Hello,
I know it it possible to use "-" to send the input file via stdin to prince.
This is done with following python code :

p = Popen(["prince "-","out.pdf], stdin=PIPE)
p.stdin.write(code.encode('utf-8')) # code is a string with HTML data
..

But i would like to stream my CSS-data also via stdin.
I've comme up with following python code :

p = Popen(["prince","-s","-","-","out.pdf"], stdin=PIPE)
p.stdin.write(stylesheet) # stylesheet is a string with CSS data
p.stdin.write(code.encode('utf-8')) # code is a string with HTML data

When i run this i get following error :
prince: loading style sheet: -
prince: debug: error loading resource: can't open input file: No such file or directory
prince: -: warning: can't open input file: No such file or directory

Must i conclude that the '-' argument only counts for a prince input file.
And cannot be used with a -s option.

Or i am doing something wrong ?

It's a bit tricky to send two inputs through the same pipe; how can the receiving program tell where one input ends and the next begins?

Latest builds of Prince have a new control interface which allows multiple resources to be sent over a single channel, but it is significantly more complex to use.

Perhaps you could just prepend the stylesheet at the beginning of the input document, wrapped in <style> tags? Then you could send everything in one stream.

Point taken.
My goal was to avoid using filesystems since all my HTML code is dynamically generated.
Initially my css info was a separate file and i linked to it from my html-code.
I have changed this.
I have now added a <style>-element in the main HTML-file that holds all the style data.
Tx for the tip