Quart-CORS

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Quart-CORS is an extension for Quart <https://github.com/pgjones/quart>_ to enable and control Cross Origin Resource Sharing <http://www.w3.org/TR/cors/>_, CORS (also known as access control).

CORS is required to share resources in browsers due to the Same Origin Policy <https://en.wikipedia.org/wiki/Same-origin_policy>_ which prevents resources being used from a different origin. An origin in this case is defined as the scheme, host and port combined and a resource corresponds to a path.

In practice the Same Origin Policy means that a browser visiting http://quart.com will prevent the response of GET http://api.com being read. It will also prevent requests such as POST http://api.com. Note that CORS applies to browser initiated requests, non-browser clients such as requests are not subject to CORS restrictions.

CORS allows a server to indicate to a browser that certain resources can be used, contrary to the Same Origin Policy. It does so via access-control headers that inform the browser how the resource can be used. For GET requests these headers are sent in the response. For non-GET requests the browser must ask the server for the access-control headers before sending the actual request, it does so via a preflight OPTIONS request.

The Same Origin Policy does not apply to WebSockets, and hence there is no need for CORS. Instead the server alone is responsible for deciding if the WebSocket is allowed and it should do so by inspecting the WebSocket-request origin header.

Simple (GET) requests should return CORS headers specifying the origins that are allowed to use the resource (response). This can be any origin, * (wildcard), or a list of specific origins. The response should also include a CORS header specifying whether response-credentials e.g. cookies can be used. Note that if credential sharing is allowed the allowed origins must be specific and not a wildcard.

Preflight requests should return CORS headers specifying the origins allowed to use the resource, the methods and headers allowed to be sent in a request to the resource, whether response credentials can be used, and finally which response headers can be used.

Note that certain actions are allowed in the Same Origin Policy such as embedding e.g. <img src="http://api.com/img.gif"> and simple POSTs. For the purposes of this readme though these complications are ignored.

The CORS access control response headers are,

================================ =========================================================== Header name Meaning

Access-Control-Allow-Origin Origins that are allowed to use the resource. Access-Control-Allow-Credentials Can credentials be shared. Access-Control-Allow-Methods Methods that may be used in requests to the resource. Access-Control-Allow-Headers Headers that may be sent in requests to the resource. Access-Control-Expose-Headers Headers that may be read in the response from the resource. Access-Control-Max-Age Maximum age to cache the CORS headers for the resource. ================================ ===========================================================

Quart-CORS uses the same naming (without the Access-Control prefix) for it's arguments and settings when they relate to the same meaning.

Usage

To add CORS access control headers to all of the routes in the application, simply apply the cors function to the application, or to a specific blueprint,

.. code-block:: python

app = Quart(__name__)
app = cors(app, **settings)

blueprint = Blueprint(__name__)
blueprint = cors(blueprint, **settings)

alternatively if you wish to add CORS selectively by resource, apply the route_cors function to a route, or the websocket_cors function to a WebSocket,

.. code-block:: python

@app.route('/')
@route_cors(**settings)
async def handler():
    ...

@app.websocket('/')
@websocket_cors(allow_origin=...)
async def handler():
    ...

The settings are these arguments,

================= ==================================================== Argument type

allow_origin Union[Set[Union[Pattern, str]], Union[Pattern, str]] allow_credentials bool allow_methods Union[Set[str], str] allow_headers Union[Set[str], str] expose_headers Union[Set[str], str] max_age Union[int, flot, timedelta] ================= ====================================================

which correspond to the CORS headers noted above. Note that all settings are optional and defaults can be specified in the application configuration,

============================ ======================== Configuration key type

QUART_CORS_ALLOW_ORIGIN Set[Union[Pattern, str]] QUART_CORS_ALLOW_CREDENTIALS bool QUART_CORS_ALLOW_METHODS Set[str] QUART_CORS_ALLOW_HEADERS Set[str] QUART_CORS_EXPOSE_HEADERS Set[str] QUART_CORS_MAX_AGE float ============================ ========================

The websocket_cors decorator only takes an allow_origin argument which defines the origins that are allowed to use the WebSocket. A WebSocket request from a disallowed origin will be responded to with a 400 response.

The allow_origin origins should be the origin only (no path, query strings or fragments) i.e. https://quart.com not https://quart.com/.

The cors_exempt decorator can be used in conjunction with cors to exempt a websocket handler or view function from cors.

Simple examples

To allow an app to be used from any origin (not recommended as it is
too permissive),

.. code-block:: python

    app = Quart(__name__)
    app = cors(app, allow_origin="*")

To allow a route or WebSocket to be used from another specific domain,
``https://quart.com``,

.. code-block:: python

    @app.route('/')
    @route_cors(allow_origin="https://quart.com")
    async def handler():
        ...

    @app.websocket('/')
    @websocket_cors(allow_origin="https://quart.com")
    async def handler():
        ...

To allow a route or WebSocket to be used from any subdomain (but not
the domain itself) of ``quart.com``,

.. code-block:: python

    @app.route('/')
    @route_cors(allow_origin=re.compile(r"https:\/\/.*\.quart\.com"))
    async def handler():
        ...

    @app.websocket('/')
    @websocket_cors(allow_origin=re.compile(r"https:\/\/.*\.quart\.com"))
    async def handler():
        ...

To allow a JSON POST request to an API route, from ``https://quart.com``,

.. code-block:: python

    @app.route('/', methods=["POST"])
    @route_cors(
        allow_headers=["content-type"],
        allow_methods=["POST"],
        allow_origin=["https://quart.com"],
    )
    async def handler():
        data = await request.get_json()
        ...

Contributing
------------

Quart-CORS is developed on `GitHub
<https://github.com/pgjones/quart-cors>`_. You are very welcome to
open `issues <https://github.com/pgjones/quart-cors/issues>`_ or
propose `merge requests
<https://github.com/pgjones/quart-cors/merge_requests>`_.

Testing
~~~~~~~

The best way to test Quart-CORS is with Tox,

.. code-block:: console

    $ pip install tox
    $ tox

this will check the code style and run the tests.

Help
----

This README is the best place to start, after that try opening an
`issue <https://github.com/pgjones/quart-cors/issues>`_.


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