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Question

We all know dependency injection makes packages decoupled. But I'm a little confused about best practices of dependency injection in go.
Lets assume package User needs to access Config package.
We can pass a Config object to User methods. In this way I can change the Config package functionality as long as the new code resolves the interfaces. Another approach is call Config package methods directly , In these scenario I can change Config code too as long as the methods names remains the same. Like so

Update :

What is different between these two approaches :

package User

func foo(config ConfigObject) {
   config.Foo()
}

And this one :

package User

import Config

func foo() {
   config.Foo()
}

Answer 1

Calling config.Foo on the config argument to a method means that you receive an instance of some structure (possibly implementing interface Config) and call the method Foo on that instance/interface. Think of this as of calling a method of an object in OO terms:

package user

func foo(cfg config.Config) {
   cfg.Foo()
}

Calling config.Foo having imported the config package means you are calling the function Foo of package config, not of any object/struct/interface. Think of this as pure procedural programming without any objects:

package user

import config

func foo() {
   config.Foo()
}

The latter has nothing to do with dependency injection, the former may constitute a part of it if Config is an interface.

Dependency injection, on the other hand, follows generally the same rules in Go as in other languages:

accept interfaces, supply implementations

Because in Go structs satisfy interfaces implicitly rather than explicitly (as it is the case in Java)

• the code accepting the value only needs to know about the interface and import it;
• the code implementing it does not even need to know about the interface (it can just happen that it satisfies it);
• the code that supplies the impl into a method accepting an interface, obviously, needs to know both.

For your example this means:

package config

type Config interface {
    Foo() string
}

package foo

type Foo struct{}

func (f *Foo) Foo() string {
    return "foo"
}

package boo

type Boo struct{}

func (b *Boo) Foo() string {
    return "boo" 
}

package main

func foo(cfg config.Config) string{
    return cfg.Foo()
}

func main() {
    // here you inject an instance of Foo into foo(Config)

    log.Print(foo(&foo.Foo{}))

    // here you inject an instance of Boo into foo(Config)
    log.Print(foo(&boo.Boo{})
}

Prints

2018/03/03 13:32:12 foo

2018/03/03 13:32:12 boo