Fix MockTransport not setting elapsed property

[veeceey]

Summary

Fixes MockTransport to automatically set response.elapsed to timedelta(0) for both sync and async requests, eliminating boilerplate and matching real transport behavior.

Problem

Previously, accessing response.elapsed on MockTransport responses would raise RuntimeError:

RuntimeError: '.elapsed' may only be accessed after the response has been read or closed.

Users had to add boilerplate in every handler:

def handler(request):
    resp = httpx.Response(200, json={"text": "Hello, world!"})
    resp.elapsed = timedelta(0)  # Boilerplate needed
    return resp

Solution

MockTransport now automatically sets response.elapsed = timedelta(0) after calling the handler, in both handle_request() and handle_async_request().

Test Plan

Before fix:

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

transport = httpx.MockTransport(handler)
client = httpx.Client(transport=transport)
response = client.get("https://example.com")
print(response.elapsed)  # RuntimeError

After fix:

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

transport = httpx.MockTransport(handler)
client = httpx.Client(transport=transport)
response = client.get("https://example.com")
print(response.elapsed)  # 0:00:00 ✓

Tests:

• Added test_mock_transport_elapsed() for sync clients
• Added test_async_mock_transport_elapsed() for async clients
• Both tests verify elapsed == timedelta(0) and type correctness

Related

Fixes #3712

[veeceey]

All CI passing on Python 3.9-3.13, ready for merge

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.

[jeonjw85]

[P2] Preserve elapsed values explicitly supplied by the mock handler

This unconditionally replaces timing metadata that the handler already supplied, rather than just defaulting a missing value:

def handler(request):
    response = httpx.Response(200, text="ok")
    response.elapsed = timedelta(seconds=7)
    return response

with httpx.Client(transport=httpx.MockTransport(handler)) as client:
    assert client.get("http://local.invalid/").elapsed == timedelta(seconds=7)

The assertion passes on the base but fails here because elapsed becomes zero. This breaks existing mocks that deliberately simulate response timings, including the explicit assignment shown in #3712. The async assignment has the same regression. Please preserve a supplied elapsed value when adding the default.

Automated review; reproduced through both Client and AsyncClient on Python 3.13.15, base b5addb6 versus head 324e16a.