= 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