text / template.Templates和html / template.Templates之间的区别

Question

Recently, I noticed that Templates() of html/template.Template works differently from the one of text/template.Template.

// go1.12
func main() {
    t := template.New( "" )
    println( len( t.Templates() ) )
}

The result of this code depends on whether you imported text/template or html/template. You'll notice that the text one prints 0 while the other prints 1. Becuase of this, I looked into the GoDoc and the html one's documentation said that Templates() includes itself -- but no further explanation. And I thought there must be some reason why; why does it have to be different from each other?

Answer 1

Templates returned by text/template.New() and html/template.New() are both incomplete templates without a "body", they cannot yet be used to generate any output. You can verify this if you try to execute them:

t := ttemplate.New("t")
h := htemplate.New("h")
fmt.Println(t.Execute(os.Stdout, nil))
fmt.Println(h.Execute(os.Stdout, nil))

Outputs (try it on the Go Playground):

template: t: "t" is an incomplete or empty template
template: "h" is an incomplete or empty template

Returning incomplete templates in the associated templates has no significance, and is an implementation detail. One package chose to include it, the other chose not to.

Note that if you "complete" the template definitions by actually parsing anything, both will include and return the self template in the associated templates, there is no difference in them:

t := ttemplate.Must(ttemplate.New("t").Parse("t"))
h := htemplate.Must(htemplate.New("h").Parse("h"))
fmt.Println(len(t.Templates()), len(h.Templates()))
fmt.Println(t.Execute(os.Stdout, nil))
fmt.Println(h.Execute(os.Stdout, nil))

This will output (try it on the Go Playground):

1 1
t<nil>
h<nil>