Python context manager context manager (with ... as ...)

First, the concept

Context Manager: the object is to achieve a context management protocol. Mainly used to save and restore all kinds of global state, close the file and so on. Context Manager itself is a decorator.

Context allows to automatically start and end, and a number of things. For example, when use with ... as opening a file, python automatically creates a context manager.

Second, the context management protocol

Context Management protocol comprises two methods:

contextmanager .__ enter __ (): the former with statement block of code, performs The __enter __ (), the returned value is assigned to the variable with the sentence as.

contextmanager .__ exit __ (): block of code with statement ends or an error, performs __exit __ ().

Third, with ... as an example instead of try ... except ... finally open the file

# Error writing 
the try : 
    f = Open ( " xxx " ) 
    do something 
the except : 
    do something 
a finally : 
    f.close 

# ################# 

# correct wording 
'' ' write finally, because to prevent the program can not be thrown finally close the file, 
but need to close the file has a premise that the file has been opened. In the first paragraph of the error code, 
if an exception occurs when f = open ( 'xxx'), such as the file does not exist, 
immediately you can know the execution f.close () is meaningless, after the correct solution is the first Sec Code '' ' 

the try : 
    F = Open ( " XXX") 
the except :
     Print ( " Fail to Open " )
    exit(-1)
try:
    do something
except:
    do something
finally:
    f.close()

with...as...:

# Open a file and ensure that it finally closed 
with Open ( " xxx " ) AS f: 
    the Data = f.read () 
    do something with the Data 

# to open multiple 
with Open ( " xxx " ) AS f1, Open ( " yyy " ) F2 AS: 
   do something with f1, F2

Fourth, the custom context manager

class MyContextManager(object):
    def __init__(self):
        print("init 函数被调用")

    def __enter__(self):
        print("enter函数被调用，返回一个对象")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("exit 函数被调用")

with MyContextManager():
    print("创建实例做点什么")

################
#结果：
init 函数被调用
enter函数被调用，返回一个对象
创建实例做点什么
exit 函数被调用

Python context manager context manager (with ... as ...)

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