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An instance of the Python __dict__ class does not contain all instance variables. What for?

Here is a little new Python question about instance variables.

Consider the following Python 2.7 class definition:

class Foo(object):
    a = 1

    def __init__(self):
            self.b = 2

    def __repr__(self):
            return "%s" % self.__dict__

Now, when I create an instance Foo, it Foo.__dict__contains b, but not a.

>>> x=Foo()
>>> x
{'b': 2}
>>> dir(x)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', 
 '__getattribute__', '__hash__', '__init__', '__module__', '__new__', 
 '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',
 '__str__', '__subclasshook__', '__weakref__', 'a', 'b']
>>> x.__dict__
{'b': 2}

And here I thought that I understand the Python Path very well.

What is the difference between x.aand x.b? As far as I can tell, they are both instance variables.

: , Python , Foo.a , . Hm... , , x.a, x - , - Foo.a:

>>> y=Foo()
>>> y.a = 2
>>> y
{'a': 2, 'b': 2}
>>> x
{'b': 2}
>>> x.a
1
>>> z=Foo()
>>> z
{'b': 2}
>>> z.a
1
>>> Foo.a
1
>>> x.a
1
>>> y.a
2

, Foo.a Foo, Foo.a:

>>> Foo.a=999
>>> x.a
999
>>> y.a
2
+3
3

a . .

>>> class Foo(object):
...    a = 1
...
>>> Foo.a
1

, __init__, .

+6

a , .

+3

Could this help you?

>>> class X(object):

    def __getattribute__(self, name):
        print name
        return object.__getattribute__(self, name)


>>> l = dir(X())
__dict__
__members__
__methods__
__class__
>>> l
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
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