The interface{} type in Golang is like the duck type in python. If it walks like a duck, it’s a duck – so the type is determined by an attribute of the variable. This duck typing support in python often leaves one searching for the actual type of the object that a function takes or returns. But with richer names, or a naming convention, one gets past this drawback. Golang tries to implement a more limited and stricter duck typing – the programmer can get define the type of a variable as an interface{} – which should have been called a ducktype{}. But when it comes time to determine the type of the duck, one must assert it explicitly – and in that process can receive an error indicating if there was a type mismatch.

explicit ducktype creation

var myVariableOfDuckType interface{} = “thisStringSetsMyVarToTypeString”

var mySecondVarOfDuckType interface{} = 123  // sets type of mySecondVar to int

ducktype assertion

getMystring, isOk := myVariableOfDuckType.(string) // isOk is true, assertion to string passed

getMystring, isOk := mySecondVarOfDuckType.(string) // isOk is false, assertion to string  failed

In python, int(123) and int(“123”) both return 123.

In Golang, int(mySecondVarOfDuckType) will not return 123, even though the value actually happens to be an int. It will instead return a “type assertion error”.

cannot convert val (type interface {}) to type int: need type assertion