Subclassing standard class: how to write my own __init__?

Dear All, I am trying to create a class that would extend functionality of datetime.date by implementing some functions I need, for example an optional initialisation by (year, day_of_year) instead of (year, month, day). I would like the class constructor to behave in the datetime's default way if

Sea-or-land test

Could you advise if there exists a simple package, containing function that would take geographic coordinates and return information on whether this point belongs to land or sea? No great accuracy is needed -- I am interested in just major features, like those seen on coarse world maps. -- http:/