Python context Management protocol, which is the detailed use of with

First, with obj as F:

#代码块 ...

Second, the implementation process:

1.with obj---> Trigger obj.__enter__ (), need to write __enter__ (self) in obj, write return value inside it, return to as F

2.with obj as f equals f = obj.__enter__ ()

3. Execute the program in the With code block

4. When normal execution occurs, the __exit__ () method in obj is executed sequentially, Exc_type, Exc_val, and EXC_TB values are none.

When there is an exception, the __exit__ () method

A. Return false when False, throws a system exception.

B. Return True when returning false, swallow the system exception and customize the exception information yourself.

Third, its role: automatically clean up/release the resources in the code block, no need to consider these issues

classFoo:def __init__(SELF,AA): Self.aa=AAdef __enter__(self):Print('Run Enter')        return Selfdef __exit__(self, exc_type, Exc_val, EXC_TB):Print('Run Exit')        Print(Exc_type)#exception classes such as: Nameerror        Print(Exc_val)#exception values such as: Name ' MyName ' is not defined        Print(EXC_TB)#tracking information such as: Traceback (most recent): XXXX        returnTrueWith Foo (11) as cc:Print(CC.AA)Print(CC.FF)Print('ssss')

Common Exception components

Traceback (most recent):(tracking information started)

File "d:/workspace/purepython3/exception. Py", line 1, <module>

MyName (end of tracking information)

Nameerror (Exception Class): Name ' myname ' is not defined (exception value)

Python context Management protocol, which is the detailed use of with