如何模拟已打补丁 class 的异步实例方法？

Question

（以下代码在Jupyter中可以运行。）我有一个class B，它使用class A，需要测试。

class A:
    async def f(self):
        pass

class B:
    async def f(self):
        a = A()
        x = await a.f()  # need to be patched/mocked

我有以下测试代码。它似乎嘲笑了 A 的 class 方法而不是实例方法。

from asyncio import Future
from unittest.mock import MagicMock, Mock, patch

async def test():
    sut = B()
    with patch('__main__.A') as a:  # it's __main__ in Jupyter
        future = Future()
        future.set_result('result')
        a.f = MagicMock(return_value=future)
        await sut.f()

await test()

但是，代码出现以下错误：

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
C:\Users\X~1\AppData\Local\Temp/ipykernel_36576/3227724090.py in <module>
     20         await sut.f()
     21 
---> 22 await test()

C:\Users\X~1\AppData\Local\Temp/ipykernel_36576/3227724090.py in test()
     18         future.set_result('result')
     19         a.f = MagicMock(return_value=future)
---> 20         await sut.f()
     21 
     22 await test()

C:\Users\X~1\AppData\Local\Temp/ipykernel_36576/3227724090.py in f(self)
      6     async def f(self):
      7         a = A()
----> 8         x = await a.f()  # need to be patched/mocked
      9 
     10 from asyncio import Future

TypeError: object MagicMock can't be used in 'await' expression

Answer 1

需要改变

a.f = MagicMock(return_value=future)

至

a().f = MagicMock(return_value=future)

Answer 2

在 Python 3.8+ 中，修补异步方法会为您提供 AsyncMock，因此提供结果会更直接一些。

在 the patch method itself 的文档中：

If new is omitted, then the target is replaced with an AsyncMock if the patched object is an async function or a MagicMock otherwise.

AsyncMock 允许您以更直接的方式提供 return 值：

import asyncio
from unittest.mock import patch


class A:
    async def f(self):
        return "foo"


class B:
    async def f(self):
        return await A().f()


async def main():
    print(await B().f())

    with patch("__main__.A.f", return_value="bar") as p:
        print(await B().f())


if __name__ == "__main__":
    try:
        asyncio.run(main())
    except KeyboardInterrupt:
        sys.exit(1)

..打印：

$ python example.py
foo
bar

side_effect kwarg 涵盖了您要查找的大多数类型的值 return（例如，如果您需要模拟函数 await something）。

if side_effect is a function, the async function will return the result of that function,

if side_effect is an exception, the async function will raise the exception,

if side_effect is an iterable, the async function will return the next value of the iterable, however, if the sequence of result is exhausted, StopAsyncIteration is raised immediately,

if side_effect is not defined, the async function will return the value defined by return_value, hence, by default, the async function returns a new AsyncMock object.

如何模拟已打补丁 class 的异步实例方法？

How to mock an async instance method of a patched class?

python

pytest

python-asyncio

python-unittest