Go: What is the difference between := and =?

:= — short variable declaration: creates a new variable and assigns a value. = — assignment: sets the value of an existing variable.

Short declarations are the idiomatic way to introduce variables inside functions. The type is inferred from the right side. If all variables on the left already exist, the compiler errors.

1func process() {
2    name := "Alice"        // declares name
3    age := 30              // declares age
4    name, role := "Bob", "admin"  // OK: role is new
5    name = "Charlie"      // assignment — name exists
6}

Simple assignment requires the variable to already exist in the current scope. Used after initial declaration to update values, swap variables, or receive multiple return values.

1var x int
2x = 42       // assignment
3
4var a, b int
5a, b = b, a  // swap — both already declared
6
7result, err := doWork()
8if err != nil {
9    err = fmt.Errorf("wrapping: %w", err)  // reassignment
10}

When to use:

• := — first appearance of a variable, capturing return values, if/for init.
• = — updating existing variables, reassigning after error checks, swaps.

Common mistakes:

• Variable shadowing: := inside an if block creates a new variable that hides the outer one.
• Using := at package level — only var is allowed there.
• Forgetting that := in a loop creates a new variable each iteration.
• Using := when all variables are already declared — compiler error.