Should you display what's happening in the unit test as it runs?

Question

As I am coding my unit tests, I tend to find that I insert the following lines:

Console.WriteLine("Starting InteropApplication, with runInBackground set to true...");
try
{
    InteropApplication application = new InteropApplication(true);
    application.Start();
    Console.WriteLine("Application started correctly");
}
catch(Exception e)
{
    Assert.Fail(string.Format("InteropApplication failed to start: {0}", e.ToString()));
}

//test code continues ...

All of my tests are pretty much the same thing. They are displaying information as to why they failed, or they are displaying information about what they are doing. I haven't had any formal methods of how unit tests should be coded. Should they be displaying information as to what they are doing? Or should the tests be silent and not display any information at all as to what they are doing, and only display failure messages?

NOTE: The language is C#, but I don't care about a language specific answer.

Answer 1

I personally would recommend that you output only errors and a summary of the number of tests run and how many passed. This is a completely subjective view though. Display what suits your needs.

Answer 2

Well, you should only know when a test failed and why it failed. It's no use to know what's going on, unless, for example, you have a loop and you want to know exactly where in the loop the test died.

Answer 3

I recommend against it - I think that the unit testing should work on the Unix tools philosophy - don't say anything when things are going well. I find that constructing tests to give meaningful information when they fail is best - that way you get nice short output when things work and it's easy to see what went wrong when there are problems - errors aren't lost to scroll blindness.

Answer 4

I'm not sure why you would do that - if your unit test is named well, you already know what it's doing. If it fails, you know what test failed (and what assert failed). If it didn't fail you know that it succeeded.

This seems completely subjective, but to me this seems like completely redundant information that just adds noise.

Answer 5

I like to buffer the verbose log (about last 20 lines or so), but I don't display it until it gets to some error. When the error happens, it's nice to have some context.

OTOH, unit tests should be small pieces of unrelated code with specific input and output requirements. In most cases, displaying input that caused the error (i.e. wrong output) is enough to trace the problem to its roots.

Answer 6

I would actually suggest against it (though not militantly). It couples the user interface of your tests with the test implementation (what if the tests are run through GUI viewer?). As alternative I would suggest one of the following:

1. I'm not familiar with NUnit, but PyUnit allows you to add a description of the test and when tests are run with the verbose option the description is printed. I would look into the NUnit documentation to see if this is something you can do.
   
2. Extend the TestCase class that you're inheriting from to add a function from which you call that logs what the test is trying to do. That way different implementations can handle messages in different ways.

Answer 7

This might be a bit too language specific, but when I'm writing NUnit tests I tend to do this, only I use the System.Diagnostics.Trace library instead of the console, that way the information is only shown if I decide to watch the tracing.

Answer 8

I think your making far more work for yourself. The tests either pass or fail, the failure should hopefully be the exception to the rule and you should let the unit test runner handle and throw the exception. What you're doing is adding cruft, the exception logged by the test runner will tell you the same thing.

Answer 9

I'd say you should output whatever suits your needs, but showing too much can dilute output from test runner.
BTW, your example code hardly looks as a unit test, more of a integration/system test.

Answer 10

The only time I would display what's happening is if there was some aspect of it that would be easier to test non-automatically. For example, if you've got code that takes a little while to run, and might get stuck in an infinite loop, you might want to print out a message every so often to indicate that it is still making progress.

Always make sure failure messages clearly stand out from other output, however.

Answer 11

You don't need to, if the tests are running silently then that means there was no error. There is usually no reason for tests to give any output other than if the test failed. If it's running, then it is running indicated by the test runner that the test has passed, i.e. it is "green". Running the test (together with many tests with console output) through a test runner in an IDE, you'll be spamming the console log with messages nobody will care about.

The test you've written is not a unit test, but looks more like an integration/system test because you seem to be running an application as a whole. A unit test will test a public method in a class, preferably keeping the class as isolated as possible.

Answer 12

You could have written the test method like this. It's up to your code-nose which style of test you prefer. I prefer not writing extra try-catches and Console.WriteLines.

public void TestApplicationStart()
{
  InteropApplication application = new InteropApplication(true);
  application.Start();
}

Test frameworks that I have worked with would interpret any unhandled (and unexpected) exception as a failed test.

Think about the time you took to gold-plate this test and how many more meaningful tests you could have written with that time.

Answer 13

Using console i/o kinda defies the whole purpose of a unit testing framework. you might as well code the whole test manually. If you are using a unit testing framework, your tests should be very malleable, tied to as few things as possible

Answer 14

Displaying information can be useful; if you're trying to find out why a test failed, it can be useful to be able to see more than just a stack trace, and what happened before the program reached the point where it failed.

However, in the "normal" case where everything succeeds, these messages are unnecessary clutter that distract from what you're really trying to do - ie. looking at an overview of which tests succeeded and failed.

I'd suggest redirecting your debugging messages to a log file. You can either do this by writing all your log message code to call a special "log print" function, or if you're writing a console program, you should be able to redirect stdout to a different file (I know for a fact that you can do this in both Unix and Windows). This way, you get the high level overview but the details are there if you need them.

Answer 15

I would avoid putting extra Try/Catch statements in Unit Tests. First of all, an expected exception in a unit test will already cause the test to Fail. That is the default behavior of NUnit. Essentitally, the test harness wraps each call to your test functions with that code already. Also, by just using the e.ToString() to display what happened, I believe you are losing a lot of information. By default, I believe NUnit will display not just the Exception type, but also the Call Stack, which I don't believe you're seeing with your method.

Secondly, there are times when its necessary. For instance, you can use the [ExpectedException] attribute to actually say when it occurs. Just be sure that when you test non-exception related Asserts (for instance Asserting a list count > 0, etc) that you put in a good description as the argument to the assert. That is useful.

Everything else is generally not needed. If your unit tests are so large that you start putting in WriteLines with what "step" of the test you're on, that is generally a sign that your test should really be broken out into multiple smaller tests. In other words, that you're not doing a unit test, but rather an integration test.

Answer 16

Have you looked at the xUnit style of unit test frameworks?
See Ron Jeffries site for a rather large list.

One of the principles of these frameworks is that they produce little or no output during the test run and only really an indicator of success at the end. In the case of failures its possible to get a more descriptive output of the reason for failure.
The reason for this mode is that while everything is OK you don't want to be bothered by extra output, and certainly if there is a failure you don't want to miss it because of the noise of other output.