How to build the placeholder during python interpolation

it would be easier if you would describe what you want to do exactly.

what you write is currently not possible. It would require recursive parsing of interpolated texts. There is a feature to add a one time snippet for on the spot expansion which could maybe do what you want. See help UltiSnips_Anon.

Given these global functions:

global !p
def comment(snip, text):
    return (snip.opt("&cms") % ' '+text+' ').strip()

def comment_start(snip):
    return snip.opt("&cms").split('%s')[0]+' '

def comment_end(snip):
    s = snip.opt("&cms").split('%s')
    if len(s) == 2: return ' '+s[1]
    return ''
endglobal

I would love to do this:

snippet modeline "Modeline (filetype)" b
`!p snip.rv = comment("vim:${1:ft="+snip.filetype+"}:")`
endsnippet

But this is the closest I could get:

snippet modeline "Modeline (filetype)" b
`!p snip.rv = comment_start(snip)`vim:${1:ft=`!v &ft`}:`!p snip.rv = comment_end(snip)`
endsnippet

The only problem is that the latter leaves a trailing space. Is there any way to avoid that?

I still cannot follow your problem. The code does precisely what you want programmed it to do.... What is the content of cms in your case. I assume python for my following example:
set cms=#%s

&cms.split = ['#', '']

comment_start() returns '# "
comment_end() returns ' '

I assume that you have a bug in your comment_end part. I replaced this via

def comment_end(snip):
    s = snip.opt("&cms").split('%s')
    if len(s) == 2 and s[1]: return ' '+s[1]
    return ''
endglobal

Try this and report back with further problems please.

This works. Thanks.