9-day9-生成器-列表解析-三元表达式-

迭代器

# 通过迭代器，迭代打印字典：

dict_1 = {'as': '1', 'ds': '2', 'cd': '3', 'ss': '4', }

d_dict = dict_1.__iter__()

while True:
    try:
        i = d_dict.__next__()
        print(i)
    except StopIteration:
        break

列表解析

s='hello'
s_new = []
for i in s:
    res = i.upper()
    s_new.append(res)
    print(s_new)


# 列表解析方法
s = 'hello'
res = [i.upper() for i in s]
print(res)

# 列表解析加判断
l = [1, 31, 73, 56, 55, 22]
res = [i for i in l if i > 50]

生成器

# 函数体内包含yield关键字，该函数执行的结果是生成器函数


#
def foo():
    print('==============> first')
    yield 1
    print('==============> second')
    yield 2
    print('==============> third')
    yield 4
    print('===============> forth')


g = foo()
for i in g:
    print(i)

l = foo().__iter__()
print(l.__next__())
print(l.__next__())
print(l.__next__())


# yield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，尔return返回一次
# 2 为函数封装好了 __iter__ 和__next__ 方法，把函数执行结果做成了迭代器
# 3. 遵循迭代器的取值方法 obj.__next__() 除法函数的执行，函数暂停与再继续的状态是由yield保存


def countdown(n):
    print('start countdown')

    while n > 0:
        yield n
        n -= 1
    print('stop countdown')

g = countdown(5)

for i in g:
    print(i)


# 实现 tail -f 功能

import time
def tail():
    with open('a.txt', 'r', encoding='utf-8') as f:
        f.seek(0, 2)
        while True:
            line = f.readline()
            if line:
                print(line,end='')
            else:
                time.sleep(0.3)
tail()


# 实现tail | grep 功能
import time
def tail(filename):
    with open(filename, 'r', encoding='utf-8') as f:
        f.seek(0, 2)
        while True:
            line = f.readline()
            if line:
                yield line
            else:
                time.sleep(0.3)


def grep(lines,pattern):
    print('lines', lines)
    for line in lines:
        if pattern in line:
            print(line)

g = tail('a.txt')

grep(g,'errpr')



# 实现tail | grep 功能

import time

def tail(filename):
    with open(filename, 'r', encoding='utf-8') as f:
        f.seek(0, 2)
        while True:
            line = f.readline()
            if line:
                yield line
            else:
                time.sleep(0.3)


def grep(lines, pattern):
    print('lines', lines)
    for line in lines:
        if pattern in line:
            print(line)

g = tail('a.txt')

grep(g, 'error')


# 设计实现 tail + grep + grep

import time

def tail(filename):
    with open(filename, 'r', encoding='utf-8') as f:
        f.seek(0, 2)
        while True:
            line = f.readline()
            if line:
                yield line
            else:
                time.sleep(0.4)


t = tail('a.txt')


def grep(lines, pattern):
    for line in lines:
        if pattern in line:
            yield line


g = grep(t, 'alex')

g_2 = grep(g, '3714')

for xx in g_2:
    print(xx)