python基础——列表推导式
1 列表推导式定义
列表推导式能非常简洁的构造一个新列表:只用一条简洁的表达式即可对得到的元素进行转换变形
2 列表推导式语法
基本格式如下:
[expr for value in collection ifcondition]
过滤条件可有可无,取决于实际应用,只留下表达式
列表推导式例子:
l=["egg%s"%i for i in range(10)]
print(l)
类似于这段for代码:
egg_list=[]
for i in range(10):
egg_list.append("egg%s"%i)
print(egg_list)
列表推导式还可以加更多的if判断for循环
l=['egg%s' %i for i in range(1,101)]
l=['egg%s' %i for i in range(1,101) if i >50 ]
l=['egg%s' %i for i in range(1,101) if i >50 if i<60]
print(l)
3 列表推导式优点
方便,改变了编程习惯,属于声明式编程
举例:
l=[1,2,3,4]
s="hello"
l1=[(num,i) for num in l for i in s]
print(l1)
输出结果为:
[(1, 'h'), (1, 'e'), (1, 'l'), (1, 'l'), (1, 'o'), (2, 'h'), (2, 'e'), (2, 'l'), (2, 'l'),
(2, 'o'), (3, 'h'), (3, 'e'), (3, 'l'), (3, 'l'), (3, 'o'), (4, 'h'), (4, 'e'), (4, 'l'),
(4, 'l'), (4, 'o')]
这个列表推导式相当于:
l=[1,2,3,4]
s="hello"
l1=[]
for num in l:
for i in s:
t=(num,i)
l1.append(t)
print(l1)
4 列表表达式例子
import os
g=os.walk("C:\python_fullstack_wen\day24\wen")
file_path_list=[]
for i in g:
for j in i[-1]:
file_path_list.append("%s\\%s"%(i[0],j))
print(file_path_list) g=os.walk("C:\python_fullstack_wen\day24\wen")
file_path_list=["%s\\%s"%(i[0],j) for i in g for j in i[-1]]
print(file_path_list)
输出结果:
['C:\\python_fullstack_wen\\day24\\wen\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen1
\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen1\\jie1\\yan1.txt',
'C:\\python_fullstack_wen\\day24\\wen\\wen1\\yan1\\yan1.txt',
'C:\\python_fullstack_wen\\day24\\wen\\wen2\\yan2.txt']
['C:\\python_fullstack_wen\\day24\\wen\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen1\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen1\\jie1\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen1\\yan1\\yan1.txt', 'C:\\python_fullstack_wen\\day24\\wen\\wen2\\yan2.txt']