Why are assertEquals() parameters in the order (expected, actual)?

Question

Why do so many assertEquals() or similar function take the expected value as first parameter and the actual one as second ?
This seems counter-intuitive to me,

Answer 1

I agree with the consensus that consistency is #1, but the behavior of comparing dictionaries may be a helpful datapoint if you're evaluating this question.

When I see a "+" on a diff, I read this as "the procedure being tested added this." Again, personal preferences apply.

Note: I used alphabetized keys and made the dictionary longer so that only a middle key would change for clarity of the example. Other scenarios display more obfuscated diffs. Also noteworthy, assertEqual uses assertDictEqual in >=2.7 and >=3.1

exl.py

from unittest import TestCase


class DictionaryTest(TestCase):

    def test_assert_order(self):
        self.assertEqual(
            {
                'a_first_key': 'value',
                'key_number_2': 'value',
                'z_last_key': 'value',
                'first_not_second': 'value',
            },
            {
                'a_first_key': 'value',
                'key_number_2': 'value',
                'z_last_key': 'value',
                'second_not_first': 'value',
            }
        )

Output:

$ python -m unittest exl
F
======================================================================
FAIL: test_assert_order (exl.DictionaryTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "exl.py", line 18, in test_assert_order
    'second_not_first': 'value',
AssertionError: {'a_first_key': 'value', 'z_last_key': 'value', 'key_number_2': 'value', 'first_ [truncated]... != {'a_first_key': 'value', 'z_last_key': 'value', 'key_number_2': 'value', 'second [truncated]...
  {'a_first_key': 'value',
-  'first_not_second': 'value',
   'key_number_2': 'value',
+  'second_not_first': 'value',
   'z_last_key': 'value'}

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)