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

One thing to also remember, there's nothing stopping you from writing functions to avoid the boilerplate.

For example I have a command line app that uses log and I wrote this function:

func captureOutput(f func()) string {
    var buf bytes.Buffer
    log.SetOutput(&buf)
    f()
    log.SetOutput(os.Stderr)
    return buf.String()
}

Then used it like this:

output := captureOutput(func() {
    client.RemoveCertificate("www.example.com")
})
assert.Equal(t, "removed certificate www.example.com\n", output)

Using this assert library: http://godoc.org/github.com/stretchr/testify/assert.

You can do one of three 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...

The third is to use fmt.Fprintf with an io.Writer that is os.Stdout in production code, but bytes.Buffer in tests.