Now I have to add something to the current behaviour. After the call to sub_func1 I have to modify the file by moving one line to another position in the file. And then sub_func2 is called and everything goes on as before. I think that this kills solution 1.

Hmmm, so what magical properties does opening a file have that allows you to move a single line to another position, that you don't seem to be able to do with an open file handle?