undefined main - testing code unable to acess main package

Question

In the below code:

../folder1/some_test.go

package main_test

import "testing"

func TestF(t *testing.T) {
    main.F()
}

---

../folder1/some_file.go

package main

func F() {

}

---

main.F() gives undefined main

$ go version
go version go1.14.3 linux/amd64

Renaming package name from main_test to main resolve the problem.

Why main_test package name is not allowed for testing code, in same folder?

Answer 1

You must import the package to use the package.

go.mod:

module example.app

main.go:

package main

func F() {}

func main() {}

main_test.go

package main_test

import (
    "testing"
    "example.app"  // import the main package
)

func TestF(t *testing.T) {
    main.F()
}

The code above assumes that main*.go are in the same directory as go.mod for package example.app. Replace example.app with the name of your module. Adjust the paths to match your application. For example, if the the main*.go files are in the directory cmd/example below the directory containing go.mod, use the import path example.app/cmd/example instead of example.app.

Note that a test can import a main package this way, but non-test code cannot.

Answer 2

The problem is that main_test is a different package from main.

To access functions in main you need to import main and access the functions like this: main.F()

Also, note f starts with lowercase and thus is not exported from the package main. To access it in main_test it needs to be exported (which can be done by changing it to start with a capital letter: F).

Alternatively, you can change the test file to be in package main.

Edit with a note: When importing main note that import paths are by directory name. Usually developers in Go put their packages in directories with the same name as the package (e.g. put main in a directory named main). In your case the package and directory names are different so the import will be import ".../folder1" not import ".../main". You'll still be able to use main.F() to access the function:

package main_test

import (
   "testing"
   "../folder1" // use the full path to folder1 from the root of your module
)

func TestF(t *testing.T) {
    main.F()
}