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

Seems to behave differently to
def f(a, L=None):
    L = []
    L.append(a)
    return L
Which behaves the same as (as far as I noticed) to the below code in the documentation (In the tutorial under 4. More Control Flow Tools)
def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

I am using CPython 3.5.1, what is the point of "if L is None:" in the lowermost above example? And why is None treated differently to []?

This is not a bug, see https://docs.python.org/3.6/faq/programming.html#why-are-default-values-shared-between-objects

In the first case L is evaluated once at compile time.
In the second case L is always set to a new empty list, regardless of what you pass as second argument to f.
In the third case L is set to a new empty list only if you don't pass a second argument (or if you pass None).