When you pass list to the function in python, unexpected output is seen

Question

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


A(3)
A(2, [45, 56, 12])
A(2)

Why does this code return the following output :

[0, 1, 4]
[45, 56, 12, 0, 1]
[0, 1, 4, 0, 1]