Go's `:=` vs `=`: When to Use Which Assignment Operator?

:= vs = in Go: A Comprehensive Guide

As a novice in Go programming, you might have noticed an intriguing duality between := and =. While := was assumed to be the Go equivalent of Python's =, using = in Go also seems to function.

Understanding Assignment Operator (=)

The = operator in Go serves as an assignment operator, assigning values to variables. Its general syntax is:

variable = expression

For instance:

var x = 10
y := 20

In the above example, x is assigned the value 10, and y is assigned the value 20.

Exploring Variable Declarations with :=

:= serves as a short variable declaration, which takes the following form:

name := expression

The type of name is automatically inferred based on the expression. For instance:

a := 10 // a is of type int
b := 3.14 // b is of type float64

Key Distinction: Declaration vs Assignment

The crucial difference between := and = lies in their roles. := is a declaration, introducing a new variable, while = is an assignment, assigning a value to an existing variable. This means that := must always declare at least one new variable.

Usage Scenarios

:= can be used in the following scenarios:

• Declaring a new variable
• Multiple variable declarations with different types
• Declaring local temporary variables in statements like if, for, and switch

Exceptions

• := cannot appear outside functions.
• := cannot be used to declare variables whose types or values are already specified in the surrounding lexical block.

Additional Resources

• [Variable Declarations](https://go.dev/ref/spec#Variable_declarations)
• [Short Variable Declarations](https://go.dev/ref/spec#Short_variable_declarations)

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