How to test the main package functions in golang?

I want to test a few functions that are included in my main package, but my tests don't appear to be able to access those functions.

My sample main.go file looks like:

package main

import (
    "log"
)

func main() {
    log.Printf(foo())
}

func foo() string {
    return "Foo"
}

and my main_test.go file looks like:

package main

import (
    "testing"
)

func Foo(t testing.T) {
    t.Error(foo())
}

when I run go test main_test.go I get

# command-line-arguments
.\main_test.go:8: undefined: foo
FAIL    command-line-arguments [build failed]

As I understand, even if I moved the test file elsewhere and tried importing from the main.go file, I couldn't import it, since it's package main.

What is the correct way of structuring such tests? Should I just remove everything from the main package asides a simple main function to run everything and then test the functions in their own package, or is there a way for me to call those functions from the main file during testing?

Solution 1:

when you specify files on the command line, you have to specify all of them

Here's my run:

$ ls
main.go     main_test.go
$ go test *.go
ok      command-line-arguments  0.003s

note, in my version, I ran with both main.go and main_test.go on the command line

Also, your _test file is not quite right, you need your test function to be called TestXXX and take a pointer to testing.T

Here's the modified verison:

package main

import (
    "testing"
)

func TestFoo(t *testing.T) {
    t.Error(foo())
}

and the modified output:

$ go test *.go
--- FAIL: TestFoo (0.00s)
    main_test.go:8: Foo
FAIL
FAIL    command-line-arguments  0.003s

Solution 2:

Unit tests only go so far. At some point you have to actually run the program. Then you test that it works with real input, from real sources, producing real output to real destinations. For real.

If you want to unit test a thing move it out of main().

Solution 3:

This is not a direct answer to the OP's question and I'm in general agreement with prior answers and comments urging that main should be mostly a caller of packaged functions. That being said, here's an approach I'm finding useful for testing executables. It makes use of log.Fataln and exec.Command.

1. Write main.go with a deferred function that calls log.Fatalln() to write a message to stderr before returning.
2. In main_test.go, use exec.Command(...) and cmd.CombinedOutput() to run your program with arguments chosen to test for some expected outcome.

For example:

func main() {
    // Ensure we exit with an error code and log message
    // when needed after deferred cleanups have run.
    // Credit: https://medium.com/@matryer/golang-advent-calendar-day-three-fatally-exiting-a-command-line-tool-with-grace-874befeb64a4
    var err error
    defer func() {
        if err != nil {
            log.Fatalln(err)
        }
    }()

    // Initialize and do stuff

    // check for errors in the usual way
    err = somefunc()
    if err != nil {
        err = fmt.Errorf("somefunc failed : %v", err)
        return
    }

    // do more stuff ...

 }

In main_test.go,a test for, say, bad arguments that should cause somefunc to fail could look like:

func TestBadArgs(t *testing.T) {
    var err error
    cmd := exec.Command(yourprogname, "some", "bad", "args")
    out, err := cmd.CombinedOutput()
    sout := string(out) // because out is []byte
    if err != nil && !strings.Contains(sout, "somefunc failed") {
        fmt.Println(sout) // so we can see the full output 
        t.Errorf("%v", err)
    }
}

Note that err from CombinedOutput() is the non-zero exit code from log.Fatalln's under-the-hood call to os.Exit(1). That's why we need to use out to extract the error message from somefunc.

The exec package also provides cmd.Run and cmd.Output. These may be more appropriate than cmd.CombinedOutput for some tests. I also find it useful to have a TestMain(m *testing.M) function that does setup and cleanup before and after running the tests.

func TestMain(m *testing.M) {
    // call flag.Parse() here if TestMain uses flags
    os.Mkdir("test", 0777) // set up a temporary dir for generate files

    // Create whatever testfiles are needed in test/

    // Run all tests and clean up
    exitcode := m.Run()
    os.RemoveAll("test") // remove the directory and its contents.
    os.Exit(exitcode)