为何Python程序仅在单元测试时抛出ModuleNotFoundError？

项目结构 #

• project/source_code/constants.py
• project/source_code/calculator.py
• project/tests.py

constants.py 代码 #

# In 'constants.py'
MIN_VALUE = 0
MAX_VALUE = 100

calculator.py 代码 #

# In 'calculator.py'
import constants

class Calculator:
    #Create initialiser (not used at the moment)
    def __init__(self):
        return

    def get_sum(self, number1, number2):
        return number1 + number2

def main():
    calculator = Calculator()
    print(calculator.get_sum(10, 10))
    return

if __name__ == "__main__":
    main()
    print("")
    exit(0)

tests.py 代码 #

# In 'tests.py'
import unittest
import source_code.calculator as Calculator

class Test_Calculator(unittest.TestCase):
    def setUp(self):
        self.calculator = Calculator()
        return

    def test_calculator(self, number1, number2, errorMessage):
        self.assertEqual(self.get_sum(number1, number2), expected_LBTT, errorMessage)
        return

    def main(self):
        test_calculator(10, 10, "The calculated sum is wrong.")
        return

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

错误现象 #

单独运行calculator.py时一切正常，但运行tests.py或执行python -m unittest命令时，出现以下错误：

> ======================================================================
> **ERROR: tests (unittest.loader._FailedTest.tests)
> **----------------------------------------------------------------------
> ImportError: Failed to import test module: tests
> Traceback (most recent call last):
> File "C:\Users\Home\AppData\Local\Programs\Python\Python311\Lib\unittest\loader.py", line 407, in _find_test_path
> module = self._get_module_from_name(name)
>              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> File "C:\Users\Home\AppData\Local\Programs\Python\Python311\Lib\unittest\loader.py", line 350, in _get_module_from_name
> __import__(name)
> File "C:\Users\Home\Desktop\BJSS\project\tests.py", line 2, in <module>
> import source_code.calculator as Calculator
> File "C:\Users\Home\Desktop\BJSS\project\source_code\calculator.py", line 1, in <module>
> import constants
> ModuleNotFoundError: No module named 'constants'
>
>
> ----------------------------------------------------------------------
> Ran 1 test in 0.000s
>
> FAILED (errors=1)

原因分析 #

两种运行方式的差异在于Python的模块搜索路径：

• 单独运行calculator.py时，Python自动把当前文件所在的project/source_code/目录加入搜索路径，所以能直接找到同目录下的constants.py。
• 运行tests.py或python -m unittest时，Python的搜索路径是project/目录（测试文件所在目录），此时source_code/下的constants.py不在搜索范围内，自然找不到。

解决方法 #

1. 修正calculator.py的导入语句 #

把calculator.py里的import constants改成相对导入或绝对导入：

• 相对导入（推荐，适用于同目录模块）：

from . import constants

• 绝对导入（以project为根目录）：

from source_code import constants

2. 确保运行时的工作目录正确 #

运行测试前，切换到project/目录的上级目录（比如BJSS/），然后用模块方式执行测试：

python -m project.tests

或者临时把project/目录加入Python环境变量PYTHONPATH：
Windows命令行：

set PYTHONPATH=C:\Users\Home\Desktop\BJSS\project

之后再运行测试命令。

3. 修复tests.py中的其他错误 #

除了导入问题，你的测试代码还有几个语法/逻辑错误，修正后才能正常运行：

# In 'tests.py'
import unittest
from source_code.calculator import Calculator

class Test_Calculator(unittest.TestCase):
    def setUp(self):
        self.calculator = Calculator()

    # 测试方法必须以test_开头，且只能有self参数
    def test_sum_calculation(self):
        # 调用实例的get_sum方法，且预期值要明确
        self.assertEqual(self.calculator.get_sum(10, 10), 20, "The calculated sum is wrong.")

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

内容的提问来源于stack exchange，提问作者David