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

Issue

I am new to Go programming language.

I noticed something strange in Go: I thought that it used := and substitutes = in Python, but when I use = in Go it is also works.

What is the difference between := and =?

Solution

= is assignment. more about assignment in Go: Assignments

The subtle difference between = and := is when = used in variable declarations.

General form of variable declaration in Go is:

var name type = expression

the above declaration creates a variable of a particular type, attaches a name to it, and sets its initial value. Either the type or the = expression can be omitted, but not both.

For example:

var x int = 1
var a int
var b, c, d = 3.14, "stackoverflow", true

:= is called short variable declaration which takes form

name := expression

and the type of name is determined by the type of expression

Note that: := is a declaration, whereas = is an assignment

So, a short variable declaration must declare at least one new variable. which means a short variable declaration doesn’t necessarily declare all the variables on its left-hand side, when some of them were already declared in the same lexical block, then := acts like an assignment to those variables

For example:

 r := foo()   // ok, declare a new variable r
 r, m := bar()   // ok, declare a new variable m and assign r a new value
 r, m := bar2()  //compile error: no new variables

Besides, := may appear only inside functions. In some contexts such as the initializers for "if", "for", or "switch" statements, they can be used to declare local temporary variables.

More info:

variable declarations

short variable declarations

Answered By – simon_xia

Answer Checked By – Marie Seifert (GoLangFix Admin)

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