Python, negative values aren't out of range in list

Question

class world:
    def __init__(self, screen_size):
        self.map = [[0 for col in range(500)] for row in range(500)]
        self.generate()

    def generate(self):
        for x in range(0, len(self.map[0])):
            for y in range(0, len(self.map)):
                kind = random.randint(0, 100)
                if kind <= 80:
                    self.map[x][y] = (random.randint(0, 255),random.randint(0, 255),random.randint(0, 255))
                else:
                    self.map[x][y] = (random.randint(0, 255),random.randint(0, 255),random.randint(0, 255))
        print self.map[50][50], self.map[-50][-50]
printing => (87, 92, 0) (31, 185, 156)

How is this possible that negative values aren't out of range? It should throw IndexError.

Answer 1

Using negative numbers starts at the back of the list and counts down, that's why they still work.

Answer 2

When you index into a list, the negative values mean N values from the end. So, -1 is the last item, -5 is the 5th from the end, etc. It's actually quite useful once you are used to it.

Answer 3

I think this can be best explained with a demonstration:

>>> a = [1, 2, 3, 4]
>>> a[-1]
4
>>> a[-2]
3
>>> a[-3]
2
>>> a[-4]
1
>>> # This blows up because there is no item that is
>>> # 5 positions from the end (counting backwards).
>>> a[-5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>>

As you can see, negative indexes step backwards through the list.

To explain further, you can read section 3.7 of this link which talks about negative indexing with lists.