retain values between fun calls

[Gary Wessle]

Hi

the second argument in the functions below suppose to retain its value
between function calls, the first does, the second does not and I
would like to know why it doesn't? and how to make it so it does?

thanks

# it does
def f(a, L=[]):
    L.append(a)
    return L
print f('a')
print f('b')


# it does not
def f(a, b=1):
    b = a + b
    return b
print f(1)
print f(2)