何时使用log over fmt进行调试和打印错误？

Question

I know that we can print variable or error using log and fmt. for example if I want to print variable I can do this :

h := "world"
fmt.Printf("hello = %v
", h)
log.Printf("halo = %v
", h)

the output would be :

hello = world
2016/12/30 09:13:12 halo = world

and usually in the error handling I found log like this

if err != nil {
    log.Println("Error : something terrible happen -> ", err)
    return err
}

but from above case I could also use fmt to print the error like this

fmt.Printf("Error : something terrible happen -> %v
",err.Error())

Is it a good practice to use fmt instead of log for printing the error? And then I always use fmt instead of log for printing the variable when debugging.

Answer 1

Select between log and fmt using these facts:

• The log functions print to stderr by default and can directed to an arbitrary writer. The fmt.Printf function prints to stdout.
• The log functions can print timestamp, source code location and other info.
• The log functions and fmt.Printf are both thread safe, but concurrent writes by fmt.Printf above an OS dependent size can be interleaved.

The answer to the three sub questions are "it depends".