How to test a function's output (stdout/stderr) in unit tests

Question

I have a simple function I want to test:

func (t *Thing) print(min_verbosity int, message string) {
    if t.verbosity >= minv {
        fmt.Print(message         


        

Answer 1

You can do one of two things. The first is to use Examples.

The package also runs and verifies example code. Example functions may include a concluding line comment that begins with "Output:" and is compared with the standard output of the function when the tests are run. (The comparison ignores leading and trailing space.) These are examples of an example:

func ExampleHello() {
        fmt.Println("hello")
        // Output: hello
}

The second (and more appropriate, IMO) is to use fake functions for your IO. In your code you do:

var myPrint = fmt.Print

func (t *Thing) print(min_verbosity int, message string) {
    if t.verbosity >= minv {
        myPrint(message) // N.B.
    }
}

And in your tests:

func init() {
    myPrint = fakePrint // fakePrint records everything it's supposed to print.
}

func Test...

Another option is to use fmt.Fprintf with an io.Writer that is os.Stdout in production code, but may be say bytes.Buffer in tests.