Allow custom openapi() to be async

fastapi/applications.py

@@ -1,3 +1,4 @@
+import asyncio
 from enum import Enum
 from typing import (
     Any,
@@ -1058,7 +1059,10 @@ class FastAPI(Starlette):
                     if root_path and self.root_path_in_servers:
                         self.servers.insert(0, {"url": root_path})
                         server_urls.add(root_path)
-                return JSONResponse(self.openapi())
+                if asyncio.iscoroutinefunction(self.openapi):
+                    return JSONResponse(await self.openapi())
+                else:
+                    return JSONResponse(self.openapi())
 
             self.add_route(self.openapi_url, openapi, include_in_schema=False)
         if self.openapi_url and self.docs_url:

tests/test_application.py

@@ -1,5 +1,6 @@
 import pytest
 from dirty_equals import IsDict
+from fastapi import FastAPI
 from fastapi.testclient import TestClient
 
 from .main import app
@@ -52,6 +53,18 @@ def test_enum_status_code_response():
     assert response.json() == "foo bar"
 
 
+def test_allow_async_openapi():
+    async def async_openapi():
+        return {"foo": "bar"}
+
+    mod_app = FastAPI()  # use fresh instance to not affect other tests
+    mod_app.openapi = async_openapi
+    mod_client = TestClient(mod_app)
+    response = mod_client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    assert response.json() == {"foo": "bar"}
+
+
 def test_openapi_schema():
     response = client.get("/openapi.json")
     assert response.status_code == 200, response.text