I want to understand the purchase.py's program(openerp\addons\purchase) meaning

e.g......purchase.py contain as follow:

class purchase_order(osv.osv):

def _amount_all(self, cr, uid, ids, field_name, arg, context=None):
    res = {}
    cur_obj=self.pool.get('res.currency')
    for order in self.browse(cr, uid, ids, context=context):
        res[order.id] = {
            'amount_untaxed': 0.0,
            'amount_tax': 0.0,
            'amount_total': 0.0,
        }............................

what's this ? I need some data let me know more about the system ?