【4.5】列表推导式、生成器表达式、字典推导式

　　• 列表推导式

 1 #!/user/bin/env python
 2 # -*- coding:utf-8 -*-
 3 # 列表推导式性能高于列表操作
 4 
 5 odd_list = []
 6 for i in range(21):
 7     if i%2 == 1:
 8         odd_list.append(i)
 9 
10 # 提取1-20之间的奇数
11 odd_list = [i for i in range(21) if i % 2 == 1]
12 print(odd_list)
13 
14 # 提取1-20之间的奇数，并且平方
15 
16 
17 def pow_item(item):
18     return item*item
19 
20 
21 odd_list = [pow_item(i) for i in range(21) if i % 2 == 1]
22 print(odd_list)

[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
[1, 9, 25, 49, 81, 121, 169, 225, 289, 361]

　　• 生成器表达式

 1 #!/user/bin/env python
 2 # -*- coding:utf-8 -*-
 3 # 生成器表达式
 4 
 5 odd_gen = (i for i in range(21) if i % 2 == 1)
 6 print(type(odd_gen))
 7 print(odd_gen)
 8 
 9 for item in odd_gen:
10     print(item)
11 
12 odd_list = list(odd_gen)
13 print(type(odd_list))
14 print(odd_list)

<class 'generator'>
<generator object <genexpr> at 0x0000015819C82660>
1
3
5
7
9
11
13
15
17
19

　　• 字典推导式

 1 #!/user/bin/env python
 2 # -*- coding:utf-8 -*-
 3 # 字典推导式
 4 my_dict = {'zy1': 11, 'zy2': 22, 'zy3': 33}
 5 reversed_dict = {value: key for key, value in my_dict.items()}
 6 print(reversed_dict)
 7 
 8 # 集合推导式
 9 my_set = {key for key, value in my_dict.items()}
10 print(type(my_set))
11 print(my_set)

{11: 'zy1', 22: 'zy2', 33: 'zy3'}
<class 'set'>
{'zy3', 'zy2', 'zy1'}