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

:= — short variable declaration: creates a new variable and assigns a value in one step. = — assignment: updates the value of an already-declared variable.

Short declarations are the standard way to introduce variables inside functions. The type is inferred from the right-hand side. At least one variable on the left must be new.

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

Assignment updates existing variables. Use it after declaration for reassignment, swapping, or receiving values.

1var count int
2for i := 0; i < 10; i++ {
3    count++  // assignment
4}
5
6var a, b int
7a, b = b, a  // swap

When to use:

• := — first use, capturing returns, if/for init.
• = — updating, reassigning, swapping.

Common mistakes:

• Shadowing: := in nested blocks hides outer variables.
• Using := when all variables exist — error.
• := at package level — not allowed.
• Shadowing in for loops: new variable per iteration.