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Test WebSocket callbacks before the first HTTP request #4085

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Test WebSocket callbacks before the first HTTP request #4085

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d62f731
Test WebSocket callbacks before the first HTTP request
wleeaf Oct 23, 2026
1f97674
Merge branch 'dev' into fix-fastapi-websocket-setup
T4rk1n Oct 26, 2026
0d0fc1d
Remove the test callback from the global registry if the test fails
wleeaf Oct 27, 2026
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45 changes: 44 additions & 1 deletion tests/backend_tests/test_fastapi_before_request.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
and Quart."""
import pytest

from dash import Dash, Input, Output, _get_app, hooks, html
from dash import Dash, Input, Output, _callback, _get_app, callback, hooks, html

pytest.importorskip("fastapi")

Expand Down Expand Up @@ -187,3 +187,46 @@ def broken():
):
pass
assert "hook bug" in caplog.text


def test_fabr006_websocket_callback_before_any_http_request():
# dash.callback is only copied into callback_map by _setup_server, a
# before_request hook, so the handshake must run it too (gh-4082).
from starlette.testclient import TestClient

app = Dash(__name__, backend="fastapi")
app.layout = html.Div([html.Button(id="ws-btn"), html.Div(id="ws-out")])

@T4rk1n T4rk1n Oct 26, 2026

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If this test ever regresses, _setup_server() never runs, so ws-out.children stays in _callback.GLOBAL_CALLBACK_MAP and gets pulled into whatever app the next test builds. Can you pop it in a finally (or a small fixture), like tests/unit/test_main_module_alias.py does, so one failure doesn't cascade?

@wleeaf wleeaf Oct 27, 2026

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The test now removes ws-out.children from GLOBAL_CALLBACK_MAP and GLOBAL_CALLBACK_LIST in a finally, like test_main_module_alias.py. With _setup_server patched to a no-op, only this test fails now, and a test that runs after it sees no leftover callback (0d0fc1d).


@callback(Output("ws-out", "children"), Input("ws-btn", "n_clicks"), websocket=True)
def echo(n_clicks):
return f"ws {n_clicks}"

request = {
"type": "callback_request",
"requestId": "1",
"rendererId": "r",
"payload": {
"output": "ws-out.children",
"outputs": {"id": "ws-out", "property": "children"},
"inputs": [{"id": "ws-btn", "property": "n_clicks", "value": 1}],
"changedPropIds": ["ws-btn.n_clicks"],
},
}
try:
with TestClient(app.server) as client:
with client.websocket_connect(
"/_dash-ws-callback", headers={"origin": "http://testserver"}
) as ws:
ws.send_json(request)
payload = ws.receive_json()["payload"]
assert payload["status"] == "ok"
assert payload["data"]["response"] == {"ws-out": {"children": "ws 1"}}
finally:
# Without _setup_server, the callback stays in the global registry and
# would be added to the app of the next test.
_callback.GLOBAL_CALLBACK_MAP.pop("ws-out.children", None)
_callback.GLOBAL_CALLBACK_LIST[:] = [
spec
for spec in _callback.GLOBAL_CALLBACK_LIST
if spec["output"] != "ws-out.children"
]