如何Mock Python函数以避免其在模块导入时被调用？

原代码（无法修改） #

app/annoying_file.py：

def annoying_function():
    '''Does something that generates exception due to some hardcoded cloud stuff'''
    raise ValueError() # Simulate the original function raising error due to no cloud connection


annoying_variable = annoying_function()


def normal_function():
    '''Works fine by itself'''
    return True

初始测试代码 #

def test_normal_function(): 
    from app.annoying_file import normal_function

    assert normal_function() == True

测试失败的错误堆栈 #

failed: def test_normal_function():
    
>       from app.annoying_file import normal_function

test\test_annoying_file.py:6: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
app\annoying_file.py:6: in <module>
    annoying_variable = annoying_function()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def annoying_function():
        '''Does something that generates exception due to some hardcoded cloud stuff'''
>       raise ValueError()
E       ValueError

app\annoying_file.py:3: ValueError

尝试的Mock方案及失败 #

尝试通过pytest-mock提前patch函数，但依然失败：

def test_normal_function(mocker):
    mocker.patch("app.annoying_file.annoying_function", return_value="foo")
    from app.annoying_file import normal_function

    assert normal_function() == True

错误堆栈：

failed: thing = <module 'app' (<_frozen_importlib_external._NamespaceLoader object at 0x00000244A7C72FE0>)>
comp = 'annoying_file', import_path = 'app.annoying_file'

    def _dot_lookup(thing, comp, import_path):
        try:
>           return getattr(thing, comp)
E           AttributeError: module 'app' has no attribute 'annoying_file'

....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1238: AttributeError

During handling of the above exception, another exception occurred:

mocker = <pytest_mock.plugin.MockerFixture object at 0x00000244A7C72380>

    def test_normal_function(mocker):
>       mocker.patch("app.annoying_file.annoying_function", return_value="foo")

test\test_annoying_file.py:5: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
.venv\lib\site-packages\pytest_mock\plugin.py:440: in __call__
    return self._start_patch(
.venv\lib\site-packages\pytest_mock\plugin.py:258: in _start_patch
    mocked: MockType = p.start()
....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1585: in start
    result = self.__enter__()
....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1421: in __enter__
    self.target = self.getter()
....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1608: in <lambda>
    getter = lambda: _importer(target)
....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1251: in _importer
    thing = _dot_lookup(thing, comp, import_path)
....\.pyenv\pyenv-win\versions\3.10.5\lib\unittest\mock.py:1240: in _dot_lookup
    __import__(import_path)
app\annoying_file.py:6: in <module>
    annoying_variable = annoying_function()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def annoying_function():
        '''Does something that generates exception due to some hardcoded cloud stuff'''
>       raise ValueError()
E       ValueError

app\annoying_file.py:3: ValueError

失败原因：patch操作需要先导入目标模块，而模块导入时会立即执行顶层的annoying_variable = annoying_function()，导致异常提前触发，patch无法生效。

解决方案 #

方案一：手动加载模块并提前注入Mock #

利用importlib.util手动加载模块，在执行模块代码前替换掉annoying_function，避免顶层代码抛出异常：

import importlib.util
import sys
from unittest.mock import Mock

def test_normal_function():
    # 模块路径和名称
    module_path = "app/annoying_file.py"
    module_name = "app.annoying_file"

    # 1. 获取模块spec
    spec = importlib.util.spec_from_file_location(module_name, module_path)
    # 2. 创建空模块对象
    module = importlib.util.module_from_spec(spec)
    # 3. 提前注入mock的annoying_function到模块中
    module.annoying_function = Mock(return_value="mocked_value")
    # 4. 注册模块到sys.modules，避免重复导入
    sys.modules[module_name] = module
    
    try:
        # 5. 执行模块代码（此时调用的是mock函数，不会抛出异常）
        spec.loader.exec_module(module)
        # 6. 测试目标函数
        assert module.normal_function() == True
    finally:
        # 清理sys.modules，不影响其他测试
        del sys.modules[module_name]

方案二：使用mock.patch强制创建属性拦截 #

通过patch的create=True参数，在模块导入前动态创建annoying_function属性：

from unittest.mock import patch

def test_normal_function():
    # 在导入模块前patch，create=True允许创建不存在的模块属性
    with patch("app.annoying_file.annoying_function", return_value="foo", create=True):
        from app.annoying_file import normal_function
        assert normal_function() == True

原理：patch会在模块加载时，自动给app.annoying_file模块添加mock的annoying_function，顶层代码执行时调用的是mock函数，不会触发异常。

内容的提问来源于stack exchange，提问作者TheArturro