使用未定义的整数键入映射

I'm using Gorilla mux for my handlers and using mux.Vars. I'm trying to write a test for one of the handlers that uses mux.Vars so what I do is

var vars = map[string]string{
    "id": user.ID,
}
context.Set(req, 0, vars)

In mux the key (an integer) is undefined so by default 0. I've logged the key when mux.Vars gets called and it prints 0. I should be able to key into this map

 map[0:map[id:522d14f5b1b92235d6000002]]

by doing map[key] but that returns nil. However, I get the correct value back if I hardcode map[0]. Any thoughts?

I'm not entirely sure I understand the question, but it looks like you might be confusing mux.Vars with mux.context. The two are separate entities. The former returns route variables that are parsed from the URL path. For instance, you could do:

r := mux.NewRouter()
r.HandleFunc("/blah/{foo}/", MyHandler)

...

func MyHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    ...
}

The latter contains context variables you set yourself. For instance:

func MyHandler(w http.ResponseWriter, r *http.Request) {
    context.Set(r, 0, map[string]string{"id": "myid"})
    myMap := context.Get(r, 0)
    ...
}

You might check out some usage examples of how others use both to see what is most appropriate for your use case:

mux.Vars: https://sourcegraph.com/github.com/gorilla/mux/symbols/go/github.com/gorilla/mux/Vars mux.context: https://sourcegraph.com/github.com/gorilla/context/symbols/go/github.com/gorilla/context