Er, for open files Geany should detect the argument lists of functions and also show them when calltips are requested (by typing a '(' after the function name or by pressing Ctrl-Shift-Space). At least with the SVN version this should work, not sure about the latest release.
God dammit, indeed it is!! And it tells the class, in which it was found! But I haven't found Ctrl+Shift+Space command in manual. And I didn't know, that I could use it, since I am so much used to Ctrl+Space!
Ok, just one more thing and you are officially the best tool for Python development ever: how can toggle those tags? I mean, there are arrows and I can click and they are toggled. But I need to have it from the keyboard, because moving hand from the keyboard to the mouse just won't work.