How is context.Context an interface and how does its design demonstrate Go best practices?

// The full context.Context interface:
type Context interface {
    Deadline() (deadline time.Time, ok bool) // when will this context expire?
    Done() <-chan struct{}                   // closed when context is cancelled
    Err() error                              // nil, context.Canceled, or context.DeadlineExceeded
    Value(key any) any                       // request-scoped values
}

// All functions that may block or need cancellation accept context as FIRST param
func FetchUser(ctx context.Context, id int) (*User, error) {
    req, _ := http.NewRequestWithContext(ctx, "GET",
        fmt.Sprintf("/users/%d", id), nil)
    resp, err := http.DefaultClient.Do(req)
    if err != nil { return nil, err }
    defer resp.Body.Close()
    var u User
    return &u, json.NewDecoder(resp.Body).Decode(&u)
}

// Creating contexts
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel() // ALWAYS defer cancel

user, err := FetchUser(ctx, 42)
if err != nil {
    if errors.Is(err, context.DeadlineExceeded) {
        log.Println("request timed out")
    }
}

// Checking cancellation in a loop
func processItems(ctx context.Context, items []Item) error {
    for _, item := range items {
        select {
        case <-ctx.Done(): return ctx.Err() // cancelled
        default: process(item)
        }
    }
    return nil
}