In res.partner method we can returns the default price unity on run time and we can access the default price unity in Odoo server. For example you can see below code.

from openerp import api
from openerp.osv import osv, fields
from openerp.tools.translate import _

_logger = logging.getLogger('rent')

class Product(osv.osv):
    
    def check_rent_price(self, cr, uid, ids, context=None):
        """
        We check that the rent price is neither empty or 0 if the product can be rent.
        """
        products = self.browse(cr, uid, ids)

        for product in products:
            if product.can_be_rent:
                if not product.rent_price or product.rent_price <= 0:
                    return False
        return True
     
    def default_price_unity(self, cr, uid, ids,  context=None):
        """
        Returns the default price unity (the first in the list).
        """   
        unity1 = self.pool('product.uom').search(cr, uid, [])[16]
        print"===========unity1==========",unity1
        unity = self.pool('product.uom').browse(cr, uid, unity1)
        if not unity:
            _logger.warning("It seems that there isn't a reference unity in the 'Duration' UoM category. "
                            "Please check that the category exists, and there's a refernce unity.")
        if unity: 
            return unity.id
        else:
            return False
    
      
#     def action_view_stock_movess(self, cr, uid, ids, context=None):
#         result = {}
#         products = self._get_products(cr, uid, ids, context=context)
#         print"========products22========",products
#         result = self._get_act_window_dict(cr, uid, 'rent.act_product_stock_move_opens', context=context)
#         print"========result22========",result
#         if products:
#             result['context'] = "{'default_product_id': %d}" % products[0]
#             print"========result['context']===========",result['context']
#         result['domain'] = "[('product','in',[" + ','.join(map(str,ids)) + "])]"
#         return result



#     @api.multi
#     @api.depends('product_variant_ids.rent_count')
#     def _rent_count(self):
#         for product in self:
#             product.rent_count = sum([p.rent_count for p in product.product_variant_ids])
#      
 
    @api.multi
    def action_view_rent(self):
        self.ensure_one()
        action = self.env.ref('rent.action_product_rent_list')
        product_ids = self.product_variant_ids.ids
 
        return {
            'name': action.name,
            'help': action.help,
            'type': action.type,
            'view_type': action.view_type,
            'view_mode': action.view_mode,
            'target': action.target,
            'context': "{'search_default_product_id': " + ','.join(map(str, product_ids)) + ", 'default_product_id': " + str(product_ids[0]) + "}",
            'res_model': action.res_model,
            'domain': action.domain,
        }
    

    _name = 'product.template'
    _inherit = 'product.template'

    _columns = {
   #             'rent_count' :fields.function(_rent_count, ' Rent'),  
                'can_be_rent' : fields.boolean('Can be rented', help='Enable this if you want to rent this product.'),
                'rent_price' : fields.float('Rent price', help='The price is expressed for the duration unity defined in the company configuration.'),
                'rent_price_unity' : fields.many2one('product.uom',  'Rent Price Unity',help='Rent duration unity in which the price is defined.'),

    }

    _defaults = {
        'can_be_rent' : False,
        'rent_price' : 1.0,
        'rent_price_unity' : default_price_unity,

    }

    _constraints = [(check_rent_price, _('The Rent price must be a positive value.'), ['rent_price']),]

Product()