What is the correct format specifier to print error object in Go: %s or %v?

Question

Here is my program.

package main

import (
    "errors"
    "fmt"
)

func main() {
    a := -1
    err := assertPositive(a)
    fmt.Printf("error: %s; int: %d\n", err, a)
    fmt.Printf("error: %v; int: %d\n", err, a)
}

func assertPositive(a int) error {
    if a <= 0 {
        return errors.New("Assertion failure")
    }
    return nil
}

Here is the output.

error: Assertion failure; int: -1
error: Assertion failure; int: -1

In this program, it makes no difference whether I use %s or %v to print the error object.

I have two questions.

Is there any scenario while printing errors where it would make a difference for %s and %v?
What is the correct format specifier to use in this case?

Use %s (but please look up what %v and %s do in the package description of fmt). — Volker
– Volker, Commented May 10, 2017 at 9:06
@Volker I know what %v and %s does. I would like to understand why you recommend %s for error objects. Can you share the rationale please? — Lone Learner
– Lone Learner, Commented May 10, 2017 at 13:54
Less indirection. What gets printed is the output of the Error() method and that's a string: Clearer. — Volker
– Volker, Commented May 10, 2017 at 14:26

user1529891 · Accepted Answer · 2017-05-10 16:36:08Z

52

According to docs:

%v  the value in a default format
...
%s  the uninterpreted bytes of the string or slice

Also, more information about error:

The error type is an interface type. An error variable represents any value that can describe itself as a string.

So, treat it as %s.

answered May 10, 2017 at 16:36

user1529891

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4 Comments

zaemz Over a year ago

The only thing that I'd say is perhaps confusing about this is that the Golang team uses %v in their blogs: blog.golang.org/error-handling-and-go. Otherwise I think it's probably okay to just use %s as well.

zaemz Over a year ago

Also more discussion in this issue here: github.com/golang/go/issues/29934, which is somewhat relevant

Jeff Learman Over a year ago

Is it worth mentioning the special handling for format strings ending in ": %s" followed by an error? That's very non-intuitive (if it was retained in the language; I'm not sure.) See @NathanLutterman's citation.

m_ocean Over a year ago

@Nathan Lutterman %v properly formats <nil> errors. %s outputs something like this when the error is nil: %!s(<nil>)

Shreevari SP · Accepted Answer · 2023-08-08 11:52:23Z

-2

%w is what is the correct format specifier according to https://pkg.go.dev/fmt#Errorf

answered Aug 8, 2023 at 11:52

Shreevari SP

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1 Comment

m_ocean Over a year ago

It's the wrapping directive. You can't use it in fmt.Println or fmt.Printf. It servers a different purpose rather than simply formatting

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