Add httpx.MockTransport()

[lovelydinosaur]

Closes #1303

Docs as follows...

---

Mock transports

During testing it can often be useful to be able to mock out a transport, and return pre-determined responses, rather than making actual network requests.

The httpx.MockTransport class accepts a handler function, which can be used to map requests onto pre-determined responses:

def handler(request):
    return httpx.Response(200, json={"text": "Hello, world!"})


# Switch to a mock transport, if the TESTING environment variable is set.
if os.environ['TESTING'].upper() == "TRUE":
    transport = httpx.MockTransport(handler)
else:
    transport = httpx.HTTPTransport()

client = httpx.Client(transport=transport)

For more advanced use-cases you might want to take a look at the third-party mocking library, RESPX, or the pytest-httpx library.

[florimondmanca]

Superb! Excitingly clean. :-)

[lundberg]

Hooray 🎉

Latest release of RESPX (0.16.2) now has optional support for not patching the httpcore transports, and passing a RESPX router handler to the MockTransport.

import httpx
import respx


mock_router = respx.MockRouter(using=None)
route = mock_router.get("https://example.org/").mock(return_value=httpx.Response(204))

transport = httpx.MockTransport(mock_router.handler)

with httpx.Client(transport=transport) as client:
    response = client.get("https://example.org/")
    assert response.status_code == 204
    assert route.called

[florimondmanca]

💯

[simonw]

Last minute suggestion: it looks like the handler function currently cannot be an async def ... function, even for the async version of the mock transport:

httpx/httpx/_transports/mock.py

Lines 43 to 56 in 1816393

	request = Request(
	method=method,
	url=url,
	headers=headers,
	stream=stream,
	)
	await request.aread()
	response = self.handler(request)
	return (
	response.status_code,
	response.headers.raw,
	response.stream,
	response.ext,
	)

I can imagine cases where I might want to be able to write a handler function that uses await ... - to access an async database layer for example.

Maybe apply something like the await me maybe pattern here, so users can define a regular function or an async function for their handler?

[florimondmanca]

@simonw Yes, that sounds pretty sensible, thanks for noticing! :) I also think async mock views could be useful to me in some cases. I'm not sure we need to impose "async def for async client", so something like await me maybe sounds sensible...