSL, or you might try Waitress:

$ pip install requests waitress
$ waitress-serve --port=8000 things:app

To test this example, open another terminal and run:

$ http localhost:8000/1/things authorization:custom-token

You can also view the application configuration from the CLI via the falcon-inspect-app script that is bundled with the framework:

falcon-inspect-app things_advanced:app

A More Complex Example (ASGI)

Here’s the ASGI version of the app from above. Note that it uses the httpx package in lieu of requests.

# examples/things_advanced_asgi.py

import json
import logging
import uuid

import falcon
import falcon.asgi
import httpx


class StorageEngine:

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

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


class StorageError(Exception):

    @staticmethod
    async def handle(ex, req, resp, params):
        # TODO: Log the error, clean up, etc. before raising
        raise falcon.HTTPInternalServerError()


class SinkAdapter:

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

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

        async with httpx.AsyncClient() as client:
            result = await client.get(url, params=params)

        resp.status = result.status_code
        resp.content_type = result.headers['content-type']
        resp.text = result.text


class AuthMiddleware:

    async 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(title='Auth token required',
                                          description=description,
                                          challenges=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(title='Authentication required',
                                          description=description,
                                          challenges=challenges,
                                          href='http://docs.example.com/auth')

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


class RequireJSON:

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


class JSONTranslator:
    # NOTE: Normally you would simply use req.get_media() and resp.media for
    # this particular use case; this example serves only to illustrate
    # what is possible.

    async def process_request(self, req, resp):
        # NOTE: Test explicitly for 0, since this property could be None in
        # the case that the Content-Length header is missing (in which case we
        # can't know if there is a body without actually attempting to read
        # it from the request stream.)
        if req.content_length == 0:
            # Nothing to do
            return

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

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

        except (ValueError, UnicodeDecodeError):
            description = ('Could not decode the request body. The '
                           'JSON was incorrect or not encoded as '
                           'UTF-8.')

            raise falcon.HTTPBadRequest(title='Malformed JSON',
                                        description=description)

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

        resp.text = json.dumps(resp.context.result)


def max_body(limit):

    async 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.HTTPPayloadTooLarge(
                title='Request body is too large', description=msg)

    return hook


class ThingsResource:

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

    async 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 = await 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(
                title='Service Outage',
                description=description,
                retry_after=30)

        # NOTE: Normally you would use resp.media for this sort of thing;
        # this example serves only to demonstrate how the context can be
        # used to pass arbitrary values between middleware components,
        # hooks, and resources.
        resp.context.result = result

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

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

        proper_thing = await self.db.add_thing(doc)

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


# The app instance is an ASGI callable
app = falcon.asgi.App(middleware=[
    # AuthMiddleware(),
    RequireJSON(),
    JSONTranslator(),
])

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

# If a responder ever raises 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')

You can run the ASGI version with any ASGI server, such as uvicorn:

$ pip install falcon httpx uvicorn
$ uvicorn things_advanced_asgi:app

Contributing

Thanks for your interest in the project! We welcome pull requests from developers of all skill levels. To get started, simply fork the master branch on GitHub to your personal account and then clone the fork into your development environment.

If you would like to contribute but don’t already have something in mind, we invite you to take a look at the issues listed under our next milestone. If you see one you’d like to work on, please leave a quick comment so that we don’t end up with duplicated effort. Thanks in advance!

Please note that all contributors and maintainers of this project are subject to our Code of Conduct.

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.

The core Falcon project maintainers are:

• Kurt Griffiths, Project Lead (kgriffs on GH, Gitter, and Twitter)

• John Vrbanac (jmvrbanac on GH, Gitter, and Twitter)

• Vytautas Liuolia (vytas7 on GH and Gitter, and vliuolia on Twitter)

• Nick Zaccardi (nZac on GH and Gitter)

• Federico Caselli (CaselIT on GH and Gitter)

Please don’t hesitate to reach out if you have any questions, or just need a little help getting started. You can find us in falconry/dev on Gitter.

See also: CONTRIBUTING.md

Legal

Copyright 2013-2022 by Individual and corporate contributors as noted in the individual source files.

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.