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

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.

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