Go Context Values
Context values are small pieces of data stored inside a context. A function that receives the context can read the data back out.
Store a value and read it back
WithValue stores the name under the key "name". Value reads it back with the same key.
package main
import (
"context"
"fmt"
)
func main() {
ctx := context.WithValue(context.Background(), "name", "Mina")
fmt.Println(ctx.Value("name"))
}Two key types hold two values
The keys nameKey and idKey are different types, so the same word can key two slots. Each Value call reads its own slot back.
package main
import (
"context"
"fmt"
)
type nameKey string
type idKey string
func main() {
ctx := context.Background()
ctx = context.WithValue(ctx, nameKey("item"), "notebook")
ctx = context.WithValue(ctx, idKey("item"), 42)
fmt.Println(ctx.Value(nameKey("item")))
fmt.Println(ctx.Value(idKey("item")))
}A missing key reads back nil
The context holds a name and nothing under the key "city". The read prints <nil>.
package main
import (
"context"
"fmt"
)
func main() {
ctx := context.WithValue(context.Background(), "name", "Mina")
fmt.Println(ctx.Value("city"))
}Example
package main
import (
"context"
"fmt"
)
type requestKey string
func last(ctx context.Context) {
fmt.Println("Request:", ctx.Value(requestKey("id")))
}
func middle(ctx context.Context) {
last(ctx)
}
func first(ctx context.Context) {
middle(ctx)
}
func main() {
ctx := context.WithValue(context.Background(), requestKey("id"), "req-42")
first(ctx)
}Key points
- Use context.WithValue to store a value inside a context.
- WithValue returns a new context that carries the value.
- The Value method reads a stored value back by its key.
- A key is often a value of a named type made just for keys.
- Two different key types can hold two values in one context.
- A key that was never stored reads back as nil.
In this example, main stores the request id in the context under a key of the named type requestKey. First passes the context to middle, and middle passes it to last. Last reads the id back and prints it at the end of the chain.
Note
Context values suit small request data, not the main inputs of a function.
Try it Yourself
Edit the code, then press Run.
Running is simulated · Go Playground
Exercise
What does ctx.Value return for a key that was never stored?