Fetch value from net/context in Golang

Question

I am using Golang net/context package for passing on the ID's wrapped in a context object from one service to another. I am able to pass the context object successfully but to actually retrieve the value of a particular key, the context.Value(key) always returns a nil. I am not sure why, but this is what I have done so far:

if ctx != nil {
        fmt.Println(ctx)
        fmt.Println("Found UUID, forwarding it")

        id, ok := ctx.Value(0).(string)  // This always returns a nil and thus ok is set to false
        if ok {
            fmt.Println("id found %s", id)
            req.headers.Set("ID", id)
        }
    }

ctx is of the type context.Context and on printing it I get:

context.Background.WithValue(0, "12345")

I am interested in fetching the value "12345" from the context. From the Golang net/context documentation (https://blog.golang.org/context) , the Value() takes in a key of interface{} type and returns a interface{} and hence I typecast is to .(string). Could anyone help with this ?

I get <nil> for ctx.Value(0). But when I print ctx I get the whole thing: context.Background.WithValue(0, "12345") — psbits
What is the type of the key '0'? go will default the type of 0 to int so When you call "Value(0)" it will call Value(int(0)) instead of Value(whatevertype0actuallyis(0)) — Benjamin Kadish
type key int const ( id key = iota ) key is set to 0 because of iota. — psbits
iota just lets you create integer enums efficiently it doesn't have a type golang.org/ref/spec#Iota — Benjamin Kadish

JimB JimB · Accepted Answer · 2016-06-16T20:22:53

Your context key is isn't an int, which is the default type the untyped constant 0 will be assigned to when passed to Value in an interface{}.

c := context.Background()

v := context.WithValue(c, int32(0), 1234)
fmt.Println(v.Value(int64(0)))  // prints <nil>
fmt.Println(v.Value(int32(0)))  // print 1234

You need to set and extract the value with the correct type as well. You need to define a single type that you will always use as keys. I often times define helper functions to extract the context values and do the type assertion, which in your case could also serve to normalize the key type.

Fetch value from net/context in Golang

1 Answers