【脚本语言系列】关于Python基础知识推导式，你需要知道的事

如何使用推导式(Comprehension)

• 列表（list）推导式
  var_list = [ out_element for out_element in in_list if out_element judge sth]

# -*- coding:utf-8 -*-
div_list = [element for element in range(25) if element % 4 is 0]
print div_list

[0, 4, 8, 12, 16, 20, 24]

# -*- coding:utf-8 -*-
pow2_list = []
for x in range(25):
    pow2_list.append(x**2)
print pow2_list

pow2_list = [x**2 for x in range(25)]    
print pow2_list


pow3_list = []
for x in range(25):
    pow3_list.append(x**3)
print pow3_list

pow3_list = [x**3 for x in range(25)]
print pow3_list

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576]
[0, 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 2197, 2744, 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648, 12167, 13824]
[0, 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 2197, 2744, 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648, 12167, 13824]

• 字典(dict)推导式
  var_dict = { key_element:value_elements for key_element in key_list}

# -*- coding:utf-8 -*-
class_freq_dict = {"a":12, "b":23, "A":34, "B":45}
noclass_freq_dict = {key.lower(): class_freq_dict.get(key.lower(),0)+class_freq_dict.get(key.upper(),0)for key in class_freq_dict.keys()}
print noclass_freq_dict

{'a': 46, 'b': 68}

# -*- coding:utf-8 -*-
class_freq_dict = {"a":12, "b":23, "A":34, "B":45}
swap_dict = {value: key for key, value in class_freq_dict.items()}
print swap_dict

{34: 'A', 12: 'a', 45: 'B', 23: 'b'}

• 集合(set)推导式
  var_set = { out_element for out_element in in_set if out_element judge sth}

# -*- coding:utf-8 -*-
pow2_set = {x**2 for x in range(25)}
print pow2_set

pow3_set = {x**3 for x in range(25)}
print pow3_set

set([0, 1, 4, 9, 256, 144, 529, 25, 576, 196, 289, 36, 169, 49, 441, 64, 324, 81, 484, 225, 100, 16, 361, 400, 121])
set([0, 1, 4096, 12167, 8, 2197, 512, 10648, 27, 8000, 9261, 3375, 4913, 1331, 13824, 2744, 1728, 64, 5832, 6859, 343, 216, 729, 1000, 125])

什么是推导式(Comprehension)

推导式（解析式）是从一个数据序列构建另一个新的数据序列的结构体。
共有3种推导：
* 列表推导式
* 字典推导式
* 集合推导式