This is called the mutable default argument bug that is commonly made by people new to Python. When you give a mutable as a default argument, the same object gets used across instances when the default argument is required to be used. The get a better understand check the Important warning section in http://docs.python.org/tutorial/controlflow.html#default-argument-values
In your code, the instance a used the mutable default argument (a empty list object) in it's init call and when you created the instance of b, which in turn called Base's init method, again used the very same object that a used in it's init. On simpler words a.l and b.l point to the same list object.
A very similar discussion - "Least Astonishment" and the Mutable Default Argument
