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
To improve is to change, to be perfect is to change often.