__init__.py

import colander
from datetime import datetime, date
from decimal import Decimal

def obj2json(values):
        for key, val in values.items():
            if isinstance(val, datetime):
                values[key] = val.strftime('%Y-%m-%d %H:%M:%S')
            elif isinstance(val, date):
                values[key] = val.strftime('%Y-%m-%d')
            elif isinstance(val, Decimal):
                values[key] = float(val)
            elif isinstance(val, colander._null):
                values[key] = ""
        return values