Quart-CORS

Quart-CORS is an extension for Quart to enable and control Cross Origin Resource Sharing, CORS (also known as access control).

CORS is required to share resources in browsers due to the 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,

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,

@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),

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

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

@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,

@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,

@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()
    ...