Golang test stdout

Question

I am trying to test some functions that print ANSI escape codes. e.g.

// Print a line in a color
func PrintlnColor(color string, a ...interface{}) {
    fmt.Print("\x1b[31m")
    fmt.Print(a...)
    fmt.Println("\x1b[0m")
}

I tried using Examples to do it, but they don't seem to like escape codes.

Is there any way to test what is written to stdout?

What's the purpose for testing stdout? It's much easier, to verify that your function produces the output you want before it's written to stdout. — JimB, Nov 14 '14 at 20:20
The problem is stdout is the only output. The function is going to return `n int, err error` just like `fmt.Println` — giodamelio, Nov 14 '14 at 20:34
you could replace `os.Stdout` with something akin to a `io.MultiWriter` that returned an `*os.File`, but it's easier to just refactor your code to make it testable. You don't need to test `fmt.Println` and `os.Stdout`, they have their own unit tests. — JimB, Nov 14 '14 at 20:47
Yeah, write an `FprintlnColor` and then make `PrintlnColor` call it with `Stdout`. It can write to a `bytes.Buffer` for tests. — twotwotwo, Nov 15 '14 at 00:33

jmaloney · Accepted Answer · 2014-11-15T23:08:59.190

Using fmt.Fprint to print to io.Writer lets you control where the output is written.

var out io.Writer = os.Stdout

func main() {
    // write to Stdout
    PrintlnColor("foo")

    buf := &bytes.Buffer{}
    out = buf

    // write  to buffer
    PrintlnColor("foo")

    fmt.Println(buf.String())
}

// Print a line in a color
func PrintlnColor(a ...interface{}) {
    fmt.Fprint(out, "\x1b[31m")
    fmt.Fprint(out, a...)
    fmt.Fprintln(out, "\x1b[0m")
}

Go play

Golang test stdout

1 Answers1