在函数内部时变量变为nil

Question

This question already has an answer here:

• How to use global var across files in a package? 2 answers
• global error variable remains nil after initialization 2 answers

Why the db variable is nil in the getBooks function?

package main

import (
    ...
)

var db *sql.DB

func init() {
    gotenv.Load()
}

func main() {

    db, err := sql.Open("postgres", os.Getenv("ELEPHANTSQL_URL"))
    err = db.Ping()
    fmt.Println(db, err)

    router := mux.NewRouter()
    router.HandleFunc("/books", getBooks).Methods("GET")
    log.Fatal(http.ListenAndServe("localhost:8000", router))
}

func getBooks(w http.ResponseWriter, r *http.Request) {
    if db == nil {
        log.Print("!!!!!!!!!!")
        os.Exit(2)
    }
}

</div>

Answer 1

With := you're declaring a new db variable inside the main() scope instead of assigning to the db variable at the package scope.

You must either use just = or use another name than db.

Try this:

var err error
db, err = sql.Open("postgres", os.Getenv("ELEPHANTSQL_URL"))