Unit testing Stdout in Go

Figuring out how to unit test functions that print to the terminal can be something of a daunting task. Thankfully, it's actually pretty simple due to the magic of interfaces in Go!

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This post is going to be short and sweet on how to test output by capturing it in a bytes.Buffer. Below, I have a function that accepts a string, prints out a statement, and then returns.

func SayHello(w io.Writer, name string) {
    fmt.Fprintf(w, "Hi, my name is %s\n", name)
}

The critical component of this function is the io.Writer interface variable being passed in. Because it is an interface, this function will accept any data type that implements the method Write(b []byte) (n int, err error).

This interface is implemented in the io package and the entire code can be seen here. Most importantly, here is the code below for the interface:

type Writer interface {
    Write(p []byte) (n int, err error)
}

So how do we print out this function to the terminal? We pass in os.Stdout, which has its own Write method! This can be seen here.

To print this out to our terminal, just add the following in the main() function.

func main() {
    SayHello(os.Stdout, "Shannon")
}

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Now, we can move onto unit testing this specific function, which just outputs a name and greeting. In order to test it, we need to capture its output and then turn it into a string from a slice of bytes. To do this, we'll utilize the bytes.Buffer struct, which has a Write method to satisfy the Writer interface.

Here is the full test code in order to compare these:

func TestSayHello(t *testing.T) {
    tt := []struct {
        description string
        name        string
        expect      string
    }{
        {"Empty", "", "Hi, my name is \n"},
        {"Name1", "Shannon", "Hi, my name is Shannon\n"},
        {"Name2", "Shug", "Hi, my name is Shug\n"},
    }

    for _, tc := range tt {
        t.Run(tc.description, func(t *testing.T) {
            var output bytes.Buffer
            SayHello(&output, tc.name)
            if tc.expect != output.String() {
                t.Errorf("got %s but expected %s", output.String(), tc.expect)
            }
        })
    }
}

You'll notice that we are declaring a Buffer here: var output bytes.Buffer and then passing that buffer into the SayHello function. However, we are actually passing in the address of the buffer. This is due to the Buffer method for Write being a pointer receiver, which can be seen here.

And that's it! Run go test -v to see that all tests pass, and you have successfully captured the output of a command in order to unit test the output without actually returning the string value.

Comments

[Mesut GENEZ]

So how do we capture print inside a void function that has no "writer" parameter?

func SayHello(name string) {
fmt.Print("Hi, my name is", name)
}

[Aleksey Reprintsev]

You can do it with example function from testing package:

func ExampleSayHello() {
    fmt.Println("Alex")
    // Output: Hi, my name is Alex"
}

(btw, the result may be unexpected for your function: go.dev/play/p/J4zY09qEU4X )

Or use os.Pipe() as described in this discussion, but be aware of buffer overflows and data race.