nums = [i for i in range(1,21)]
for num in nums:
nums.remove(num)
应该是nums在循环中元素一直在减少，当删除nums的第1个元素后，其后续元素都前移一位，再删除第2个元素时实际删除的原第3个元素，
以此类推，只删除了单数。
[_1_, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 10, 12, 14, 15, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 10, 12, 14, 16, 17, 18, 19, 20]
[2, 4, 6, 8, 10, 12, 14, 16, 18, 19, 20]
斜体数字是每次删除的数字
每次删除的时候列表的元素索引往前去了一位