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pytest-asyncio-httpx-mocking

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When masking httpx.AsyncClient with unittest.mock in Pytest, AsyncMock must be used instead of MagicMock for async methods like post/get to prevent TypeError when awaited.

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httpx.AsyncClient Mocking with AsyncMock

症状

  • 在测试中使用 patch.object(httpx.AsyncClient, 'post', return_value=mock_response)。
  • 运行时，代码中包含 response = await client.post(...) 的地方抛出 TypeError: object MagicMock can't be used in 'await' expression。

根本原因

原因 1: 异步函数的返回值必须是 Coroutine

  • 问题: httpx.AsyncClient.post 是一个 async def 方法，调用它会返回一个可等待（awaitable）的协程。
  • 为什么发生: 默认的 patch 或 MagicMock 没有自动推断对象的异步特性时，它只是同步地返回了 return_value。当事件循环试图 await 这个同步的 MagicMock 对象时，就会报错。
  • 解决方案: 在 patch 参数里显式使用 new=AsyncMock(return_value=...) 或 new_callable=AsyncMock。

代码解决方案

❌ 错误写法:

from unittest.mock import patch, MagicMock

mock_response = MagicMock(status_code=200)
# 当被 await 时会触发 TypeError!
with patch.object(httpx.AsyncClient, 'post', return_value=mock_response):
    await my_crawler.fetch()

✅ 正确写法:

from unittest.mock import patch, MagicMock, AsyncMock

mock_response = MagicMock(status_code=200) # Response 对象本身及其方法通常是同步的
# 正确！覆盖掉原来的方法，使其行为成为一个 AsyncMock
with patch.object(httpx.AsyncClient, 'post', new=AsyncMock(return_value=mock_response)):
    await my_crawler.fetch()

使用 side_effect 模拟循序多次请求：

with patch.object(httpx.AsyncClient, 'get', new=AsyncMock(side_effect=[mock_1, mock_2])):
    ...

关键经验

  • 针对任何 async def 的 Mock，必须保证它被调用时能走协程语境。
  • 严格区分 异步的请求方法 与 同步的响应对象：httpx.AsyncClient.get 是异步的（需 AsyncMock），但它返回的 Response 对象上的 .json() 是同步的（需 MagicMock 即可）。