call by objects problem

[landlord1984]

def f(x,l=[]):
    for i in range(x):
        l.append(i*i)
    print(l) 

f(2)
f(3,[3,2,1])
f(3)

Output:

[0, 1]
[3, 2, 1, 0, 1, 4]
[0, 1, 0, 1, 4]

I can understand the first two implementation of f(), but not the third. Can someone help explain why the third output starting with 0,1, which is the memory left by the first f() result?

[buran]

read this
http://docs.python-guide.org/en/latest/writing/gotchas/

you should NOT use mutable default arguments