nose.tools.eq_ vs assertEqual

Question

The Problem:

We've been using nose test runner for quite a while.

From time to time, I see our tests having eq_() calls:

eq_(actual, expected)

instead of the common:

self.assertEqual(actual, expected)

The question:

Is there any benefit of using nose.tools.eq_ as opposed to the standard unittest framework's assertEqual()? Are they actually equivalent?

---

Thoughts:

Well, for one, eq_ is shorter, but it has to be imported from nose.tools which makes the tests dependent on the test runner library which can make it more difficult to switch to a different test runner, say py.test. On the other hand, we are also using @istest, @nottest and @attr nose decorators a lot.

Answer 1

They aren't equivalent to unittest.TestCase.assertEqual.

nose.tools.ok_(expr, msg=None)

Shorthand for assert. Saves 3 whole characters!

nose.tools.eq_(a, b, msg=None)

Shorthand for assert a == b, "%r != %r" % (a, b)

https://nose.readthedocs.org/en/latest/testing_tools.html#nose.tools.ok_

These docs are however slightly misleading. If you check the source you'll see eq_ actually is:

def eq_(a, b, msg=None):
    if not a == b:
        raise AssertionError(msg or "%r != %r" % (a, b))

https://github.com/nose-devs/nose/blob/master/nose/tools/trivial.py#L25

This is pretty close to the base case of assertEqual:

def _baseAssertEqual(self, first, second, msg=None):
    """The default assertEqual implementation, not type specific."""
    if not first == second:
        standardMsg = '%s != %s' % _common_shorten_repr(first, second)
        msg = self._formatMessage(msg, standardMsg)
        raise self.failureException(msg)  # default: AssertionError

https://github.com/python/cpython/blob/9b5ef19c937bf9414e0239f82aceb78a26915215/Lib/unittest/case.py#L805

However, as hinted by the docstring and function name, assertEqual has the potential of being type-specific. This is something you lose with eq_ (or assert a == b, for that matter). unittest.TestCase has special cases for dicts, lists, tuples, sets, frozensets and strs. These mostly seem to facilitate prettier printing of error messages.

But assertEqual is a class member of TestCase, so it can only be used in TestCases. nose.tools.eq_ can be used wherever, like a simple assert.