# 原始列表
nums = [5, 12, 7, 9, 21, 7, 14]
# 统计数字7出现的次数
count_seven = nums.count(7)          
print("数字7出现的次数：",  count_seven          )
# 判断99是否存在，不存在则添加
if 99 not in nums:
    nums.append(99)           
print("添加99后的列表：", nums             )
# 删除第一次出现的数字7
nums. remove             (7)   # 只删除第一个7
print("删除第一个7后的列表：", nums)
# 取出第2个到第4个元素（索引1到3）
sub_list = nums[1:4]        
print("第2~4个元素组成的列表：",   sub_list             )

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