Python开发基础 day8 生成器 三元表达式 列表解析 生成器表达式

#生成器函数：函数体内包含有yield关键字，该函数执行的结果是生成器

 

def foo():
    print('first------>')
    yield 1
    print('second----->')
    yield 2
    print('third----->')
    yield 3
    print('fouth----->')

g=foo()

# print(g)
# from collections import Iterator
# print(isinstance(g,Iterator))

 

#生成器就是迭代器

# print(g.__next__())
# print(g.__next__())
# print(g.__next__())
# print(g.__next__())

# for i in g: #obj=g.__iter__() #obj.__next__()
#     print(i)

 

'''
yield的功能：
    1.与return类似，都可以返回值，但不一样的地方在于yield返回多次值，而return只能返回一次值
    2.为函数封装好了__iter__和__next__方法，把函数的执行结果做成了迭代器
    3.遵循迭代器的取值方式obj.__next__（），触发的函数的执行，函数暂停与再继续的状态都是由yield保存的
'''

 

def countdown(n):
    print('starting countdown')
    while n > 0:
        yield n
        n-=1
    print('stop countdown')
g=countdown(5)
# print(g)
# print(g.__next__())
# print(g.__next__())
# print(g.__next__())
# print(g.__next__())
# print(g.__next__())
# print(g.__next__())

#
# for i in g:
#     print(i)


#tail -f a.txt
import time
def tail(filepath,encoding='utf-8'):
    with open(filepath,encoding=encoding) as f:
    f.seek(0,2)
    while True:
        # f.seek(0, 2) #不行
        line=f.readline()
        if line:
            # print(line,end='')
            yield line
        else:
            time.sleep(0.5)

g=tail('a.txt')
print(g)
print(g.__next__())
#
# for i in g:
    # print(i)

#tail -f a.txt | grep 'error'


def grep(lines,pattern):
    for line in lines:
        if pattern in line:
            # print(line)
            yield line
# tail_g=tail('a.txt')
# print(g)

# grep_g=grep(tail_g,'error')

# print(grep_g)
#
# print(grep_g.__next__())

# for i in grep_g:
    # print(i)


#tail -f a.txt |grep 'error' |grep '404'

# g1=tail('a.txt')
#
# g2=grep(g1,'error')
#
# g3=grep(g2,'404')
# for i in g3:
     #print(i)

 

 

 

 

#生成器函数补充

def countdown(n):
    while n > 0:
        yield n
        n-=1


# g=countdown(5)
# print(g.__next__())
# print(g.__next__())
#
# print('='*20)
# for i in g:
    # print(i)
#
# print('*'*20)
# for i in g:
    # print(i)

 

# for i in countdown(5):
    # print(i)
# print('*'*20)
# for i in countdown(5):
    # print(i)
# print('*'*20)
# for i in countdown(5):
    # print(i)


# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())
# print(countdown(5).__next__())

 

# print(countdown(5),countdown(5),countdown(5))

三元表达式：

 

x=2
y=3

# if x > y:
     # print(x)
# else:
    # print(y)


# res='aaaaa' if x > y else 'bbbbbbb'
#
# print(res)


# def max2(x,y):
    # # if x > y:
        # # return x
    # # else:
        # # return y
#
    # return x if x > y else y
#
# print(max2(1,2))

列表解析：

 

 

s='hello'

# l=[]
# for i in s:
#     res=i.upper()
#     l.append(res)
#
# print(l)

 

 


# l=[]
# for i in range(10000):
#     l.append(i)
# print(l)


# l=[1,2,3,4]
# l_new=[]
# for i in l:
#     res=i**2
#     l_new.append(res)
# print(l_new)

 

#列表解析：

# s='hello'
# res=[i.upper() for i in s]
# print(res)


# l=[1,31,73,84,57,22]
# l_new=[]
# for i in l:
#     if i > 50:
#         l_new.append(i)
# print(l_new)

# res=[i for i in l if i > 50]
# print(res)

 

# for i in obj1:
#     if 条件1:
#         for i in obj2：
#             if 条件2：
#                 for i in obj3:
#                     if 条件3：
# ...
# l=[1,31,73,84,57,22]
# # print([i for i in l if i > 50])
# # print([i for i in l if i < 50])
# print([i for i in l if i > 20 and i < 50])

 

#生成器表达式

# [i for i in range(1000000000000000000000000000000000000000000)]

g=(i for i in range(100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000))

print(g)
print(next(g)) #next(g) == g.__next__()
print(next(g)) #next(g) == g.__next__()
print(next(g)) #next(g) == g.__next__()
print(next(g)) #next(g) == g.__next__()
print(next(g)) #next(g) == g.__next__()

 

 

#len('hello') 'hello'.__len__()

# print(len('hello'))
# print('hello'.__len__())
# iter(g) #g.__iter__()