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Hey I'm just getting started with unittests and mocks in Python. Trying to test a function that takes a single column dataframe and returns a float after some calculation based on the dataframe values.

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

I'm running into an issue where the result is a MagicMock object instead of a float. I've tried looking for answers, but not able to wrap my head around how to properly set the return value for the mock object.

Output from running test.

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)
Andrew Parmar
  • 71
  • 4

1 Answers1

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From what I could tell, the problem arises in repeated __getitem__ calls. The following works though:

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)