使用patch.object（）和side_effect

我写了一个测试，可以强制fetchone()返回None，通过使用：

• 函数with mock.patch()来获取mock对象，它避免了与数据库的真实的连接（与代码中的完全一样）
• 函数with mock.path.object()模拟方法cursor（）
• side_effect而不是return_value，以强制fetchone()返回None

适合您需要的测试应该如下：

import unittest
from unittest import mock

import mysql.connector

def fetch_a_row():
    # establishing the connection
    conn = mysql.connector.connect(user='root',password='password',host='127.0.0.1',database='mydb')
    # Creating a cursor object using the cursor() method
    cursor = conn.cursor()
    # return the fetchone() output
    return cursor.fetchone()

class TestMysqlConn(unittest.TestCase):

    def test_01(self):
        # following patch() is the same you have used in your code
        with mock.patch('mysql.connector.connect') as mock_connect:
            mock_connect_instance = mock_connect.return_value
            # mock the method cursor() of the mock object mock_connect
            with mock.patch.object(mock_connect_instance, 'cursor') as mock_cursor:
                mock_cursor_instance = mock_cursor.return_value
                # force fetchone() to return None by side_effect
                mock_cursor_instance.fetchone.side_effect = [None]
                # check if fetch_a_row() return None
                self.assertEqual(None, fetch_a_row())

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

在代码中，我编写了函数fetch_one_row()，它模拟了你的产品代码。

EDIT：我已经编辑了答案，因为OP添加了他的函数fetch_a_row()的代码。

使用上下文管理器测试

您已经编辑了您的问题，并使用上下文管理器添加了函数fetch_a_row()的代码。
当您使用上下文管理器时，调用方法__enter__()（参见this link），此方法返回对象conn。因此，使用上下文管理器，我必须修改测试函数如下：

import unittest
from unittest import mock

import mysql.connector

def fetch_a_row():
    # establishing the connection
    with mysql.connector.connect(user='root',password='password',host='127.0.0.1',database='mydb') as conn:
        # Creating a cursor object using the cursor() method
        cursor = conn.cursor()
        cursor.execute("SELECT * FROM mytable")
        # return the fetchone() output
        return cursor.fetchone()

class TestMysqlConn(unittest.TestCase):

    def test_01(self):
        with mock.patch('mysql.connector.connect') as mock_connect:
            mock_connect_instance = mock_connect.return_value
            with mock.patch.object(mock_connect_instance, '__enter__') as mock_connect_context_manager:
                mock_connect_context_manager_instance = mock_connect_context_manager.return_value
                with mock.patch.object(mock_connect_context_manager_instance, 'cursor') as mock_cursor:
                    mock_cursor_instance = mock_cursor.return_value
                    mock_cursor_instance.fetchone.side_effect = [None]
                    self.assertEqual(None, fetch_a_row())

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