python - if语句

1. 示例

cars = ['audi', 'bmw', 'subaru', 'toyota']

for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())

2. 条件测试

布尔值：True和False

# 检查相等或不等
==  # 变量相等 
!=  # 变量不等
<  <=  ==  >=  >  # 数值比较

# 多个条件
and 
or

# 检查特定值是否在列表中
'audi' in cars
# 检查特定值是否不在列表中
if 'audi' not in cars:
    print('not in')

3. if语句

3.1 简单if语句

age = 19
if age >= 18:
    print('you area old enough to vote!')

3.2 if-else语句

age = 13
if age >= 18:
    print('you area old enough to vote!')
else:
    print('sorry! you area too young to vote.')

3.3 if-elif-else语句

age = 12
if age < 4:
    print('Your admission cost is $0.')
elif age < 18:
    print('Your admission cost is $25.')
else:
    print('Your admission cost is $40.')

age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
else:
    price = 40

print(f'Your admission cost is ${price}.')

3.4 多个elif

age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
elif age < 65:
    price = 40
else:
    price = 20

print(f'Your admission cost is ${price}.')

3.5 省略else代码块

age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
elif age < 65:
    price = 40
elif age >= 65:
    price = 20

print(f'Your admission cost is ${price}.')

3.6 测试多个条件

if-elif-else最多只会执行其中某一个条件，若要执行多个条件下语句，需要多个if语句。

requested_toppings = ['mushrooms', 'extra cheese']

if 'mushrooms' in requested_toppings:
    print("Adding mushrooms.")
if 'pepperoni' in requested_toppings:
    print("Adding pepperoni.")
if 'extra cheese' in requested_toppings:
    print("Adding extra cheese.")
    
print("\nFinished making your pizza!")

4. 使用if处理列表

4.1 检查特殊元素

requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']

for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':   # 青椒已经用完了
        print("Sorry, we are out of green peppers right now.")
    else:
        print(f"Adding {requested_topping}.")

print("\nFinished making your pizza.")

4.2 确定列表不是空的

requested_toppings = []

if requested_toppings:           # if语句用列表名用作条件表达式时，列表为空则False，列表非空则True
    for requested_topping in requested_toppings:
        print(f"Adding {requested_topping}.")
else:
    print("Are you sure you want a plain pizza?")

4.3 使用多个列表

available_toppings = ['mushrooms', 'olives', 'green peppers', 'pepperoni', 'pineapple', 'extra cheese']

requested_toppings = ['mushrooms', 'french fries', 'extra cheese']

for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print(f"Adding {requested_topping}.")
    else:
        print(f"Sorry, we don't have {requested_topping}.")

print("\nFinished making your pizza.")

5. if语句格式

在==、>=和<=等关系运算符两边各添加一个空格。