Why doesn't fmt.Println work in Google app engine

Question

I built a simple web app using google app engine and golang. in code below, I use fmt.Println twice to print out somehting for debugging purpose. I have no problem running the app. everything works except nothing print out on the terminal.

func HomeHandler(w http.ResponseWriter, r *http.Request) {
    c := appengine.NewContext(r)
    q := datastore.NewQuery("Post").Ancestor(goblogKey(c)).Order("-CreatedOn").Limit(10)

    //posts := make([]entity.Post, 0, 10)
    var posts []entity.Post

    if _, err := q.GetAll(c, &posts); err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    fmt.Println(string(len(posts)) + "...........")

    postList := []dto.Post{}

    for _, val := range posts {
        newpost := dto.Post{
            Post:     val,
            BodyHTML: template.HTML(val.Body),
        }

        fmt.Println(val.Title)

        postList = append(postList, newpost)
    }

    page := dto.PageData{Title: "Home", Posts: postList}
    templates.ExecuteTemplate(w, "index", page)
}

Answer 1

In the real appengine enviroment you can't se anything output to stdout.
Appengine context give you away to log (that you can check in you appengine admin's page and in console playground).

func HomeHandler(w http.ResponseWriter, r *http.Request) {
    c := appengine.NewContext(r)
    c.Debugf("The message: %s", "foo")
    ...

Read more: https://developers.google.com/appengine/docs/go/reference#Context