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import fnmatch
from functools import wraps
def asgi_cors_decorator(
allow_all=False, hosts=None, host_wildcards=None, callback=None
hosts = hosts or []
host_wildcards = host_wildcards or []
# We need hosts and host_wildcards to be b""
hosts = set(h.encode("utf8") if isinstance(h, str) else h for h in hosts)
host_wildcards = [
h.encode("utf8") if isinstance(h, str) else h for h in host_wildcards
if any(h.endswith(b"/") for h in (hosts or [])) or any(
h.endswith(b"/") for h in (host_wildcards or [])
assert False, "Error: CORS origin rules should never end in a /"
def _asgi_cors_decorator(app):
async def app_wrapped_with_cors(scope, recieve, send):
async def wrapped_send(event):
if event["type"] == "http.response.start":
original_headers = event.get("headers") or []
access_control_allow_origin = None
if allow_all:
access_control_allow_origin = b"*"
elif hosts or host_wildcards or callback:
incoming_origin = dict(scope.get("headers") or []).get(
if incoming_origin:
matches_hosts = incoming_origin in hosts
matches_wildcards = any(
fnmatch.fnmatch(incoming_origin, host_wildcard)
for host_wildcard in host_wildcards
matches_callback = False
if callback is not None:
matches_callback = callback(incoming_origin)
if matches_hosts or matches_wildcards or matches_callback:
access_control_allow_origin = incoming_origin
if access_control_allow_origin is not None:
# Construct a new event with new headers
event = {
"type": "http.response.start",
"status": event["status"],
"headers": [
for p in original_headers
if p[0] != b"access-control-allow-origin"
+ [
await send(event)
await app(scope, recieve, wrapped_send)
return app_wrapped_with_cors
return _asgi_cors_decorator
def asgi_cors(app, allow_all=False, hosts=None, host_wildcards=None, callback=None):
return asgi_cors_decorator(allow_all, hosts, host_wildcards, callback)(app)
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