How does django AddFunc call context content

How does django AddFunc call context content

tmpl.AddFunc("activeClass", func(routeName string) string {
//get context url??
return routeName
})

Start with the tmpl.AddFunc example shown in the issue and identify the relevant Iris template documentation or entry point. Clarify what “context url” should refer to and define the documentation change needed; the issue names no file or test, so completion criteria are not established.

Written by the indexing model from the issue text.