Equivalent of setdefault in Go?

I can do:

_, ok := some_go_map[a_key]

to test for existence of key.

But I've been spoiled by Python's dict's setdefault method (if a key does not have value set in a "map" [dict == associative array], set it to a given default, then get it; otherwise just get).

Wondering if there's some idiom in Go to achieve the same thing?

Note that Go's default behavior is to return the "zero value" for the value type (e.g., 0 or "") when a looked-up key's missing, so if the default you want happens to be that, you're all set already.

Riffing off Buddy and larsmans' answers, here's code that attaches a new method to a named Dict type, so you can either use d[key] for Go's built-in behavior or d.SetDefault(key, val)--

http://play.golang.org/p/5SIJSWNWO7

package main

import "fmt"

type Dict map[string]float64

func (d Dict) SetDefault(key string, val float64) (result float64) {
	if v, ok := d[key]; ok {
		return v
	} else {
		d[key] = val
		return val
	}
}

func main() {
	dd := Dict{}
	dd["a"] = 3
	fmt.Println(dd.SetDefault("a", 1))
	fmt.Println(dd.SetDefault("b", 2))
}

You can always define it yourself:

func setDefault(h map[string]int, k string, v int) (set bool, r int) {
	if r, set = h[k]; !set {
		h[k] = v
		r = v
		set = true
	}
	return
}

But no, it's not in the stdlib. Usually, you'd just do this inline.

As far as I know, there isn't something like that built in. You could add a type to help out

http://play.golang.org/p/pz34p7w6fP

package main

import "fmt"

type Default struct {
	vals     map[string]int
	defaults int
}

func (d *Default) Get(key string) int {
	if val, ok := d.vals[key]; ok {
		return val
	}
	return d.defaults
}

func main() {
	someMap := Default{map[string]int{"foo": 1, "bar": 2}, 5}
	fmt.Println(someMap.Get("foo"))
	fmt.Println(someMap.Get("doesn't exist"))
}