Python assertItemsEqual/assertCountEqual AttributeError

Question

I ran into an issue when using unittest.TestCase.assertItemsEqual (or assertCountEqual in Python 3) that confused me for a bit, and I wasn't able to find a solution on here, so I'm posting my fix here for posterity:

The following unit test fails under both Python 2 and 3:

import six
import unittest

class Foo(object):
    def __init__(self, a=1, b=2):
        self.a = a
        self.b = b
    def __repr__(self):
        return '({},{})'.format(self.a, self.b)
    def __eq__(self, other):
        return self.a == other.a and self.b == other.b
    def __lt__(self, other):
        return (self.a, self.b) < (other.a, other.b)
    __hash__ = None

class Test(unittest.TestCase):
    def test_foo_eq(self):
        self.assertEqual(sorted([Foo()]), sorted([Foo()]))
        six.assertCountEqual(self, [Foo()], [Foo()])
        six.assertCountEqual(self, [Foo(1,2), Foo(2,3)], [Foo(2,3), Foo(1,2)])

unittest.main()

The error message looks like:

======================================================================
ERROR: test_foo_eq (__main__.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tsanders/scripts/one_offs/test_unittest_assert_items_equal2.py", line 20, in test_foo_eq
    six.assertCountEqual(self, [Foo(1,2), Foo(2,3)], [Foo(2,3), Foo(1,2)])
  File "/home/tsanders/.local/lib/python2.7/site-packages/six.py", line 673, in assertCountEqual
    return getattr(self, _assertCountEqual)(*args, **kwargs)
  File "/usr/lib64/python2.7/unittest/case.py", line 929, in assertItemsEqual
    differences = _count_diff_all_purpose(first_seq, second_seq)
  File "/usr/lib64/python2.7/unittest/util.py", line 116, in _count_diff_all_purpose
    if other_elem == elem:
  File "/home/tsanders/scripts/one_offs/test_unittest_assert_items_equal2.py", line 11, in __eq__
    return self.a == other.a and self.b == other.b
AttributeError: 'object' object has no attribute 'a'

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

FAILED (errors=1)

Answer 1

I had to look at the source for the unittest module to solve this.

When the elements of the lists being compared are not hashable, the assertItemsEqual/assertCountEqual function falls back to a different algorithm to compare the lists. That algorithm uses an empty object() as a sentinel, which isn't equality-comparible to an object of type Foo.

The fix was to modify my __eq__ function as follows:

def __eq__(self, other):
    try:
        return self.a == other.a and self.b == other.b
    except AttributeError:
        return False