Is there autovivification for Go?

Question

Is there autovivification for Go?

As @JimB correctly noticed, my definition is not that strict. About my goal: In Python we have a very elegant "emulation" for an autovivification:

class Path(dict):
    def __missing__(self, key):
            value = self[key] = type(self)()
            return value

Is there a similar solution for Go?

Answer 1

Go maps will return a zero value for the type if the key doesn't exist, or the map is nil

https://play.golang.org/p/sBEiXGfC1c

var sliceMap map[string][]string

// slice is a nil []string
slice := sliceMap["does not exist"]


var stringMap map[string]string

// s is an empty string
s := stringMap["does not exist"]

Since a map with numeric values return will return 0 for missing entries, Go lets you use the increment and decrement operators on non-existent keys:

counters := map[string]int{}
counters["one"]++

Answer 2

Also extending JimB's answer, with the combination of map, interface{} and type assertion, you can dynamically create any complex structures:

type Obj map[interface{}]interface{}

func main() {
    var o Obj

    o = Obj{
        "Name": "Bob",
        "Age":  23,
        3:      3.14,
    }
    fmt.Printf("%+v\n", o)

    o["Address"] = Obj{"Country": "USA", "State": "Ohio"}
    fmt.Printf("%+v\n", o)

    o["Address"].(Obj)["City"] = "Columbus"
    fmt.Printf("%+v\n", o)

    fmt.Printf("City = %v\n", o["Address"].(Obj)["City"])
}

Output (try it on the Go Playground):

map[Name:Bob Age:23 3:3.14]
map[Age:23 3:3.14 Address:map[Country:USA State:Ohio] Name:Bob]
map[3:3.14 Address:map[Country:USA State:Ohio City:Columbus] Name:Bob Age:23]
City = Columbus