How to force a list to a fixed size?

You might want to use a collections.deque object with the maxlen constructor argument instead:

>>> l = collections.deque(maxlen=5)
>>> l.append('apple')
>>> l.append('orange')
>>> l.append('grape')
>>> l.append('banana')
>>> l.append('mango')
>>> print(l)
deque(['apple', 'orange', 'grape', 'banana', 'mango'], maxlen=5)
>>> l.append('kiwi')
>>> print(l)  # only 5 items in list
deque(['orange', 'grape', 'banana', 'mango', 'kiwi'], maxlen=5)