How to use golang template missingkey option?

Question

I would expect

t, err := template.New("t1").Option("missingkey=error").Parse("{{index . \"/foo/bar\"}}")        
err = t.Execute(os.Stdout, d)

to return an error if the map 'd' doesn't have a key '/foo/bar', but everything is just fine. Am I missing something?

Here's the go playground: http://play.golang.org/p/Oqg1Dy4h1k

Benjamin R · Accepted Answer · 2021-04-22 08:41:35Z

2

The missingkey option does not work with index. You will only get the desired result when you access the map using .<field-name>:

t, err := template.New("t1").Option("missingkey=error").Parse(`{{.foo}}`)
err = t.Execute(os.Stdout, d)

You can get around this by defining your own index function that returns an error when a key is missing:

package main

import (
    "errors"
    "fmt"
    "os"
    "text/template"
)

func lookup(m map[string]interface{}, key string) (interface{}, error) {
    val, ok := m[key]
    if !ok {
        return nil, errors.New("missing key " + key)
    }
    return val, nil
}

func main() {
    d := map[string]interface{}{
        "/foo/bar": 34,
    }
    fns := template.FuncMap{
        "lookup": lookup,
    }
    t, err := template.New("t1").Funcs(fns).Parse(`{{ lookup . "/foo/baz" }}`)
    if err != nil {
        fmt.Printf("ERROR 1: %q\n", err)
    }
    err = t.Execute(os.Stdout, d)
    if err != nil {
        fmt.Printf("ERROR 2: %q\n", err)
    }
}

https://play.golang.org/p/0_ZZ2Pwa1uZ

edited Apr 22, 2021 at 8:41

Benjamin R

6191 gold badge8 silver badges27 bronze badges

answered Feb 24, 2016 at 20:40

user142162

Sign up to request clarification or add additional context in comments.

2 Comments

user5976738 Over a year ago

Thanks. I edited my question to show why I'm using index (and you answered while I was in the middle of it). The key has a slash in it. This puts a wrinkle in my plans.

user142162 Over a year ago

@user5976738: see my updated answer for a workaround

kataras · Accepted Answer · 2019-12-25 22:18:11Z

Hello @user5976738 and all.

I have a different solution for that problem which does not require a template function at all and it uses the text/template/parse package.

This workaround may be very useful for some of you. Example of usage:

if missingKeys, ok := IsMissingKeys(err); ok{
   for _, key := range missingKeys{ println(key) }
}

Source Code:

package main

import (
    "fmt"
    "os"
    "strings"
    "strconv"
    "text/template"
    templateparse "text/template/parse"
)

type errMissingKeys struct {
    Keys         []string
    TemplateName string
}

func (err errMissingKeys) Error() string {
    return fmt.Sprintf("template: %s: map has no entry for keys: %s", err.TemplateName, strings.Join(err.Keys, ", "))
}

// IsMissingKeys reports whether an "err" is caused by template missing keys.
//
// Usage:
// if missingKeys, ok := IsMissingKeys(err); ok{
//     for _, key := range missingKeys{ println(key) }
// }
func IsMissingKeys(err error) ([]string, bool) {
    if err != nil {
        if v, ok := err.(errMissingKeys); ok {
            return v.Keys, true
        }
    }

    return nil, false
}

func main() {
    data := map[string]interface{}{
        "bar": 34,
    }

    tmpl, err := template.New("myTemplate").Option("missingkey=error").Parse(`{{.baz}} {{.other}}`)
    if err != nil {
        fmt.Println(err.Error())
        os.Exit(1)
    }

    err = tmpl.Execute(os.Stdout, data)
    if err != nil {
        if strings.Contains(err.Error(), "map has no entry for key ") {
            // Check if a field is not a "data" match,
            // which will lead on execute error on first undefined (missingkey=error).
            // So collect all these keys so we can have a typed error with all unknown keys listed.
            var missingKeys []string
            for _, n := range tmpl.Root.Nodes {
                if n.Type() == templateparse.NodeAction {
                    key := strings.TrimFunc(n.String(), func(r rune) bool {
                        return r == '{' || r == '}' || r == ' ' || r == '.'
                    })

                    if key == "" {
                        continue
                    }

                    if _, ok := data[key]; !ok {
                        missingKeys = append(missingKeys, strconv.Quote(key))
                    }
                }
            }

            // if just one then keep the original error which contains the full context,
            // else set to custom error type which will keep the missing keys for caller's further use.
            if len(missingKeys) > 1 {
                err = errMissingKeys{TemplateName: tmpl.Name(), Keys: missingKeys}
            }
        }
        fmt.Println(err.Error())
        os.Exit(1)
    }
}

Playground link: https://play.golang.org/p/JMwBpU2KyP2

Best Regards, Gerasimos Maropoulos - Creator of https://iris-go.com

Collectives™ on Stack Overflow

How to use golang template missingkey option?

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related