列表生成式和生成器表达式

一.列表生成式

egg_list=[]
# for i in range(100):
#     egg_list.append('egg%s' %i)
# print(egg_list)

l=[ 'egg%s' %i for i in range(100) if i >50]
print(l)

 

三元表达式：

name='alex'
name='linhaifeng'
res='SB' if name == 'alex' else 'shuai'
print(res)

　

列表生成式例子：

1.

l=[1,2,3,4]
s='hello'

l1=[(num,s1) for num in l if num > 2 for s1 in s]
print(l1)

[(3, 'h'), (3, 'e'), (3, 'l'), (3, 'l'), (3, 'o'), (4, 'h'), (4, 'e'), (4, 'l'), (4, 'l'), (4, 'o')]

相当于　　

l1=[]
for num in l:
    for s1 in s:
        t=(num,s1)
        l1.append(t)
print(l1)


2.

import os

g=os.walk('C:\\egon')
l1=['%s\\%s' %(i[0],j) for i in g for j in i[-1]]
print(l1)

#相当于
# g=os.walk('C:\\egon')
# 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)







['C:\\egon\\egon.txt', 'C:\\egon\\egon1.txt', 'C:\\egon\\a\\a.txt', 'C:\\egon\\a\\a1\\a1.txt', 'C:\\egon\\a\\a1\\a2\\a2.txt', 'C:\\egon\\b\\b.txt']

 

二.生成器表达式

g=l=('egg%s' %i for i in range(100))
print(g)
print(next(g))
print(next(g))



<generator object <genexpr> at 0x0000000000690E60>
egg0
egg1

 

f=open('a.txt')           ##过于占用内存
l=[]
for line in f:
    line=line.strip()
    l.append(line)
print(l)

#或者
l1=[line.strip() for line in f ]      ##声明式编程
print(l1)

['sfasdfasdf', '1234432', '343243', '324124', '3124124', '321441234']

　　

表达式形式

money_l=[]
with open('b.txt') as f:
    for line in f:
        goods=line.split()
        res=float(goods[-1])*float(goods[-2])
        money_l.append(res)
print(money_l)




[30.0, 1000000.0, 6000.0, 90000.0, 30.0]

生成器形式

f=open('b.txt')
g=(float(line.split()[-1])*float(line.split()[-2]) for line in f)
print(sum(g))



#求和


#b.txt
apple 10 3
tesla 1000000 1
mac 3000 2
lenovo 30000 3
chicken 10 3

 

res = []
with open('b.txt') as f:
    for line in f:
        #print(line)
        l=line.split()
        #print(l)
        d={}
        d['name']=l[0]
        d['price']=l[1]
        d['count']=l[2]
        res.append(d)
print(res)



[{'name': 'apple', 'price': '10', 'count': '3'}, {'name': 'tesla', 'price': '1000000', 'count': '1'}, {'name': 'mac', 'price': '3000', 'count': '2'}, {'name': 'lenovo', 'price': '30000', 'count': '3'}, {'name': 'chicken', 'price': '10', 'count': '3'}]

 

模拟数据库的查询

##列表形式
with open('b.txt') as f:
    res=[line.split() for line in f]
    print(res)


[['apple', '10', '3'], ['tesla', '1000000', '1'], ['mac', '3000', '2'], ['lenovo', '30000', '3'], ['chicken', '10', '3']]

##生成器形式
with open('b.txt') as f:
    res=(line.split() for line in f)
    print(res)
    dic_g=({'name':i[0],'price':i[1],'count':i[2]} for i in res)
    print(dic_g)
    apple_dic=next(dic_g)
    print(next(dic_g))
    print(apple_dic['count'])


<generator object <genexpr> at 0x0000000000B20E60>
<generator object <genexpr> at 0x0000000000B20F68>
{'price': '1000000', 'name': 'tesla', 'count': '1'}
3

 

过滤查询

 

#取出单价>10000
with open('b.txt') as f:
    res=(line.split() for line in f)
    dic_g=({'name':i[0],'price':i[1],'count':i[2]} for i in res if float(i[1]) > 10000)
    print(dic_g)
    for i in dic_g:
        print(i)
    print(list(dic_g))



<generator object <genexpr> at 0x0000000000D10EB8>
{'count': '1', 'name': 'tesla', 'price': '1000000'}
{'count': '3', 'name': 'lenovo', 'price': '30000'}
[]