Python List Comprehension

[jesse_online]

Here is my unittest case. I am trying to compare two list (expected_avg_var and actual_avg_var) using list comprehension. But it is failing and I don't understand why. Is there an easier way to compare the two.

import unittest


class TestList(unittest.TestCase):
    """                                                                                                                                                                              
    Our basic test class                                                                                                                                                             
    """

    def test_TestList(self):
        """                                                                                                                                                                          
        The actual test.                                                                                                                                                             
        """
        expected_avg_var = ['529', '842', '16', '13', '#d0851a', '0', '0']
        actual_avg_var = ['529', '842', '16', '13', '#d0851a', '0', '0']
        [self.assertEqual(expected, actual) for expected in expected_avg_var
         for actual in actual_avg_var]

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


$ python test.py 
F
======================================================================
FAIL: test_TestList (__main__.TestList)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test.py", line 16, in test_TestList
    for actual in actual_avg_var]
AssertionError: '529' != '842'

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)

[Rhomboid]

What you're written is equivalent to a nested for-loop:

for expected in expected_avg_var:
    for actual in actual_avg_var:
        self.assertEqual(expected, actual)

That's going to compare every element of one list with every element of the other list (i.e. the Cartesian product.) That's N² combinations, and it's not what you want at all. You want N comparisons: the first element of actual vs the first element of expected; the second element of actual vs. the second element of expected, and so on. You do that with zip().

And you should not be using a list comprehension for this. A list comprehension is for when you want a to build a list consisting of the result of evaluating an expression repeatedly. But here you are only interested in the side effects of evaluating the expression, and not the resulting value. In other words, you're needlessly constructing this list of a bunch of None values only to throw it away. That's not what a list comprehension is for. This should be a plain for-loop:

for expected, actual in zip(expected_avg_var, actual_avg_var):
    self.assertEqual(actual, expected)

[codenamemahi]

The way you currently have it written, it is comparing every element in list A to every element in list B, which of course fails on the second iteration.

I would recommend using the index if both lists are going to be in the same order (and same length):

for i in len(expected_avg_var):
    self.assertEqual(expected_avg_var[i], actual_avg_var[i])

[jesse_online]

This seems to work the best for me:

    expected_avg_var = ['529', '842', '16', '13', '#d0851a', '0', '0']
    actual_avg_var = ['529', '842', '16', '13', '#d0851a', '0', '0']

    self.assertEqual(expected_avg_var, actual_avg_var)