python - 模拟在被测方法中实例化的 Python 类

Question

我有一个正在测试的系统(下面的 Class Printer)，它使用另一个类(下面的 Class ContentContainer)。在一个方法中(下面的方法 retrieve_and_show_content)，这个类被实例化。在对该方法(下面的方法 test_printer_03)的测试中，我想实例化一个 mock 而不是真正的类。然而，它不是这样工作的。

我读了here我应该将 name 指向的对象更改为另一个对象。似乎我要替换的对象的名称只是ContentContainer，而我实际要替换的对象的名称是TestMockClass.ContentContainer。这个观察是否正确？如果是这样，我该如何更改？如果我简单地删除补丁语句中的前缀 TestMockClass，我会得到一个 TypeError: Need a valid target to patch。您提供了:“ContentContainer”。

#TestMockClass.py
import unittest
from mock import Mock, patch

class Printer():
    def __init__(self, name, cc):
        self.name = name
        self.cc = cc
    def show_content(self):
        text = '{0} says: {1}'.format(self.name, self.cc.content())
        return text
    def retrieve_and_show_content(self):
        cc_tmp = ContentContainer()
        text = '{0} says: {1}'.format(self.name, cc_tmp.content())
        return text

class ContentContainer():
    def __init__(self):
        self.method_counter()
    def content(self):
        return 'Content from ContentContainer'
    def method_counter(self):
        pass

class Test(unittest.TestCase):

    '''No mocking'''
    def test_printer_01(self):
        cc = ContentContainer()
        sut = Printer('P01', cc)
        result = sut.show_content()
        expected_result = 'P01 says: Content from ContentContainer'
        self.assertEqual(result, expected_result, 
                         msg = '\nRetrieved:\n{0} \nExpected:\n{1}'.format(result, expected_result))
        result = sut.retrieve_and_show_content()
        expected_result = 'P01 says: Content from ContentContainer'
        self.assertEqual(result, expected_result, 
                         msg = '\nRetrieved:\n{0} \nExpected:\n{1}'.format(result, expected_result))

    '''Create a mock object, which is the input of the method under test'''
    def test_printer_02(self):
        mock_cc = Mock()
        mock_cc.content.return_value = 'Mocked content'
        sut = Printer('P02', mock_cc)
        result = sut.show_content()
        expected_result = 'P02 says: Mocked content'
        self.assertEqual(result, expected_result, 
                         msg = '\nRetrieved:\n{0} \nExpected:\n{1}'.format(result, expected_result))
        self.assertFalse(mock_cc.method_counter.called, 'Method method_counter shall not be called')

    '''Create a mock class, which is instantiated inside the method under test'''
    @patch('TestMockClass.ContentContainer')
    def test_printer_03(self, mock_cc):
        mock_cc.content.return_value = 'Mocked content'
        sut = Printer('P03', mock_cc)
        result = sut.retrieve_and_show_content()
        expected_result = 'P03 says: Mocked content'
        self.assertEqual(result, expected_result, 
                         msg = '\nRetrieved:\n{0} \nExpected:\n{1}'.format(result, expected_result))
        self.assertFalse(mock_cc.method_counter.called, 'Method method_counter shall not be called')

if __name__ == "__main__":
    unittest.main()

运行此单元测试时，输出为:

AssertionError: 
Retrieved:
P03 says: Content from ContentContainer 
Expected:
P03 says: Mocked content

Answer 1

两件事:

1. 因为 ContentContainer 现在与测试在同一个文件中，您实际上需要修补 __main__.ContentContainer:

   @patch('__main__.ContentContainer')
   

2. 因为 ContentContainer 是一个类，而您在该类的实例上调用 content，您实际上希望模拟 content 在那个实例上，而不是在类里面。因此你需要做:mock_cc.return_value.content.return_value = 'Mocked content'不是类(class))。这是因为调用一个类会创建一个实例。所以，实例就是类调用的返回值。

因此测试应该如下所示:

@patch('__main__.ContentContainer')
def test_printer_03(self, mock_cc):
    mock_cc.return_value.content.return_value = 'Mocked content'
    ...