Question

I have a question regarding the subclassing behaviour in Python 2.7.

If I subclass from the built-in dict type, it seems that __ dict __ is always empty.
Where does Python save the key/value pairs?

>>> class Foobar(dict):
...     pass
... 
>>> foobar = Foobar()
>>> foobar.__dict__
{}
>>> foobar['test'] = 1
>>> foobar.__dict__
{}
>>> 

Answer 1

A partial answer is that you’re misunderstanding the purpose of __dict__. __dict__ is used to store attributes, not items, and it’s present in most user-defined objects. Indeed, if you subclass dict in the appropriate way, __dict__ will have values in it.

>>> class Foo(dict):
...     def __init__(self, *args, **kwargs):
...         super(Foo, self).__init__(*args, **kwargs)
...         self.banana = 'banana'
...         self['banana'] = 'not really a banana'
... 
>>> f = Foo()
>>> f.__dict__
{'banana': 'banana'}
>>> f.banana
'banana'
>>> f['banana']
'not really a banana'