缺少的是保存您的 Usernames 实例的集合。对于这种特殊情况，您可能需要一个字典。

The missing piece is a collection to hold your Usernames instances. For this particular scenario, you probably want a dictionary.

>>> myDict = {}
>>> myDict['foo'] = 5
>>> 'foo' in myDict
True
>>> myDict['foo']
5
>>> myDict.get('bar', 'nope')
'nope'
>>>