Capture or assign golang template output to variable

Question

Within a template, how can I achieve this?

{{$var := template "my-template"}}

I just get "unexpected <template> in operand".

Cant

Answer 1

There is no "builtin" action for getting the result of a template execution, but you may do it by registering a function which does that.

You can register functions with the Template.Funcs() function, you may execute a named template with Template.ExecuteTemplate() and you may use a bytes.Buffer as the target (direct template execution result into a buffer).

Here is a complete example:

var t *template.Template

func execTempl(name string) (string, error) {
    buf := &bytes.Buffer{}
    err := t.ExecuteTemplate(buf, name, nil)
    return buf.String(), err
}

func main() {
    t = template.Must(template.New("").Funcs(template.FuncMap{
        "execTempl": execTempl,
    }).Parse(tmpl))
    if err := t.Execute(os.Stdout, nil); err != nil {
        panic(err)
    }
}

const tmpl = `{{define "my-template"}}my-template content{{end}}
See result:
{{$var := execTempl "my-template"}}
{{$var}}
`

Output (try it on the Go Playground):

See result:

my-template content

The "my-template" template is executed by the registered function execTempl(), and the result is returned as a string, which is stored in the $var template variable, which then is simply added to the output, but you may use it to pass to other functions if you want to.

Kesselring

Thanks, the problem is that I don't have access to the go code that is processing the template: (i'm using consul-template) – Cant

How can I know the buffer is big enough? – Grazynagreabe

@NikolaiEhrhardt bytes.Buffer increases its capacity on demand, it does not run out of space. – Kesselring

Yes I assumed that, but did not study its write-method ... – Grazynagreabe