5 第五章 if语句

一个简单示例

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

for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())
# Audi
# BMW
# Subaru
# Toyota

条件测试

条件测试: 每条if 语句的结果要么是True ，要么是False ；

Python根据条件测试的值为True 还是False 来决定是否执行if 语句中的代码

A == B：A等于B，则结果为True，否则为False

A != B: A不等于B，则结果为True，否则为False

requested_topping = 'mushrooms'

if requested_topping != 'anchovies':
	print("Hold the anchovies!")
# Hold the anchovies!

条件语句中可包含各种数学比较，如小于(<)、小于等于(<=)、大于(>)、大于等于(>=)

# 使用and 检查多个条件
age = 30
if age > 18 and age < 60:
  print("good")
# good

# 使用 or 检查多个条件
age = 9
if age > 18 or age < 10:
  print("good_2")
# good_2

要判断特定的值是否已包含在列表中，可使用关键字in；

要判断特定的值是否未包含在列表中，可使用关键字not in

布尔表达式的结果要么为True ，要么为False ；

布尔值通常用于记录条件，如游戏是否正在运行，或用户是否可以编辑网站的特定内容

if语句

if conditional_test:
	do something
# 如果条件测试的结果为True ，Python就会执行紧跟在if语句后面的代码；
# 否则Python将忽略这些代码

age = 19
if age >= 18:
  print("You are old enough to vote!")
  print("Have you registered to vote yet?")
# You are old enough to vote!
# Have you registered to vote yet?

# if-else 结构
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!")
# Sorry, you are too young to vote.
# Please register to vote as soon as you turn 18!

# if-elif-else 结构
age = 12

if age < 4:
	price = 0
elif age < 18:
	price = 5
else:
	price = 10
    
print("Your admission cost is $" + str(price) + ".")
# Your admission cost is $5.

# 使用多个elif 代码块
age = 12

if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 10
else:
	price = 5
    
print("Your admission cost is $" + str(price) + ".")

# 省略else 代码块
age = 12

if age < 4:
	price = 0
elif age < 18:
	price = 5
elif age < 65:
	price = 10
elif age >= 65:
	price = 5
    
print("Your admission cost is $" + str(price) + ".")
# 该处的elif 代码块在顾客的年龄超过65（含）时，就将价格设置为5美元；
# 这比使用else 代码块更清晰些
# 经过这样的修改后，每个代码块都仅在通过了相应的测试时才会执；
# else 是一条包罗万象的语句，只要不满足任何if 或elif 中的条件测试，其中的代码就会执行，这可能会引入无效甚至恶意的数据；
# 如果知道最终要测试的条件，应考虑使用一个elif代码块来代替else代码块；
# 这样，你就可以肯定，仅当满足相应的条件时，你的代码才会执行

# 测试多个条件
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!")
# Adding mushrooms.
# Adding extra cheese.
# Finished making your pizza!

使用if语句处理列表

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("Adding " + requested_topping + ".")
        
print("\nFinished making your pizza!")
# Adding mushrooms.
# Sorry, we are out of green peppers right now..
# Adding extra cheese.

# Finished making your pizza!

# 确定列表是否未空
requested_toppings = []

if requested_toppings:
	for requested_topping in requested_toppings:
		print("Adding " + requested_topping + ".")
	print("\nFinished making your pizza!")
else:
	print("Are you sure you want a plain pizza?")
# Are you sure you want a plain pizza?

# 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("Adding " + requested_topping + ".")
	else:
		print("Sorry, we don't have " + requested_topping + ".")
        
print("\nFinished making your pizza!")
# Adding mushrooms.
# Sorry, we don't have french fries.
# Adding extra cheese.
# Finished making your pizza!

设置if语句的格式

在条件测试的格式设置方面，PEP 8提供的唯一建议是：

在诸如== 、>= 和<= 等比较运算符两边各添加一个空格，

例如，if age < 4: 要比 if age<4: 好；

这样的空格不仅不会影响Python对代码的解读，还会让代码阅读起来更容易

小结

条件测试；if、if-else、if-elif-else结构；
Python在代码格式方面的建议，这可确保即便你编写的程序越来越复杂，但其代码依然易于阅读和理解