Why is assertDictEqual needed if dicts can be compared by `==`?

Question

I have always used assertDictEqual, because sometimes when I didn't use it I got information that equal dicts are not the same.

But I know that dicts can be compared by == operator:

>>> {'a':1, 'b':2, 'c': [1,2]} == {'b':2, 'a':1, 'c': [1,2]}
True

Where may I actually need assertDictEqual?

Answer 1

Basically, it allows unittest to give you more information about why the test failed ("diagnostics", to use the language from "Growing Object-Oriented Software Guided by Tests" by Steve Freeman and Nat Pryce). Compare these two tests:

import unittest


class DemoTest(unittest.TestCase):

    D1 = {'a': 1, 'b': 2, 'c': [1, 2]}
    D2 = {'a': 1, 'b': 2, 'c': [1]}

    def test_not_so_useful(self):
        self.assertTrue(self.D1 == self.D2)

    def test_useful(self):
        self.assertDictEqual(self.D1, self.D2)


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

And their outputs:

======================================================================
FAIL: test_not_so_useful (__main__.DemoTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "path/to/file.py", line 10, in test_not_so_useful
    self.assertTrue(self.D1 == self.D2)
AssertionError: False is not true

vs.

======================================================================
FAIL: test_useful (__main__.DemoTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "path/to/file.py", line 13, in test_useful
    self.assertDictEqual(self.D1, self.D2)
AssertionError: {'a': 1, 'b': 2, 'c': [1, 2]} != {'a': 1, 'b': 2, 'c': [1]}
- {'a': 1, 'b': 2, 'c': [1, 2]}
?                         ---

+ {'a': 1, 'b': 2, 'c': [1]} 

In the latter, you can see exactly what the difference was, you don't have to work it out yourself. Note that you can just use the standard assertEqual instead of assertDictEqual, with the same result; per the docs

...it’s usually not necessary to invoke these methods directly.