Default Value

[Ahmed Abdulshafy]

I'm reading the Python.org tutorial right now, and I found this part rather strange and incomprehensible to me>

Important warning: The default value is evaluated only once. This makes a difference when the default is a mutable object such as a list, dictionary, or instances of most classes
def f(a, L=[]):
    L.append(a)
    return L

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

This will print
[1]
[1, 2]
[1, 2, 3]

How the list is retained between successive calls? And why?