fmt.Printf("%v") panics where %#v doesn't

[Thorsten von Eicken]

I was surprised to see Printf %v panic on a struct that %#v prints fine. Is this expected / intentional / as designed?

Test case:

package main

import (

"fmt"

)

type MyError struct{ error }

func main() {

err := MyError{}

fmt.Printf("%%#v %#v\n", err)

fmt.Printf("%%+v %+v\n", err)

fmt.Printf("%%v %v\n", err)

}

Output:

%#v main.MyError{error:error(nil)}
%+v %!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
%v %!v(PANIC=runtime error: invalid memory address or nil pointer dereference)

Playground: https://play.golang.org/p/B5FeWMryN0

[Matt Harden]

I think MyError has the Error method, which %v will use but %#v will not. Then it panics because it's trying to call error(nil).Error().

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[Thorsten von Eicken]

You're probably correct, see this:

package main

import (

"fmt"

)

type MyError struct{ error }

type MyError2 struct {

foo int

err error

}

func main() {

err := MyError{}

fmt.Printf("%%#v %#v\n", err)

fmt.Printf("%%+v %+v\n", err)

fmt.Printf("%%v %v\n", err)

err2 := MyError2{}

fmt.Printf("%%#v %#v\n", err2)

fmt.Printf("%%+v %+v\n", err2)

fmt.Printf("%%v %v\n", err2)

var rr error

fmt.Printf("%%#v %#v\n", rr)

fmt.Printf("%%+v %+v\n", rr)

fmt.Printf("%%v %v\n", rr)

}

%#v main.MyError{error:error(nil)}
%+v %!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
%v %!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
%#v main.MyError2{foo:0, err:error(nil)}
%+v {foo:0 err:<nil>}
%v {0 <nil>}
%#v <nil>
%+v <nil>
%v <nil>

It's only if the error is embedded that it panics.

Playground: https://play.golang.org/p/TeNVj96zxr

[Matt Harden]

This might make it clearer as well: https://play.golang.org/p/g3KdJUQRlK

[Thorsten von Eicken]

Yup, doesn't quite make sense to me that Error() is called if it's an embedded struct and not when it's a struct field. See https://play.golang.org/p/Vp6Y-RETWZ

[Chris Manghane]

In your example, MyError2 does not have an Error() method so it is not called. From the fmt package docs (golang.org/pkg/fmt):

• 3. If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface, that will be invoked.
• If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X), the following... rules apply:
• 4. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).
  

MyError implements the error interface and MyError2 does not.