Python 在Google应用程序引擎中使用自定义属性

我正在使用google应用程序引擎/python。我已经创建了一个自定义数据类型和属性。但我在使用上有问题。这是我的定义

from datetime import datetime

class MyDateTime(datetime):
    def __str__(self):
        return self.strftime("%Y-%m-%d %H:%M")

class MyDateTimeProperty(db.Property):
    # specify datatype
    data_type = MyDateTime

    # For writing to datastore.
    def get_value_for_datastore(self, model_instance):
        return super(MyDateTimeProperty, self).get_value_for_datastore(model_instance)

    # For reading from datastore.
    def make_value_from_datastore(self, value):
        return super(MyDateTimeProperty, self).validate(value)

class MyClass(db.Model):
    date = MyDateTimeProperty()

我就是这样使用它的：

obj = MyClass(date=MyDateTime(2011,1,1))
obj.put()

我收到了以下错误消息：

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\admin\__init__.py", line 295, in post
    exec(compiled_code, globals())
  File "<string>", line 23, in <module>
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1052, in put
    self._populate_internal_entity()
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1020, in _populate_internal_entity
    self._entity = self._populate_entity(_entity_class=_entity_class)
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1092, in _populate_entity
    self._to_entity(entity)
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 1005, in _to_entity
    self.__set_property(entity, prop.name, prop.get_value_for_datastore(self))
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\db\__init__.py", line 995, in __set_property
    entity[name] = datastore_value
  File "C:\Program Files\Google\google_appengine\google\appengine\api\datastore.py", line 881, in __setitem__
    datastore_types.ValidateProperty(name, value)
  File "C:\Program Files\Google\google_appengine\google\appengine\api\datastore_types.py", line 1477, in ValidateProperty
    'Unsupported type for property %s: %s' % (name, v.__class__))
BadValueError: Unsupported type for property date: <class '__main__.MyDateTime'>

---

关键是，使用

get\u value\u for datastore

将不可存储的MyDateTime对象更改为可存储的datetime对象，并

make\u value\u from\u datastore

反之亦然。

您的
属性
子类需要定义一个
validate（）
方法，如果其数据类型不是默认验证器知道的已定义数据存储数据类型之一。在./p>中有一个示例验证器方法。如果这是实例化代码而不是示例性的，则可以考虑在模板过滤器中处理日期格式而不是自定义属性。我同意Drew的观点，这是为了在代码< STR < /代码>中获得自定义行为而跳过很多箍。
class MyDateTimeProperty(db.Property):
    # specify datatype
    data_type = MyDateTime

    # For writing to datastore.
    def get_value_for_datastore(self, model_instance):
        date = super(MyDateTimeProperty, self).get_value_for_datastore(model_instance)
        return datetime(date.year, date.month, date.day, date.hour, date.minute)

    # For reading from datastore.
    def make_value_from_datastore(self, value):
        if value is None:
            return None
        return MyDateTime(value.year, value.month, value.day, value.hour, value.minute)