Allow handler to optionally be async when MockTransport is used with …

…AsyncClient (#1449)

httpx/_transports/mock.py

@@ -1,3 +1,4 @@
+import asyncio
 from typing import Callable, List, Optional, Tuple
 
 import httpcore
@@ -47,7 +48,17 @@ async def arequest(
             stream=stream,
         )
         await request.aread()
+
         response = self.handler(request)
+
+        # Allow handler to *optionally* be an `async` function.
+        # If it is, then the `response` variable need to be awaited to actually
+        # return the result.
+
+        # https://simonwillison.net/2020/Sep/2/await-me-maybe/
+        if asyncio.iscoroutine(response):
+            response = await response
+
         return (
             response.status_code,
             response.headers.raw,

tests/client/test_async_client.py

@@ -301,3 +301,16 @@ async def test_mounted_transport():
         response = await client.get("custom://www.example.com")
         assert response.status_code == 200
         assert response.json() == {"app": "mounted"}
+
+
+@pytest.mark.usefixtures("async_environment")
+async def test_async_mock_transport():
+    async def hello_world(request):
+        return httpx.Response(200, text="Hello, world!")
+
+    transport = httpx.MockTransport(hello_world)
+
+    async with httpx.AsyncClient(transport=transport) as client:
+        response = await client.get("https://www.example.com")
+        assert response.status_code == 200
+        assert response.text == "Hello, world!"