Question

Is it possible to use assert_equal to compare objects? I keep seeing this error:

AssertionError: <ex49.parser.Sentence object at 0x01F1BAF0> !=
<ex49.parser.Sentence object at 0x01F1BB10>

The relevant code fragment:

def test_parse_subject():

    testsentence = "princess go east"
    result = lexicon.scan(testsentence)
    Sent = parse_sentence(result)
    ResultSent = Sentence(('subject', 'princess'),
                      ('verb', 'go'),
                      ('object', 'east'))
    print ResultSent.subject
    print ResultSent.verb
    print ResultSent.object
    print Sent.subject
    print Sent.verb
    print Sent.object
    assert_equal(Sent, ResultSent)

The print outputs on screen suggests that the objects have the same contents – yet the assertion error shows up. Why is this? Is there some way to use assert_equal to override this?

Answer 1

I believe you need to implement the __eq__ method on the Sentence class.

assertEqual(first, second, msg=None)¶
Test that first and second are equal. If the values do not compare equal, the test will fail.

In addition, if first and second are the exact same type and one of list, tuple, dict, set, frozenset or unicode or any type that a subclass registers with addTypeEqualityFunc() the type-specific equality function will be called in order to generate a more useful default error message (see also the list of type-specific methods).

Python unittest documentation

The correspondence between operator symbols and method names is as follows: xlt(y), x<=y calls x.le(y), x==y calls x.eq(y), x!=y and x<>y call x.ne(y), x>y calls x.gt(y), and x>=y calls x.ge(y).

Python data model documentation

An example:

import unittest


class A:

    def __init__(self, num):
        self.num = num

    def __eq__(self, other):
        return self.num == other.num


class Test(unittest.TestCase):

    def test(self):
        a1 = A(1)
        a12 = A(1)
        a2 = A(2)

        self.assertEqual(a1, a1, 'a1 != a1')
        self.assertEqual(a1, a12, 'a1 != a12')
        self.assertEqual(a1, a2, 'a1 != a2')

def main():
    unittest.TestRunner(Test())

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

Now comment the __eq__ method and see the difference.