4-1比萨：
pizzas=['apple_pizza','banana_pizza','beef_pizza']
for pizza in pizzas:
print('I like '+pizza)
print('I really like pizza')运行结果：
4-2动物：
animals=['dog','cat','tortoise']
for animal in animals:
print('A '+animal+' would make a great pet')
print('Any of these animals would make a great pet')运行结果：
4-3数到20：
for num in range(1,21):
print(num,end=' ')
print('\n')运行结果：
4-4一百万：
arr=list(range(1,1000001))
for num in arr:
print(num,end=' ')运行结果（结果太长就截一部分）：
4-5计算1~1000000的总和：
arr=list(range(1,1000001))
print(min(arr))
print(max(arr))
print(sum(arr))运行结果：
4-6奇数：
arr=list(range(1,20,2))
for x in arr:
print(x,end=' ')运行结果：
4-7 3的倍数：
arr=[3*i for i in range(1,11)]
for num in arr:
print(num,end=' ')运行结果：
4-8立方：
arr=[i**3 for i in range(1,11)]
for num in arr:
print(num,end=' ')运行结果：
4-9立方解析：
同上
4-10切片：
arr=['one','two','three','four','five']
print('The first three items in the list are:')
for num in arr[:3]:
print(num,end=' ')
print('Three items from the middle of the list are:')
for num in arr[1:4]:
print(num,end=' ')
print('The last three items in the list are:')
for num in arr[-3:]:
print(num,end=' ')运行结果：
4-11你的比萨和我的比萨：
pizzas=['apple_pizza','banana_pizza','beef_pizza']
friend_pizzas=pizzas[:]
pizzas.append('chicken_pizza')
friend_pizzas.append('big_pizza')
print("My favourite pizzas are:")
for pizza in pizzas:
print(pizza,end=' ')
print("\nMy friend's favourite pizzas are:")
for pizza in friend_pizzas:
print(pizza,end=' ') 运行结果：
4-12 使用多个循环：
同上
4-13自助餐：
foods=('beef','chicken','hanburger','apple','vegetables')
for food in foods:
print(food,end=' ') 运行结果：
尝试修改：
foods[1]='milk'结果：
替换:
print('\n')
foods=('banana','duck','hanburger','apple','vegetables')
for food in foods:
print(food,end=' ') 运行结果：
4-14：浏览https://python.org/dev/peps/pep-0008/
4-15 已经设置好自己的编辑器