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

:= — short variable declaration: declares one or more new variables and assigns values. = — assignment: sets the value of already-declared variables.

Short declarations are confined to function bodies. They infer types automatically and require at least one new variable on the left side. This makes them concise and idiomatic for most Go code.

1func fetchData() {
2    url := "https://api.example.com"  // new: url
3    resp, err := http.Get(url)         // new: resp, err
4    defer resp.Body.Close()
5    body, _ := io.ReadAll(resp.Body)   // new: body
6    fmt.Println(string(body))
7}

Assignment (=) updates existing variables. It's used after the initial declaration for reassignment, swapping, or updating state.

1var count int
2for i := 0; i < 10; i++ {
3    count++  // assignment to existing variable
4}
5
6x, y := 1, 2  // short declaration
7x, y = y, x   // swap using assignment

When to use:

• := — introducing new variables, capturing multiple return values, if/for init statements.
• = — updating existing variables, reassigning after error checks, loop accumulation.

Common mistakes:

• Shadowing: := in a nested block creates a new variable that hides the outer one.
• Using := with all existing variables — compiler error (need at least one new).
• Attempting := at package level — not allowed; use var.
• Forgetting that := in a for range creates a new loop variable each iteration.