Python编程从入门到实践练习第五章：if语句和条件测试

一、条件测试

1.1 检测多个条件（and / or）

所用关键词

1. and : 两个条件都为true
2. or ：至少满足一个条件

实例

age_o=20
age_1=22
print(age_o>=21 and age_1<=22,age_o>=21 or age_1<=22)

1.2 检测特定值是否包含在列表中

所用关键词

1. 判断特定值在列别中：in
2. 判断特定值不在列表中：not in

实例

color = ['green','red','blue','yellow','pink']
if 'pink' in color:
    print('yes!')
if 'dark' not in color:
    print('sorry!')

1.3 if语句结构

1. 单if语句
2. if-else语句
3. if-elif-else语句

二、if语句处理列表

2.1 判断列表是否为空

三种方法判断：

• 直接使用条件表达式if + list，如果list不为空，if list表达式返回true，否则返回false
• 检测列表是否==【】
• 按列别长度进行判断

list_1 = []
if list_1:
    print('This is not a empty list!')
else:
    print('This is a empty list!')

if list_1==[]:
    print('This is a empty list!')
else:
    print('This is not a empty list!')

if len(list_1)==0:
    print('This is a empty list!')
else:
    print('This is not a empty list!'

2.2 练习题

代码

print('5-8')
usernames = ['root','admin','jack','peter','diane']
for username in usernames:
    if username == 'admin':
        print('Hello admin, would you like to see a status report?')
    else:
        print('Hello Jaden, thank you for logging in again.')

print('5-9')
usernames.clear()
if usernames:
    for username in usernames:
        if username == 'admin':
            print('Hello admin, would you like to see a status report?')
        else:
            print('Hello Jaden, thank you for logging in again.')
else:
    print('We need to find some users!')

print('5-10')
current_users = ['jack','peter','will','admin','diane']
new_users = ['mary','peter','will','lili','frank']
for new_user in new_users:
    for current_user in current_users:
        if new_user == current_user:
            print(f'Dear {
      
      new_user.title()}, you need to change a username!')
            break;
    else:
        print(f'Dear {
      
      new_user.title()}, this username is ready to use!')
else:
    print('All the usernames are finished checked!')

print('5-11')
numbers = list(range(1,10))
for number in numbers:
    if number==1:
        print('first')
    elif number == 2:
        print('second')
    elif number==3:
        print('third')
    else:
        print(f'{
      
      number}th')

输出

注：清空列表的方法

1. 使用clear方法
2. 直接让列表==[]
3. 使用del语句：del [:]，不指定起始和结束索引，把所有元素全部删除
   清空列表