if

python

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

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

要检查是否两个条件都为True，可使用关键字and将两个条件测试合而为一。

关键字or也能够让你检查多个条件，但只要至少一个条件满足，就能通过整个测试。仅当两个测试都没有通过时，使用or的表达式才为False。

要判断特定的值是否已包含在列表中，可使用关键字in。

python

requested_toppings = ['mushrooms','onions','pineapple']

# True
'mushrooms'in requested_toppings

# False
'pepperoni'in requested_toppings

还有些时候，确定特定的值未包含在列表中很重要。在这种情况下，可使用关键字not in。

python

banned_users = ['andrew','carolina','david']
user = 'marie'

if user not in banned_users:
    print(f"{user.title()},you can post a response if you wish.")

布尔表达式：

python

game_active = True
can_edit = False

最简单的if语句只有一个测试和一个操作：

python

if conditional_test:
    do something

if-else语句：

python

age = 17
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
else:
    print("Sorry,you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")

python

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

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

print("\nFinished making your pizza!")

if-elif-else结构：

python

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.")

python

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}.")

else 代码块是可以省略的：

python

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}.")

在if语句中将列表名用作条件表达式时，Python将在列表至少包含一个元素时返回True，并在列表为空时返回False（这点和JavaScript中的空数组的布尔值判断逻辑不一样）。

遍历键值对：

python

user = {
    'username': 'efermi',
}
for key,value in user.items():
    print(f"\nkey: {key}")
    print(f"Value: {value}")

遍历键：

python

# .keys() 是可以省略的，默认就是遍历所有键
for name in fvorite_languages.keys():
    print(name.title())

遍历值：

python

for language in favorite_languages.values():
    print(language.title())

# 对值进行去重后再遍历
for language in set(favorate_languages.values):
    print(language.title())

if ​

if