First a little about the "template" concept:
A template.Template
value is "the representation of a parsed template". But the wording here is a little "imperfect". A template.Template
value may be (and usually is) a collection of multiple, associated templates. template.Template
has an unexported field:
tmpl map[string]*Template // Map from name to defined templates.
This tmpl
field holds all other associated templates, templates that are visible to the template, and which can be referred to by their names.
You can read more about this in this answer: Go template name
Back to the Template.ParseFiles()
method. This method parses multiple templates from the files passed to it as parameters. The templates parsed form the files will be named after the file names (without folders, just the file name), and they will be added to the internal, associated templates map of the t
template designated by the method receiver.
The parsed templates will not be appended. Multiple, separate template.Template
values will be created for them, but they will be associated (so they can refer to each other, e.g. they can include each other).
Let's see an example. Let's assume we have these 2 template files:
a.html
is:
I'm a.
And b.html
:
I'm b.
And an example code:
t := template.New("a.html")
if _, err := t.ParseFiles("a.html", "b.html"); err != nil {
panic(err)
}
if err := t.Execute(os.Stdout, nil); err != nil {
panic(err)
}
This example creates a new, empty template named a.html
, then parses 2 files: a.html
and b.html
.
What will be the result? t
will denote the a.html
template, because we created it prior with that specific name. Running the code, the output will be:
I'm a.
Now if we change the first line to:
t := template.New("x.html")
And leave the rest unchanged, running it we see something similar:
panic: template: "x.html" is an incomplete or empty template
The reason is that t
denotes a template named x.html
but it's empty, as we didn't parse anything "into" it, and the parsed files also didn't match the name x.html
. So attempting to execute it results in an error.
Now if we try to execute one of its associated, named template:
if err := t.ExecuteTemplate(os.Stdout, "a.html", nil); err != nil {
panic(err)
}
It succeeds, and again gives:
I'm a.