如何在MagicMock对象上设置值。因此被测函数不会返回MagicMock对象

Question

嘿，我刚刚开始用Python进行单元测试和模拟。尝试测试一个函数，该函数接受单列数据帧，并在根据数据帧值进行一些计算后返回一个浮点数。

import unittest
from unittest.mock import MagicMock

def avg_annual_fcf_growth_rate(fcf_data_frame):
    delta = fcf_data_frame.iloc[9][0] - fcf_data_frame.iloc[0][0]
    avg_growth_rate = (delta**(1/9))-1 
    return avg_growth_rate

class Test_DCF(unittest.TestCase):

    def test_fcf_calculation(self):
        mock_fcf = MagicMock()
        mock_fcf.iloc[9][0].return_value = 100
        mock_fcf.iloc[0][0].return_value = 10
        result = avg_annual_fcf_growth_rate(mock_fcf)
        expected = ((100-10)**(1/9)) - 1
        self.assertEqual(result, expected)

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

#Sample dataframe 
         Free Cash Flow USD Mil
2008-12                  5114.0
2009-12                 10909.0
2010-12                 11915.0
2011-12                 12285.0
2012-12                 11175.0
2013-12                 16617.0
2014-12                 16825.0
2015-12                 15409.0
2016-12                 19581.0
2017-12                 34068.0

我遇到了一个问题，结果是一个MagicMock对象而不是一个浮点数。我试着寻找答案，但无法理解如何正确设置模拟对象的返回值。

运行测试的输出。

F
======================================================================
FAIL: test_fcf_calculation (__main__.Test_DCF)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "path/to/intrinsic_value_dcf_test.py", line 18, in test_fcf_calculation
    self.assertEqual(result, expected)
AssertionError: <MagicMock name='mock.iloc.__getitem__().[56 chars]104'> != 0.6486864043382532

----------------------------------------------------------------------
Ran 1 test in 0.004s

FAILED (failures=1)

Answer 1

据我所知，问题出现在重复的__getitem__调用中。不过，下面的方法是可行的：

def test_fcf_calculation(self):
    mock_fcf = MagicMock()

    mock_fcf.iloc.__getitem__.return_value = MagicMock(**{'__getitem__.side_effect': [100, 10]})

    result = avg_annual_fcf_growth_rate(mock_fcf)
    expected = ((100-10)**(1/9)) - 1
    print(mock_fcf.mock_calls)
    self.assertEqual(result, expected)