如何在Go中包装FPrintf？

I am trying to write a functional that wraps FPrintf however I keep getting weird characters out.

Here is an reproducer

https://play.golang.org/p/yZgNnpovEa

The idea is to be able to have a conditional Printf which I can test the output (Thus use of FPrintf so I can test the output towards the input). Is there any way to get around this?

I have seen How to ignore extra fields for fmt.Sprintf but all answers there assume the user is expecting only %s while in my case I want to be as flexible as Printf and the only other one is downvoted.

Is this just not possible and can anybody give a reasonable explanation why?

The problem is you actually send an empty slice to fmt.Fprintf. Additional check of params' length should fix the problem.

func (p ConditionalPrinter) printF(s string, params ...interface{}) {
    if p.print {
        if len(params) == 0 {
            fmt.Fprintf(p.writer, s)
        } else {
            fmt.Fprintf(p.writer, s, params)
        }
    }
}

Or this:

func (p ConditionalPrinter) printF(s string, params ...interface{}) {
    switch {        
    case !p.print:
        return
    case len(params) == 0:
        fmt.Fprintf(p.writer, s)
    default:
        fmt.Fprintf(p.writer, s, params)
    }
}

See Playground link