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

:= — short variable declaration: creates a new variable and assigns a value in one step. = — 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 you use := and all variables on the left already exist, the compiler errors — you need at least one new variable.

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. It's 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 statements.
• = — updating existing variables, reassigning after error checks, multiple-return 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 — use = if you want to accumulate.