python: class attributes and instance attributes

Question

I\'m new to python and learned that class attributes are like static data members in C++. However, I got confused after trying the following code:

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Answer 1

>>> class Foo:
...     a=1
... 
>>> f1=Foo()
>>> f2=Foo()
>>> f1.a    # no instance attribute here in f1, so look up class attribute in Foo
1
>>> f1.a=5  # create new instance attribute in f1
>>> f1.a    # instance attribute here. Great, let's use this.
5
>>> f2.a    # no instance attribute in f2, look up class attribute in Foo
1
>>>
>>> class Foo:
...     a=[]
... 
>>> f1=Foo()
>>> f2=Foo()
>>> f1.a            # no instance attribute - look up class attribute in Foo
[]
>>> f1.a.append(5)  # no instance attribute, modify class attribute in-place
>>> f1.a            # no instance attribute - look up class attribute in Foo
[5]
>>> f2.a            # same here
[5]