Python list.remove() skips next element in list

Question

Python, but not programming, newbie here. I'm programming with lists and have run into an interesting problem.

width = 2
height = 2

# Traverse the board
def traverse(x, y):
    # The four possible directions
    squares = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
    print squares

    # Remove impossible squares
    for square in squares:
        print "now accessing", square
        if (square[0] < 1 or square[0] > width or 
            square[1] < 1 or square[1] > height or
            square == (1, height)):
            squares.remove(square)
            print "removed", square
    print(squares)

# Testing traverse
traverse(1,1)

This results in the following output:

[(0, 1), (2, 1), (1, 0), (1, 2)]
now accessing (0, 1)
removed (0, 1)
now accessing (1, 0)
removed (1, 0)
[(2, 1), (1, 2)]

It completely skips the elements (2,1) and (1,2) -- without even checking them! I found answers here saying I shouldn't be modifying a list while traversing it, and yup, that definitely makes sense. Rookie mistake. But can someone tell me WHY it doesn't work? What's behind the veil of Python lists?

Answer 1

Others have explained that you should not remove elements from an array you are iterating over; however, if you traverse the array backwards, there is no problem.

The most compact way to solve this problem (assuming you don't need a copy of the original array after you are done) is to use the reversed() function, as in

for square in reversed(squares)

This will start iterating from the end of the array, and work backwards. Taking out elements in this way will not affect the rest of the code, since you are not changing the order of elements you have not yet visited. I think this is the most elegant way to solve this. I learnt this trick here