Where I can find app which takes screenshots? For N900.
The app should save the screenshots to automatically
named files (e.g. date-time or numbered).
It would be a good idea to assign the screenshot
action to the half-press of the camera button.

If this app can be coded as Python script, how
should I start coding?

Check Irfanviews capture feature for what I want.
No "save as" popups. Running at background.