5.1 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())

5.2.1 检查是否相等

car = ‘bmw‘
if car == ‘bmw‘:
print("Yes!")
else:
print("No!")

car = ‘audi‘
if car == ‘bmw‘:
print("Yes!")
else:
print("No!")

5.2.2 检查是否相等时考虑大小写

car = ‘Audi‘
if car != ‘audi‘: # 2个大小写不同的值会被视为不相等
print("Yes!")
else:
print("No!")

car = ‘Audi‘
if car.lower() == ‘audi‘: # 判定并不会更改car中的值，car中还是‘Audi‘
print("Yes!")
else:
print("No!")

5.2.3 检查是否不相等

requested_topping = ‘mushrooms‘
if requested_topping != ‘anchovies‘:
print("Hold the anchovies!")

5.2.4 比较数字

age = 18
if age == 18:
print("Yes!")

answer = 17
if answer != 42:
print("That is not the correct answer. Please try again!")

age = 19
if age < 21:
print("Yes!")
else:
print("No!")
if age <= 21:
print("Yes!")
else:
print("No!")
if age > 21:
print("Yes!")
else:
print("No!")
if age >= 21:
print("Yes!")
else:
print("No!")

5.2.5 检查多个条件

1. 使用and检查多个条件(检查是否两个人都不小于21岁)

age_0 = 22
age_1 = 18
if (age_0 >= 21) and (age_1 >= 21):
print("Yes!")
else:
print("No!")
age_1 = 22
if (age_0 >= 21) and (age_1 >= 21):
print("Yes!")
else:
print("No!")

2. 使用or检查多个条件（只要至少1个条件满足，就能通过测试；2个测试都没有通过时，表达式结果为False）

age_0 = 22
age_1 = 18
if (age_0 >= 21) or (age_1 >= 21):
print("Yes!")
else:
print("No!")

age_0 = 18
age_1 = 18
if (age_0 >= 21) or (age_1 >= 21):
print("Yes!")
else:
print("No!")

5.2.6 检查特定值是否包含在列表中

requested_toppings = [‘mushrooms‘, ‘onions‘, ‘pineapple‘]
if ‘mushrooms‘ in requested_toppings:
print("True!")

if ‘pepperoni‘ in requested_toppings:
print("True!")
else:
print("False!")

5.2.7 检查特定值是否不包含在列表中

banned_users = [‘andrew‘, ‘carolina‘, ‘david‘]
user = ‘marie‘
if user not in banned_users:
print(user.title() + ", you can post a response if you wish.")

5.2.8 布尔表达式（结果要么为True，要么为False）

5-1 条件测试：预期 + 答案

car = ‘subaru‘
print("Is car == ‘subaru‘? I predict true.")
print(car == ‘subaru‘)
print("Is car == ‘audi‘? I predict false.")
print(car == ‘audi‘)

5-2 更多的条件测试：每个都来一个true和false

检查两个字符串是否相等

string_1 = ‘Big data‘
string_2 = ‘Big data‘
string_3 = ‘Big Data‘
print("string_1 == string_2:")
print(string_1 == string_2)
print("string_1 == string_3:")
print(string_1 == string_3)

使用函数lower() 的测试

print("string_1.lower() == string_3.lower()??
print(string_1.lower() == string_3.lower())

检查两个数字

age_1 = 66
age_2 = 68
print("age_1 == age_2:")
print(age_1 == age_2)
print("age_1 != age_2:")
print(age_1 != age_2)
print("age_1 > age_2:")
print(age_1 > age_2)
print("age_1 < age_2:")
print(age_1 < age_2)
print("age_1 >= age_2:")
print(age_1 >= age_2)
print("age_1 <= age_2:")
print(age_1 <= age_2)

使用关键字and 和or 的测试

print("False and True:")
print((age_1 == age_2) and (age_1 != age_2))
print("False or True:")
print((age_1 == age_2) or (age_1 != age_2))

检查特定值是否包含在列表中

requested_toppings = [‘mushrooms‘, ‘onions‘, ‘pineapple‘]
print("Is ‘mushrooms‘ in requested_toppings?")
print(‘mushrooms‘ in requested_toppings)
print("Is ‘ice cream‘ in requested_toppings?")
print(‘ice cream‘ in requested_toppings)
print("Is ‘mushrooms‘ not in requested_toppings?")
print(‘mushrooms‘ not in requested_toppings)
print("Is ‘ice cream‘ not in requested_toppings?")
print(‘ice cream‘ not in requested_toppings)

5.3.1 简单的if 语句

if conditional_test: # 如果测试通过了，将执 行if 语句后面所有缩进的代码行，否则将忽略它们。

do something

age = 23
if age >= 18:
print("You are old enough to vote!")
print("Have you registered to vote yet?")

5.3.2 if-else语句

age = 16
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!")

5.3.3 if-elif-else 结构

age = 13
if age < 4:
print("Your admission cost is $0.")
elif age < 18:
print("Your admission cost is $5.")
else:
print("Your admission cost is $10.")

age = 13
if age < 4:
price = 0
elif age < 18:
price = 5
else:
price = 10
print("Your admission cost is $" + str(price) + ".")

5.3.4 使用多个elif 代码块

age = 13
if age < 4:
price = 0
elif age < 18:
price = 5
elif age < 65:
price = 10
else: # age >= 65
price = 5
print("Your admission cost is $" + str(price) + ".")

5.3.5 省略else 代码块

age = 13
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) + ".")

5.3.6 测试多个条件

如果顾客点了两种配料，就需要确保在其比萨中包含这些配料

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.")

5.4.1 检查特殊元素

requested_toppings = [‘mushrooms‘, ‘green peppers‘, ‘extra cheese‘]
for requested_topping in requested_toppings:
print("Adding " + requested_topping + ".")
print("Finished making your pizza!")

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("Finished making your pizza!")

5.4.2 确定列表不是空的

requested_toppings = []
if requested_toppings:
for requested_topping in requested_toppings:
print("Adding " + requested_topping + ".")
print("Finished making your pizza!")
else:
print("Are you sure you want a plain pizza?")

5.4.3 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("Adding " + requested_topping + ".")
else:
print("Sorry, we don‘t have " + requested_topping + ".")
print("Finished making your pizza!")

5-3 外星人的颜色#1

alien_color = ‘green‘
if alien_color == ‘green‘:
print("You just earned 5 points!")

alien_color = ‘red‘
if alien_color == ‘green‘:
print("You just earned 5 points!")

5-4 外星人的颜色#2

alien_color = ‘green‘
if alien_color == ‘green‘:
print("You just earned 5 points!")
else:
print("You just earned 10 points!")

alien_color = ‘yellow‘
if alien_color == ‘green‘:
print("You just earned 5 points!")
else:
print("You just earned 10 points!")

5-5 外星人的颜色#3

alien_color = ‘red‘
if alien_color == ‘green‘:
print("You just earned 5 points!")
elif alien_color == ‘yellow‘:
print("You just earned 10 points!")
else:
print("You just earned 15 points!")

5-6 人生的不同阶段

age = 16
if age < 2:
print("You‘re a baby!")
elif age < 4:
print("You‘re a toddler!")
elif age < 13:
print("You‘re a kid!")
elif age < 20:
print("You‘re a teenager!")
elif age < 65:
print("You‘re an adult!")
else:
print("You‘re an elder!")

5-7 喜欢的水果

favorite_fruits = [‘apples‘, ‘oranges‘, ‘bananas‘]
if ‘apples‘ in favorite_fruits:
print("You really like apples!")
if ‘peaches‘ in favorite_fruits:
print("You really like peaches!")
if ‘bananas‘ in favorite_fruits:
print("You really like bananas!")
if ‘kiwis‘ in favorite_fruits:
print("You really like kiwis!")
if ‘blueberries‘ in favorite_fruits:
print("You really like blueberries!")

5-8 跟用户打招呼

user_names = [‘eric‘, ‘willie‘, ‘admin‘, ‘erin‘, ‘ever‘]
for username in user_names:
if username == ‘admin‘:
print("Hello admin, would you like to see a status report?")
else:
print("Hello " + username.title() + ", thank you for logging in again.")

5-9 处理没有用户的情形

user_names = []
if user_names:
for username in user_names:
if username == ‘admin‘:
print("Hello admin, would you like to see a status report?")
else:
print("Hello " + username.title() + ", thank you for logging in again.")
else:
print("We need to find some users!")

5-10 检查用户名

current_users = [‘eric‘, ‘willie‘, ‘admin‘, ‘erin‘, ‘Ever‘]
new_users = [‘sarah‘, ‘willie‘, ‘PHIL‘, ‘ever‘, ‘Iona‘]
current_users_lower = [user.lower() for user in current_users] # 等价于下面三行

current_users_lower = []

for user in current_users:

current_users_lower.append(user.lower())

for new_user in new_users:
if new_user.lower() in current_users_lower:
print("Sorry " + new_user + ", that name is taken.")
else:
print("Great, " + new_user + " is still available.")

5-11 序数：序数表示位置

numbers = list(range(1, 10))
for number in numbers:
if number == 1:
print("1st")
elif number == 2:
print("2nd")
elif number == 3:
print("3rd")
else:
print(str(number) + "th")

Python第五章

原文：https://www.cnblogs.com/XDZ-TopTan/p/13211986.html