使用pytest+mock测试时，无法正确mock被其他类调用的Leave类

我在尝试Mock被其他类调用的Leave类时测试失败，现有三个模块代码如下：

tree.py #

from branch import Branch

class Tree:

    type_of_tree = None
    branches = None

    def __init__(self, type_of_tree, branches = 0, default_leaves = 0):
        self.type_of_tree = type_of_tree
        self.branches = []
        for branch in range(branches):
            self.create_branch(default_leaves)


    def create_branch(self, leaves):
        self.branches.append(Branch(leaves))


    def get_leaves(self):
        return_value = []
        for branch in self.branches:
            return_value.extend(branch.get_leaves())
        return return_value

branch.py #

import uuid
from leave import Leave


class Branch:
    leaves = None
    identifier = None

    def __init__(self, leaves=0):
        self.identifier = str(uuid.uuid4())
        self.leaves = []
        for leave in range(leaves):
            self.create_leave()

    def create_leave(self):
        self.leaves.append(Leave())

    def get_leaves(self):
        return_value = []
        for leave in self.leaves:
            return_value.append(leave.get_identifier())
        return return_value

leave.py #

import uuid


class Leave:
    identifier = None

    def __init__(self):
        self.identifier = str(uuid.uuid4())

    def get_identifier(self):
        return self.identifier

我的测试代码如下：

from unittest.mock import patch, MagicMock
from uuid import uuid4

from tree import Tree

FAKE_UUID = str(uuid4())

class TestLeave:


    def test_leave(self):
        with patch('leave.Leave') as mock_leave:
            mock_leave.return_value = MagicMock()
            mock_leave.get_identifier.return_value = FAKE_UUID
            tree = Tree("Pine", 1, 1)
            leaves = tree.get_leaves()
            assert leaves == [FAKE_UUID]

测试未通过，查阅官方文档后仍未找到解决办法。

问题出在两个核心点：

1. Patch目标路径错误
   Branch类在自己的模块（branch.py）中导入了Leave，实际运行时使用的是branch模块内的Leave引用。因此需要patch的是branch.Leave，而非原始的leave.Leave。

2. Mock方法返回值设置错误
   get_identifier是Leave实例的方法，必须给mock_leave.return_value（即Mock出来的实例对象）的get_identifier设置返回值，而非直接给mock_leave（Mock类本身）设置。

修正后的测试代码：

from unittest.mock import patch, MagicMock
from uuid import uuid4

from tree import Tree

FAKE_UUID = str(uuid4())

class TestLeave:
    def test_leave(self):
        # 修正patch目标为branch模块中的Leave引用
        with patch('branch.Leave') as mock_leave:
            # 创建Mock实例并设置方法返回值
            mock_leave_instance = MagicMock()
            mock_leave_instance.get_identifier.return_value = FAKE_UUID
            # 将实例设为Mock类的返回值
            mock_leave.return_value = mock_leave_instance
            
            tree = Tree("Pine", 1, 1)
            leaves = tree.get_leaves()
            assert leaves == [FAKE_UUID]

修改后测试即可正常通过。

内容的提问来源于stack exchange，提问作者Rubén Pozo