Introduction to the usage of recursive calls of python generators (code example)

This article brings you an introduction to the usage of recursive calls of Python generators (code examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Generator

What is a generator: As long as the yield keyword appears in the function body, then the function code will not be executed when the function is executed, and a result will be obtained. The result is the generator

The generator is the iterator

The function of yield

yield provides us with a way to customize the iterator object

## The difference between #yield and return:

1. Yield can return multiple values ​​

2. The pause and resumption of functions are saved by yield for us

As long as you see the function If yield appears in it, then it is a generator

Example 1: As we mentioned above, if we see yield in the function, then it is a generator, and the generator is an iterator,

Then when it comes to iteration The processor should think of the value method of xx.__next__()

def test():
    print('=====>1')
    yield 1
    print('=====>2')
    yield 2
    print('=====>3')
    yield 3
g = test()　　#就相当于一个容器
print(g.__next__())
print(g.__next__())
print(next(g))

Running result:

We After knowing this way to obtain values, you will think of another simple way with the same principle is the for loop

def test():
    print('=====>1')
    yield 1
    print('=====>2')
    yield 2
    print('=====>3')
    yield 3
g = test()
for i in g:
    print(i)

Running result:

Example 2: To call the result of test1 to test2, you need to use yield to customize a generator

def test1():
    for i in range(10):
        yield i 　　#把0~9变成生成器返回给函数test1
g = test1()　　　　　#g是个生成器
def test2(g):
    for i in g:
        print(i)
test2(g)

Running results:

Example 3: Log error monitor

import time
def tail(filepath): 　　#定义一个查看文件的函数
    with open(filepath, 'rb') as f: 　　#打开形参为filepath rb是二进制读
        f.seek(0,2)  　　#把光标移动到末尾
        while True:　　#循环监控日志
            data = f.readline() 　　#读取文件末尾
            if data: 　　#加入有数据就用yield返回
                yield data
            else:#　　否则就睡眠0.05秒
                time.sleep(0.05)
def grep(file, k):  　　#定义过滤关键字函数
    for i in tail(file): 　　#循环生成器中的数据
        if k in i.decode('utf-8'):　　#因为是用二进制读取方式，所以需要解码显示
            print(i.decode('utf-8'))
grep('a.txt', '500')　　#监控a.txt最新日志，并过滤500的错误代码

Once 500 appears, it will be captured

## Another usage of yield, assign value

def test(name):
    while True:
        foot = yield
        print('%s正在吃%s' % (name, foot))

e = test('轩轩') #e是生成器
next(e)    #初始化，e.__next__()
# e.send(None)    #初始化，与上一行二选一
e.send('饺子')    #发送值传给foot
e.send('冰激凌')    #发送值传给foot

Run result:

##Recursive call

Recursive call:

In the process of calling a function, the function itself is called directly or indirectly, which is called recursive callThe two necessary stages of recursion:

1 recursion, 2 backtracking

Example: A, B, C, D, and E, 5 people eat buns. We want to know how many buns A ate, but A said it was better than B. Eat 2 more pieces. B said he ate 2 more pieces than C. C said he ate 2 more pieces than D. Ding said he ate 2 more pieces than E. E said he didn't eat it.

Then because he knew that E didn't eat it , so according to the answers of A, B, C and D, we know that A ate 8, so the process of going back and forth is recursion and backtracking

age(A) = age(B) 2

age(B) = age(C) 2

age(C) = age(D) 2

age(D) = age(E) 2

age( E) = 0

def num(n):    
if n == 1:        
return 0    
return num(n-1) + 2res = num(5) 
print(res)

Run result:

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