Mocking in GO lang - unit-testing

Question

I am new to Go and I am still trying to get my head around its concept. I am trying to create a simple unit test and want to Mock one of its service. I want to mock my_mod_2.EmpInfo in it so that I dont call the actual service.
method-1.go
package my_mod_1
import (
"awesomeProject-1/my-mod-2"
)
func CreateAndSendMail() string {
svc := my_mod_2.EmpInfo{}
name := svc.GetName()
empAddress := svc.GetAddress()
return name + " lives in " + empAddress
}
Here is Emp.go
package my_mod_2
import "fmt"
type EmpInfo struct {}
func (o EmpInfo) GetName() string{
fmt.Println("Called actual")
return "John Doe"
}
func (o EmpInfo) GetAddress() string {
return "US"
}
Here is the method-1_test.go
package my_mod_1
import (
"testing"
)
func TestCreateAndSendMail(t *testing.T) {
val := CreateAndSendMail()
if val != "John Doe lives in US" {
t.Error("Value not matched")
}
}
I am seeing Called actual in test execution. I Know I have to create a mock using interface but I am just not getting it. Can someone please help me out with this small code ?

Answer 1

First of all, you need to prepare your code to use interfaces and mocks. To do this I suggest you to declare Service interface beside CreateAndSendMail method. In this case, it is better to pass service instance to the method or use it as an instance variable of a struct to which your method is belong:
type Service interface {
GetName() string
GetAddress() string
}
func CreateAndSendMail(svc Service) string {
name := svc.GetName()
empAddress := svc.GetAddress()
return name + " lives in " + empAddress
}
or
type Service interface {
GetName() string
GetAddress() string
}
type S struct {
svc Service
}
func (s *S) CreateAndSendMail() string {
name := s.svc.GetName()
empAddress := s.svc.GetAddress()
return name + " lives in " + empAddress
}
Then, your EmpInfo will implement your Service interface implicitly. And this is a cool feature of golang interfaces.
After all our preparations, we are ready to create test. To do this, we can implement mocks by ourselves:
import (
"testing"
)
type MockSvc struct {
}
func (s *MockSvc) GetName() string {
return "Mocked name"
}
func (s *MockSvc) GetAddress() string {
return "Mocked address"
}
func TestCreateAndSendMail(t *testing.T) {
svc := &MockSvc{}
val := CreateAndSendMail(svc)
if val != "Mocked name lives in Mocked address" {
t.Error("Value not matched")
}
}
Also, we can use special tool gomock to automate mock creation process

Answer 2

Interfaces are the most used Go features that helps with testing. Interface in Go allows for Duck Typing where you can switch any type that has implemented an interface to be mocked by a different type.
From your example, the service has the following two methods: GetName() and GetAddress(). Create an interface Service with these two methods.
type Service Interface {
GetName() string
GetAddress() string
}
Now your struct EmpInfo, already implements the Service interface. Create a new MockService struct with same 2 functions.
type MockService struct {}
func (ms *MockService) GetName() string {
// Mock Code
}
func (ms *MockService) GetAddress() string {
// Mock Code
}
Then, replaces instances of EmpInfo with MockService wherever you need.
PS: Consider adding functions GetName and GetAddress with pointer to EmpInfo.