Go local variable shadowing 2020-12-12

While writing Go code, I encountered a case when using the init() function.

Consider this case:

package main

import "fmt"

var a int

func doSomething() (int, error) {
  return 10, nil
}

func init() {
  a = 1

  a, err := doSomething()

  if err != nil {
    panic(err)
  }

  fmt.Printf("In init(), a is %d\n", a)
}

func main() {
  fmt.Printf("In main(), a is %d\n", a)
  return
}

When we execute this, it prints out:

In init(), a is 10
In main(), a is 1

Weird, the init() function does set the variable a with the value 10.

But if we are not using short assignment in the init() function, and do this instead:

func init() {
  a = 1

  var err error
  a, err = doSomething()

  (the rest of the code ...)

It would print as what we expect:

In init(), a is 10
In main(), a is 10

Using a short assignment will declare a new local variable a. The local variable shadows the global variable a. So when we are updating the value ofa, we are updating the local variant, not the global one.