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

Answer 2

standard i/o Or Error is used communicate with app server used by the devleoper. In production system there's no meaning of using standard i/o. In production systems log is used to track the results. In app engine there's some limitations. like fmt, socket etc.
Its always better to use log when testing or running program in remote server.