Foreword
yield of the English word meaning production, new to Python's time was very confused, has not figure out yield usage. Recently re-learn the next, so finishing the following article, we learn for themselves and their reference, the following did not talk much to say, to take a look at the detailed introduction.
First look at an example
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
next(g)
In the above example, because the yield keyword has the function foo, so foo () function g execution result is a generator, then you can use the Next (g) or g. Next () method performs the trigger generator
Results of the implementation of
starting...
When using the next (g) trigger the execution of the generator, the program executed in the normal order from the top down, yield encountered, the program will be suspended
, and the yield values are returned back contact
Print next (g) of the results
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
print(next(g))
Program execution results
starting...
None
In the example above, perform a next (g) method, the program pauses in yield that line, then call the next (g) again, the program will continue to run down the line from the yield statement
Modify the code above, calling several times more than the next method, and printing method returns the next result
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
print(next(g))
print("*"*20)
print(next(g))
The above results of the implementation of this code is
starting...
None
********************
res: None
None
We can see, the program is actually running step by step conjecture, but the above program also has a very significant drawback: it is the above code does not have any practical significance: the value of the res is always None
In the actual development, the purpose of the form is the expression yield can get a yield value and yield this value is assigned to a variable, so have actual meaning
How it should operate in order to assign a value to a variable res it? ? That is to call generator itself send method
send method triggers a generator perform, while also pass parameters to yield the send method
to modify the above code
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
next(g)
print(g.send(5))
Results of the implementation of:
starting...
res: 5
None
To analyze the execution of the above code:
1.程序开始执行以后，因为foo函数中有yield关键字，所以foo函数并不会真的执行，而是先得到一个生成器g.
2.直到调用next方法，foo函数正式开始执行，先执行foo函数中的print方法，然后进入while循环
3.程序遇到yield关键字，程序暂停，此时next(g)语句执行完成
4.程序执行g.send(5)，程序会从yield关键字那一行继续向下运行，send会把5这个值传递给yield
5.yield接收到send方法传递过来的值，然后由yield赋值给res变量
6.由于send方法中包含next()方法，所以程序会继续向下运行执行print方法，然后再次进入while循环
7.程序执行再次遇到yield关键字，yield会返回后面的值，由于yield后面没有接任何参数，所以yield会返回None,程序再次暂停，直到再次调用next方法或send方法
Modify the code, call the send method multiple times
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
next(g)
print(g.send(5))
print("*"*20)
print(g.send(10))
print("#"*20)
print(g.send(15))
Executing a program, the following results
starting...
res: 5
None
********************
res: 10
None
####################
res: 15
None
It can be seen as the execution process of the above code performs the same operation above analysis
In the example above, if the call to send method, passing parameters to None, the results obtained will be like it? ?
From the above analysis, it is possible to know:
If the g.send()method of transmitting the parameters to yield keyword is None, then the yield value passed to the key variable to be None res
For the latter did not take any value would yield, the default value is also returned to None yield, so the program execution result will be a None two
modify the code to verify the above conjecture
def foo():
print("starting...")
while True:
res = yield
print("res:",res)
g = foo()
next(g)
print("#"*20)
print(g.send(None))
View the results of the program
starting...
####################
res: None
None
As can be seen from the execution result of the program, if the method of a producer call to send, parameters passed as None, the result will be two programs executed None
Use the form to achieve expression yield linux system "grep -rl root / etc" command
code show as below:
import os
def init(func):
def wrapper(*args, **kwargs):
g = func(*args, **kwargs)
next(g)
return g
return wrapper
@init
def get_file_path(target):
"""
get file abspath
# 阶段一：递归找文件的绝对路径，把文件的完事路径发送给阶段二
:param target:
:return:
"""
while True:
start_path = yield
g = os.walk(start_path)
for parent_dir, _, files in g:
for file in files:
file_path = r"%s\%s" % (parent_dir, file)
target.send(file_path)
@init
def opener(target):
"""
get file obj
# 阶段二：收到文件的完整路径，打开文件获取文件对象，把文件对象发送给阶段三
:param target:
:return:
"""
while True:
file_path = yield
with open(file_path, encoding='utf-8') as f:
target.send((file_path, f))
@init
def cat_file(target):
"""
read file content
# 阶段三：收到文件对象，for循环读取文件的每一行内容，把每一行内容发给阶段四
:param target:
:return:
"""
while True:
file_path, f = yield
for line in f:
file_content = target.send((file_path, line))
if file_content:
break
@init
def grep(target, pattern):
"""
grep function
# 阶段四：收到文件的一行内容，判断要查找的内容是否在这一行中，如果在，则把文件名发送给阶段五
:param target:
:param pattern:
:return:
"""
tag = False
while True:
file_path, line = yield tag
tag = False
if pattern in line:
target.send(file_path)
tag = True
@init
def printer():
"""
print file name
# 阶段五：收到文件名，打印结果
:return:
"""
while True:
filename = yield
print(filename)
path1 = "/root" # 定义要搜索的路径
path2 = "/etc" # 定义要搜索的路径
g = get_file_path(opener(cat_file(grep(printer(), "root"))))
print(g)
g.send(path1)
g.send(path2)
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summary
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