err variable shadowing — is it a bad practice?

[iakov....@gmail.com]

Hi,

Occasionally go vet complains about shadowing of the err variable.

Consider the following code.

package main

import (

"flag"

"io"

"os"

)

func main() {

inFilename := flag.String("in", "infile", "input file")

outFilename := flag.String("out", "outfile", "output file")

flag.Parse()

in, err := os.Open(*inFilename)

if err != nil {

panic(err)

}

out, err := os.Create(*outFilename)

if err != nil {

panic(err)

}

buf := make([]byte, 1024)

for {

n, err := in.Read(buf)

if err != nil && err != io.EOF {

panic(err)

}

if n == 0 {

break

}

if _, err := out.Write(buf[:n]); err != nil {

panic(err)

}

}

}

Go vet in strict mode complains:

$ go tool vet -shadow -shadowstrict ./cat.go 

./cat.go:26: declaration of "err" shadows declaration at ./cat.go:14

./cat.go:33: declaration of "err" shadows declaration at ./cat.go:26

Is it really a bad practice? What's the alternative?

Declaring all the variables in advance with explicitly specifying their types, i.e.

var out *os.File
out, err = os.Create(*outFilename)

?

Thanks,

Iakov

[Dave Cheney]

This is extremely common in go code, vet is being pedantic,

[Steven Hartland]

I believe vet shadow is known to have issues and is quite picky ;-)

Your first one I'd say could be classed as bug as you're clearly declaring n however I'd guess this is due to the for loop as declaring n out of the loop eliminating the short declaration will fix it. This is confirmed by eliminating the for which also eliminates the warning.

Your second warning (cat.go:33) I'd say is correct as you don't declare a new variable hence err is declared unnecessarily. I'm not sure if go will actually redeclare a new var under the hood, but as err is already available just reuse it.

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