python / scrapy default initialization of items

 def sp_offer_page(self, response):
            item = MallCrawlerItem()

If MallCrawlterItem is your class, it might make more sense to have it set everything to defaults.

            for key, value in self.get_default_item_dict().iteritems():
                item[key] = value

MallCrawlerItem appears to be dict-like. dict's have a method update, which can replace this loop. If MallCrawlerItem also has this method, you can use it.

            item['is_coupon'] = 1
            item['mall'] = 'abc'

Why not part of the defaults?

            # some other logic 
            yield item

This doesn't make sense here. I guess this function has been abbreviated.

    def get_default_item_dict(self):     # populate item with Default Values
        return {'mall': 'null', 'store': 'null', 'bonus': 'null',
                'per_action': 0, 'more_than': 0,
                'up_to': 0, 'deal_url': 'null', 'category': 'null', 'expiration': 'null', 'discount': 'null',
                'is_coupon': 0, 'code': 'null'}

I'd put this dictionary in a global constant, not put it inline with your code. I'd also put a line after all the commas. I think that'll be easier to read.

Item has default values. Why don't you use them?