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

:= — short variable declaration: declares a new variable AND assigns a value. = — simple assignment: assigns a value to an already-declared variable.

The short declaration operator creates the variable in the current block scope. The type is inferred from the right-hand side. At least one variable on the left must be new.

1func main() {
2    x := 42       // declares x as int
3    y, z := 1, 2  // declares y and z
4    x, w := 3, 4  // OK: x reassigned, w is new
5}

Assignment (=) requires the variable to already exist in scope. You can assign to multiple variables on the left if all are pre-declared.

1var x int
2x = 10     // assignment — OK
3
4var a, b int
5a, b = 5, 6  // multi-assignment — OK
6
7c = 7  // error: undeclared c

When to use:

• := — first time a variable appears; used in if/for init and multiple return values.
• = — updating an existing variable, reassigning after error handling.

Common mistakes:

• Shadowing outer variables: x := 1 inside an if block creates a new x.
• Using := in a for loop with an existing variable — creates a new scoped variable.
• Trying := outside a function body — not allowed at package level.