instance attribute "a" defined outside __init__

[Rio]

Hi, When running below code, I get error saying: 

instance attribute "a" defined outside __init__

    class Foo:

        var = 9

        def add(self, a, b):
            self.a = a
            self.b = b
            print a+b

        def __init__(self):
            print 10

    bar = Foo()  # create object

    bar.add(4, 7)  # call method by object.method

    print bar.var  # access class variable

why the output prints:

10
11
9

why not serially?

9
11
10