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

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)
}
}
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.

Answer 1

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.

Answer 2

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.

Answer 3

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.