Python学习札记(十七) 高级特性3 列表生成式

参考：列表生成式

Note

1.List Comprehensions，即列表生成式，是Python中内置的非常强大的list生成式。

eg.生成一个列表：[1*1, 2*2, ..., 10*10]

使用for...in的方法：

#!/usr/bin/env python3

L1 = []

for i in range(1, 11) :
	L1.append(i*i)

print(L1)

使用列表生成式：

L2 = [i*i for i in range(1, 11)]

print(L2)

output:

sh-3.2# ./listcomprehensions1.py 
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

2.列表生成式可以在后面加上条件判断：

eg.符合 (i*i)%2==0 要求

L3 = [i*i for i in range(1, 11) if (i*i)%2 == 0]

print(L3)

output:

[4, 16, 36, 64, 100]

3.也可以使用两层循环：

L4 = [i+j for i in range(1, 11) for j in range(1, 11)]

print(L4)

[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

L5 = [i+j for i in 'ABC' for j in 'abc']

print(L5)

['Aa', 'Ab', 'Ac', 'Ba', 'Bb', 'Bc', 'Ca', 'Cb', 'Cc']

4.使用列表生成式能简化代码，如输出当前目录的文件名：

import os

L6 = [i for i in os.listdir('.')] # listdir()

print(L6)

['listcomprehensions1.py']

5.对于dict来说，列表生成式也可以生成key-value的list：items()方法

dic = {'Chen': 'Student', '952693358': 'QQ', 'Never-give-up-C-X': 'WeChat'}

L7 = [x+'='+y for x, y in dic.items()]

print(L7)

['952693358=QQ', 'Chen=Student', 'Never-give-up-C-X=WeChat']

6.将字符串变成小写字符串：lower()函数

str1 = 'HeyGirl'

L8 = [i.lower() for i in str1]

print(L8)

['h', 'e', 'y', 'g', 'i', 'r', 'l']

Practice

如果list中既包含字符串，又包含整数，由于非字符串类型没有lower()方法，所以列表生成式会报错：

>>> L = ['Hello', 'World', 18, 'Apple', None]
>>> [s.lower() for s in L]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
AttributeError: 'int' object has no attribute 'lower'

使用内建的isinstance函数可以判断一个变量是不是字符串：

>>> x = 'abc'
>>> y = 123
>>> isinstance(x, str)
True
>>> isinstance(y, str)
False

提供：L1 = ['Hello', 'World', 18, 'Apple', None]

期待输出L2 = [L1中属于字符串的元素的小写]

Ans:

#!/usr/bin/env python3

L1 = ['Hello', 'World', 18, 'Apple', None]

L2 = [i.lower() for i in L1 if isinstance(i, str)]

print(L2)

sh-3.2# ./listcomprehensions2.py 
['hello', 'world', 'apple']

2017/2/6