You can even do weird things with host blocks:
##{py##
def block_init(data,value,**kw):
    data['variable']='VARIABLE'
    data['list']=[1,2,3]
    data['dictvar']={'subdict':{'item':'NESTED ITEM'}}
####
##{js##
function block_init(data) {
    data['variable']='VARIABLE';
    data['list']=[1,2,3];
    data['dictvar']={'subdict':{'item':'NESTED ITEM'}};
}
####
##>init##
Substition:   ##variable##
If block:     ##@variable##
                 only show when true
              ####
If..else:     ##@variable##
                 only show when true
              ##*##
                 only show when false
              ####
If not block: ##!variable##
                 only show when false
              ####
If not else:  ##!variable##
                 only show when false
              ##*##
                 only show when true
              ####
Choice:       ##one?variable##
                 only show when variable is "one"
              ##two?## - note, no variable specified
                 only show when variable is "two"
              ##*##
                 show when variable is something else
              ####
Blocks:       ##blockvar=blockname##
                 This block is called with ##blockvar##
              ####
Block call:   ##variable>blockname##
For..in:      ##item:list##
                 ##item##
              ####
Nested data:  ##dictvar.subdict.item##
Literal #:    ###hexcolor## is not what you want (it's beard, then #hexcolor, then beard).
              ##\####variable## will produce something like #ffffff.
Host language blocks:
##{py##
def block_custom(data,value,output,getvalue,compare):
    """
        data is a dictionary with template variables
        value is a value of variable passed with block call
        output(x) converts x into string and writes it into template
        getvalue(x) will get value of x template variable
        compare(x,y) is cross-language comparison function
    """
    output(len(data))
####
Roughly the same in JavaScript:
##{js##
function block_custom(data,value,output,getvalue,compare) {
    // the very same as above
    var n=0;
    for(var i in data) n++;
    output(n);
}
####
And now call it as ordinary block: ##variable>custom##
Yes, that's all in a single template file