为什么在ndb中需要和默认是互斥的？ [英] Why required and default are mutally exclusive in ndb?

问题描述

在旧的谷歌appengine数据存储API必需和默认可以一起用于属性定义。使用ndb我得到一个

  ValueError：重复，必需和默认是互斥的。 
  

示例代码：

  from google.appengine.ext从google.appengine.ext导入ndb 
 import db 
 $ b $ class NdbCounter（ndb.Model）：
＃引发ValueError 
 count = ndb.IntegerProperty（required = True，default = 1）
 
 class DbCounter（db.Model）：
＃不会引发ValueError 
 count = db .IntegerProperty（required = True，default = 1）
  

我想实例化一个Counter而不必指定一个值。我也想避免有人把这个值覆盖到None。上面的例子被构造。我可能没有必要的属性生活，而是添加一个增量（）方法。我仍然没有看到为什么required和default是互斥的原因。

它是一个错误还是一个特性？

解决方案

我认为你是对的。也许我在写代码的那部分时感到困惑。 required = True的意思是不允许写入None，所以应该可以将其与默认值结合使用。请在NDB跟踪器中提交功能请求： http://code.google.com/p/ appengine-ndb-experiment / issues / list

注意，对于重复的属性，事情比较复杂，重复的可能会继续与要求的或者不相容的即使实现了上述功能。

In old google appengine datastore API "required" and "default" could be used together for property definitions. Using ndb I get a

ValueError: repeated, required and default are mutally exclusive.

Sample code:

from google.appengine.ext import ndb
from google.appengine.ext import db

class NdbCounter(ndb.Model):
    # raises ValueError
    count = ndb.IntegerProperty(required=True, default=1)

class DbCounter(db.Model):
    # Doesn't raise ValueError
    count = db.IntegerProperty(required=True, default=1)

I want to instantiate a Counter without having to specify a value. I also want to avoid someone to override that value to None. The example above is constructed. I could probably live without a required attribute and instead add an increment() method. Still I don't see the reason why required and default are mutually exclusive.

Is it a bug or a feature?

解决方案

I think you are right. Perhaps I was confused when I write that part of the code. It makes sense that "required=True" means "do not allow writing the value None" so it should be possible to combine this with a default value. Please file a feature request in the NDB tracker: http://code.google.com/p/appengine-ndb-experiment/issues/list

Note that for repeated properties things are more complicated, to repeated will probably continue to be incompatible with either required or default, even if the above feature is implemented.

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