Covering Error Cases in Go Unit Tests

In a previous article I showed table-driven tests in Go, which are a compact way to run lots of test cases. In this article I will show another way to improve unit tests in Go: table-driven tests with invalid values.

To illustrate this, I've created an example library in Go that converts Roman numerals in string form to their numeric value. Of course, not all strings are valid Roman numerals. In Go, functions typically return an error value rather than throwing an exception. However, this is not done using "special" return values as in C; instead, functions return multiple values, one of which is an error value that is non-nil if an error occurred. Go provides an error type for use in these cases.

Here is an example from the Go blog:

func Sqrt(f float64) (float64, error) {
    if f < 0 {
        return 0, errors.New("math: square root of negative number")
    }
    // implementation
}

The (float64, error) indicates that the function returns both a value of type float64 and a value of type error. If the error value is non-nil, the primary return value should be ignored.

Users of this function should check the error on return:

result, err := Sqrt(-1.0)
if err != nil {
    // Don't use the result
    fmt.Println("I sent a bad input")
}

In a unit test, we want to ensure that errors that should be returned are returned. To do this, we can again leverage a table-driven test, with the input and the expected error.

For example:

var invalidTests = []struct {
    input    string
    expected error
}{
    {"XXXX", ErrInvalidFormat},
    {"VV", ErrInvalidFormat},
    {"VX", ErrInvalidFormat},
}

The test code looks like this:

func TestInvalid(t *testing.T) {
    for _, tt := range invalidTests {
        res, err := RomanToInt(tt.input)
        if err == nil {
            t.Errorf("Expected error for input %v but received %v", tt.input, res)
        }
        if err != tt.expected {
            t.Errorf("Unexpected error for input %v: %v (expected %v)", tt.input, err, tt.expected)
        }
    }
}

In the example library I simplified this since all cases resulted in the same error.

Similar to our previous use of table-driven tests, this allows us to add test cases quickly and to easily see what error is expected for a given input. As before, we need to be careful when logging any test failure to make sure we indicate which test case failed and what we were expecting instead; otherwise debugging becomes very difficult.

Now that we can test both normal and error cases, we can cover our entire Roman numeral function. In the next article I will show how to turn that excellent unit test code coverage into a green badge on a GitHub repository.