正如@kostix所说，
m是本地范围
if。试试这个代码
type Comparison struct { Left []byte Right []byte Name string}func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) { key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil) side := r.FormValue("side") comparison := new(Comparison) err := datastore.Get(c, key, comparison) check(err) // NOTE! now m is in the function's scope var m Image if( side == "left"){ m, _, err = image.Depre(bytes.NewBuffer(comparison.Left)) } else { m, _, err = image.Depre(bytes.NewBuffer(comparison.Right)) } check(err) w.Header().Set("Content-type", "image/jpeg") jpeg.Enpre(w, m, nil)}
