外观
if 语句
python
cars = ['audi','bmw','subaru','toyota']
for car in cars:
if car == 'bmw':
print(car.upper())
else:
print(car.title())1
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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_toppings1
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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.")1
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布尔表达式：
python
game_active = True
can_edit = False1
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最简单的 if 语句只有一个测试和一个操作：
python
if conditional_test:
do something1
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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!")1
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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!")1
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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.")1
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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}.")1
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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}.")1
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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）。