"The default value is only evaluated once" does not mean that a parameter with a default retains its value between invocations of the function. It means that the expression which you specify (the None part of def f(a, L=None)) is evaluated once, and the object it results in is stored in a hidden location and re-used if no value for that parameter is given at call. Parameters are still reset to the value (default or not) at every invocation.
