[Python] 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.

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