I have a recurring question when writing unit tests for code that involves constant string values.

Let's take an example of a method/function that does some processing and returns a string containing a pre-defined constant. In python, that would be something like:

STRING_TEMPLATE = "/some/constant/string/with/%s/that/needs/interpolation/"

def process(some_param):
    # We do some meaningful work that gives us a value
    result = _some_meaningful_action()
    return STRING_TEMPLATE % result

If I want to unit test process, one of my tests will check the return value. This is where I wonder what the best solution is.

In my unit test, I can:

1. apply DRY and use the already defined constant
2. repeat myself and rewrite the entire string

def test_foo_should_return_correct_url():
    string_result = process()

    # Applying DRY and using the already defined constant
    assert STRING_TEMPLATE % "1234" == string_result

    # Repeating myself, repeating myself
    assert "/some/constant/string/with/1234/that/needs/interpolation/" == url

The advantage I see in the former is that my test will break if I put the wrong string value in my constant. The inconvenient is that I may be rewriting the same string over and over again across different unit tests.