def update(self, table, kwargs, condition=None):
        """.. :py:method:: update

            All module update can user this function.
            condition only support string and dictionary.

        Usage::

            >>> update('dept', {'name': 'design', 'quantity': 3}, {'id': 'we4d'})
            update dept set name='design', quantity=3 where id='we4d';

            >>> update('dept', {'name': 'design', 'quantity': 3}, 'introduction is null')
            update dept set name='design', quantity=3 where introduction is null;

            >>> update('physician', {'$inc': {'status': -10}, 'present': 0}, {'id': 'someid'})
            update physician set status=status+-10, present=0 where id='someid';

        """
        sql = "update {} set {}"
        equations = []
        values = []
        for k, v in kwargs.iteritems():
            if k == '$inc' and isinstance(v, dict):
                for ik, iv in v.iteritems():
                    equations.append("{field}={field}+{value}".format(field=ik, value=iv))
            else:
                equations.append("{}=%s".format(k))
                values.append(v)

        sql = sql.format(table, ', '.join(equations))
        sql += self.parse_condition(condition) + ";"
        super(PGWrapper, self).execute(sql, values, result=False)