Python单元测试：MagicMock返回对象而非预期值的问题

原代码 #

待测试的类定义如下：

class SomeClass:
    _attribute = None

    def __init__(self, attribute):
        self._attribute = attribute

    @property
    def attribute(self):
        print("abc")
        return self._attribute


class SomeChildClass:
    context: SomeClass

    def __init__(self):
        pass
    
    def print_attribute(self):
        print(self.context.attribute)

测试需求 #

要测试SomeChildClass.print_attribute方法，希望完全MockSomeClass及其attribute属性，避免创建真实的SomeClass实例。

失败的尝试 #

尝试1：MagicMock实例 + PropertyMock patch类属性 #

def test_print_attribute(self):
    with patch('app.some_class.SomeClass.attribute', new_callable=PropertyMock) as mock_attr:
        mock_attr.return_value = "TEST"
        some_class = MagicMock()
        self.child_class = SomeChildClass()
        self.child_class.context = some_class
        self.child_class.print_attribute()

输出结果：

<MagicMock name='mock.attribute' id='2576201109456'>

尝试2：直接patch整个SomeClass类 #

@patch('app.some_class.SomeClass')
def test_print_attribute(self, mock_some_class):
    child_class = SomeChildClass()
    child_class.context = mock_some_class
    mock_some_class.attribute.return_value = "TEST"
    child_class.print_attribute()

输出结果：

<MagicMock name='SomeClass.attribute' id='1701032644768'>

解决方法 #

问题核心：@property是类级别的描述符，直接给MagicMock实例或类Mock设置普通return_value不会触发PropertyMock的行为，必须给Mock实例的类绑定PropertyMock。

方法1：给MagicMock实例绑定PropertyMock #

def test_print_attribute(self):
    # 创建PropertyMock并指定返回值
    mock_attr = PropertyMock(return_value="TEST")
    # 创建MagicMock实例
    some_class = MagicMock()
    # 关键：给MagicMock实例的类型设置attribute属性为PropertyMock
    type(some_class).attribute = mock_attr
    
    self.child_class = SomeChildClass()
    self.child_class.context = some_class
    self.child_class.print_attribute()  # 输出：TEST

方法2：patch整个类后给实例设置PropertyMock #

@patch('app.some_class.SomeClass')
def test_print_attribute(self, mock_some_class):
    # 获取类Mock的实例对象（调用类时返回的Mock）
    mock_instance = mock_some_class.return_value
    # 给实例的类型设置PropertyMock
    type(mock_instance).attribute = PropertyMock(return_value="TEST")
    
    child_class = SomeChildClass()
    child_class.context = mock_instance
    child_class.print_attribute()  # 输出：TEST

内容的提问来源于stack exchange，提问作者michaelgr22