【4.5】列表推导式、生成器表达式、字典推导式

　　• 列表推导式

#!/user/bin/env python
# -*- coding:utf-8 -*-
# 列表推导式性能高于列表操作

odd_list = []
for i in range(21):
    if i%2 == 1:
        odd_list.append(i)

# 提取1-20之间的奇数
odd_list = [i for i in range(21) if i % 2 == 1]
print(odd_list)

# 提取1-20之间的奇数，并且平方


def pow_item(item):
    return item*item


odd_list = [pow_item(i) for i in range(21) if i % 2 == 1]
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

#!/user/bin/env python
# -*- coding:utf-8 -*-
# 生成器表达式

odd_gen = (i for i in range(21) if i % 2 == 1)
print(type(odd_gen))
print(odd_gen)

for item in odd_gen:
    print(item)

odd_list = list(odd_gen)
print(type(odd_list))
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

#!/user/bin/env python
# -*- coding:utf-8 -*-
# 字典推导式
my_dict = {'zy1': 11, 'zy2': 22, 'zy3': 33}
reversed_dict = {value: key for key, value in my_dict.items()}
print(reversed_dict)

# 集合推导式
my_set = {key for key, value in my_dict.items()}
print(type(my_set))
print(my_set)

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