I think this is happening because a list is a mutable object while the value None is immutable.
For the first function, variable L is outside the function environment (in the function definition), and it refers to an empty list. Then you make changes to this list in the function environment, but since a list is mutable, the variable L that is outside the function environment refers to this now mutated list, and the change propagates each time you call the function.
For the second function, variable L is also outside the function environment (in the function definition), but this time it refers to None, which is immutable. Now, every change you make in the function environment will not affect what L refers to outside the function environment. The variable L inside the function environment refers to something different as you change it. First, it refers to an empty list, and then a list that gets a value appended to it. You then return this list. The next time you call the function, you call it with the variable L that is outside the function environment, which has not changed and still refers to None.
Hope this makes sense.
