Python，负值不超出列表范围

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)

负值怎么可能不超出范围？它应该抛出 IndexError。

Answer 1

使用负数从列表后面开始倒计时，这就是它们仍然有效的原因。