Odd scoping in Python

scope.py

# I don't understand the output (shown below).
# L is local to f, right?  So how are the
# contents of L remembered between invocations?

def f(a, L=[]):
    L.append(a)
    return L

print(f(1))
print(f(2))
print(f(3))

# Output:
# [1]
# [1, 2]
# [1, 2, 3]