Using OR in List Comprehension

Question

For the life of me I can't figure out why this won't work. I'm trying to get the sum of a list however if the list contains the value 13 that value should be excluded form the sum and so should the following value. For example [1,13,3] would be 1 and [13,2,5] would be 5.

I have been trying this list comprehension:

lst = [3,3,13]
sum([lst[i] for i in range(len(lst)) if not lst[i] == 13 or not lst[i-1] == 13])

The OR seems to not be working like I expect. If I remove either part of the OR statement it works exactly as it should. Example

>>> lst = [3,3,13]
>>> [lst[i] for i in range(0,3) if not lst[i] == 13]
[3, 3]
>>> [lst[i] for i in range(len(lst)) if not lst[i] == 13 or not lst[i-1] == 13]
[3, 3, 13]

Answer 1

You have two errors in your code.

1. If the last element is 13, then the first element with not be added because lst[0-1] == 13 because of Python's index wrap around.
2. As stated in the comments and in the other answer, you should be using and, instead of or. This is because of De Morgan's laws.

Then, the resulting code is

sum([lst[i] for i in range(len(lst)) if not lst[i] == 13 and not(i > 0 and lst[i-1] == 13)])