Python正课44 —— 生成式

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一：列表生成式

格式：[expression for item in iterable in condition]

原代码：

l = ['alex_dsb','lxx_dsb','wxx_dsb','xxq','egon_dsb']
new_l = []
for name in l:
    if name.endswith('dsb'):
        new_l.append(name)

改进后：

l = ['egon_is_sb','egon_is_dsb','egon_is_superdsb','xxq','egon_dsb']

new_l = [name for name in l]  # 最基础的列表生成式
new_l = [name for name in l if name.endswith('sb')]

print(new_l)

把所有小写字母变成大写

l = ['egon_is_sb','egon_is_dsb','egon_is_superdsb','xxq','egon_dsb']
new_l = [name.upper() for name in l ]
print(new_l)

去掉所有名字的_sb后缀

l = ['egon_is_sb','egon_is_sb','egon_is_superd_sb','xxq','egon_sb']

new_l = [name.replace('_sb','') for name in l]
print(new_l)

二：字典生成式

keys = ['name','age','gender']

dic = {key:None for key in keys}
print(dic)

items = [('name','egon'),('age',18),('gender','male')]

dict = {k:v for k,v in items if k!='gender'}
print(dict)

三：集合生成式

keys = ['name','age','gender']
set1 = {key for key in keys}
print(set1,type(set1))

四：元组生成式（没有元组生成式）

g = (i for i in range(10) if i > 3)
print(g,type(g))

# 输出：<generator object <genexpr> at 0x01D63450> <class 'generator'>

五：生成器表达式

g = (i for i in range(10) if i > 3)
# ！！！！！！！强调！！！！！！！
# 此时g的内部，一个值都没有
# 只有next的时候，才会产生一个值

print(g)
print(next(g))
print(next(g))
print(next(g))
print(next(g))

方式一：

with open(r'内容.txt',mode='rt',encoding='UTF-8') as f:
    res = 0
    for line in f:
        res += len(line)
    print(res)

方式二：

with open(r'内容.txt',mode='rt',encoding='UTF-8') as f:

    res = sum([len(line) for line in f])
    print(res)

方式三：效率高

with open(r'内容.txt', mode='rt', encoding='UTF-8') as f:
    # g = (len(line) for line in f)
    # print(g)
    # res = sum(g)
    # print(res)
    # res = sum((len(line) for line in f))
    # 上述可以简写为如下格式
    res = sum(len(line) for line in f)
    print(res)