How to force a list to a fixed size?

Question

In Python 3, I want to create a list that will contain the last 5 variables entered into it.

Here is an example:

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

So, in python, is there any way to achieve what is demonstrated above? The variable does not need to be a list, I just used it as an example.

Answer 1

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) deque(['orange','grape','banana','mango','kiwi'], maxlen=5) #only 5 items in list 

Answer 2

I ran into this same issue... maxlen=5 from deque was NOT a supported option due to access speed / reliability issues.

SIMPLE Solution:

l = [] l.append(x)                         # add 'x' to right side of list l = l[-5:]                          # maxlen=5 

After you append, just redefine 'l' as the most recent five elements of 'l'.

print(l) 

Call it Done.

For your purposes you could stop right there... but I needed a popleft(). Whereas pop() removes an item from the right where it was just appended... pop(0) removes it from the left:

if len(l) == 5:                     # if the length of list 'l' has reached 5      right_in_left_out = l.pop(0)    # l.popleft() else:                               #     right_in_left_out = None        # return 'None' if not fully populated 

Hat tip to James at Tradewave.net

No need for class functions or deque.

Further... to append left and pop right:

l = [] l.insert(0, x)                      # l.appendleft(x) l = l[-5:]                          # maxlen=5 

Would be your appendleft() equivalent should you want to front load your list without using deque

Finally, if you choose to append from the left...

if len(l) == 5:                     # if the length of list 'l' has reached 5      left_in_right_out = l.pop()     # pop() from right side else:                               #     left_in_right_out = None        # return 'None' if not fully populated