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

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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)
    }
}

But how can I test what the function actually sends to standard output? Test::Output does what I want in Perl. I know I could write all my own boilerplate to do the same in Go (as described here):

orig = os.Stdout
r,w,_ = os.Pipe()
thing.print("Some message")
var buf bytes.Buffer
io.Copy(&buf, r)
w.Close()
os.Stdout = orig
if(buf.String() != "Some message") {
    t.Error("Failure!")
}

But that's a lot of extra work for every single test. I'm hoping there's a more standard way, or perhaps an abstraction library to handle this.

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("removed certificate www.example.com\n", output)

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

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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.

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You could consider adding a return statement to your function to return the string that is actually printed out.

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

Now, your test could just check the returned string against an expected string (rather than the print out). Maybe a bit more in-line with Test Driven Development (TDD).

And, in your production code, nothing would need to change, since you don't have to assign the return value of a function if you don't need it.

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Comments
  • This is probably what I will do, as it's a more general solution than that provided by @Ainar-G, which requires that I control all of the code which might output something.
  • FYI this is not go-routine safe. A second go-routine could reset log back to os.Stderr while the first go-routing is still logging.
  • Examples are good for when you have deterministic output and fmt.Fprint is good for when you can control the print code (which you can't if it's in an external library or something). I think most of the time one of these things will be true. If there's a case where neither is true an option to read what is written to os.Stdout into a string would be nice.