mock-response-delay 2026.9.7

requests with responses

Wrap a callback before giving it to responses. A request with a read timeout shorter than the delay raises requests.exceptions.Timeout:

"""Time out against a slow server."""

import pytest
import requests
import responses
from requests import PreparedRequest

from mock_response_delay.for_requests import delayed_responses_callback


def slow_callback(request: PreparedRequest) -> tuple[int, dict[str, str], str]:
    """Answer any request."""
    del request
    return (200, {}, "Hello")


waits: list[float] = []

with responses.RequestsMock() as mock:
    mock.add_callback(
        method="GET",
        url="https://example.com/",
        callback=delayed_responses_callback(
            callback=slow_callback,
            delay_seconds=5.0,
            sleep_fn=waits.append,
        ),
    )

    with pytest.raises(expected_exception=requests.exceptions.Timeout):
        requests.get(url="https://example.com/", timeout=1.0)

    response = requests.get(url="https://example.com/", timeout=10.0)

assert response.text == "Hello"
assert waits == [1.0, 5.0]

requests accepts the timeout as one number, or as a (connect, read) tuple. A slow server only affects the read leg, so only the read timeout is compared with the delay.

httpx with respx or httpx.MockTransport

Wrap a handler before giving it to httpx.MockTransport, or to respx as a side effect. A request with a read timeout shorter than the delay raises httpx.ReadTimeout:

"""Time out against a slow server."""

import httpx
import pytest

from mock_response_delay.for_httpx import delayed_httpx_handler


def slow_handler(request: httpx.Request) -> httpx.Response:
    """Answer any request."""
    del request
    return httpx.Response(status_code=200, text="Hello")


waits: list[float] = []
transport = httpx.MockTransport(
    handler=delayed_httpx_handler(
        handler=slow_handler,
        delay_seconds=5.0,
        sleep_fn=waits.append,
    ),
)

with httpx.Client(transport=transport) as client:
    with pytest.raises(expected_exception=httpx.ReadTimeout):
        client.get(url="https://example.com/", timeout=1.0)

    response = client.get(url="https://example.com/", timeout=10.0)

assert response.text == "Hello"
assert waits == [1.0, 5.0]

httpx2

httpx2 has its own request, response and exception classes, so it has its own module, which works in the same way with httpx2.MockTransport:

"""Time out against a slow server."""

import httpx2
import pytest

from mock_response_delay.for_httpx2 import delayed_httpx2_handler


def slow_handler(request: httpx2.Request) -> httpx2.Response:
    """Answer any request."""
    del request
    return httpx2.Response(status_code=200, text="Hello")


waits: list[float] = []
transport = httpx2.MockTransport(
    handler=delayed_httpx2_handler(
        handler=slow_handler,
        delay_seconds=5.0,
        sleep_fn=waits.append,
    ),
)

with httpx2.Client(transport=transport) as client:
    with pytest.raises(expected_exception=httpx2.ReadTimeout):
        client.get(url="https://example.com/", timeout=1.0)

    response = client.get(url="https://example.com/", timeout=10.0)

assert response.text == "Hello"
assert waits == [1.0, 5.0]

Controlling the clock

sleep_fn defaults to time.sleep, so by default the delay is real. The examples above record the waits instead, which keeps the test instant. Something which advances a fake clock, such as a freezegun tick, works the same way.