__exit__ 在Python中

上下文管理器用于管理程序使用的资源。使用完成后，我们要释放内存并终止文件之间的连接。如果不释放它们，则会导致资源泄漏，并可能导致系统变慢或崩溃。即使我们不释放资源，上下文管理器也会隐式执行此任务。

Refer the below article to get the idea about basics of Context Manager.

Context Manager

__exit__() 方法

这是ContextManager类的一个方法。 __exit__方法负责释放当前代码片段占用的资源。在我们处理完资源之后，无论如何都必须执行此方法。此方法包含正确关闭资源处理程序的指令，以便释放资源以供操作系统中的其他程序进一步使用。

如果引发异常；它的类型、值和回溯作为参数传递给__exit__() 。否则，将提供三个None参数。如果异常被抑制，则__exit__()方法的返回值将为True ，否则为False 。

syntax: __exit__(self, exception_type, exception_value, exception_traceback)

parameters:
exception_type: indicates class of exception.
exception_value: indicates type of exception . like divide_by_zero error, floating_point_error, which are types of arithmetic exception.
exception_traceback: traceback is a report which has all of the information needed to solve the exception.

# 示例 1: .

# Python program creating a 
# context manager 
    
class ContextManager(): 
    def __init__(self): 
        print('init method called') 
            
    def __enter__(self): 
        print('enter method called') 
        return self
        
    def __exit__(self, exc_type, exc_value, exc_traceback): 
        print('exit method called') 
    
    
with ContextManager() as manager: 
    print('with statement block')

输出 ：

init method called
enter method called
with statement block
exit method called

# 示例 2：了解__exit__()的参数。我们将创建一个上下文管理器，用于划分两个数字。如果

# Python program to demonstrate
# __exit__ method
  
class Divide:
    def __init__(self, num1, num2):
        self.num1 = num1
        self.num2 = num2
  
    def __enter__(self):
        print("Inside __enter__")
        return self
  
    def __exit__(self, exc_type, exc_value, traceback):
        print("\nInside __exit__")
        print("\nExecution type:", exc_type)
        print("\nExecution value:", exc_value)
        print("\nTraceback:", traceback)
  
    def divide_by_zero(self):
        # causes ZeroDivisionError exception
        print(self.num1 / self.num2)
  
  
# Driver's code
with Divide(3, 1) as r:
    r.divide_by_zero()
  
print("................................................")
  
# will raise a ZeroDivisionError
with Divide(3, 0) as r:
    r.divide_by_zero()

输出：

Inside __enter__
3.0

Inside __exit__

Execution type: None

Execution value: None

Traceback: None
................................................
Inside __enter__

Inside __exit__

Execution type: 

Execution value: division by zero

Traceback: 
Traceback (most recent call last):
  File "gfg.py", line 32, in 
    r.divide_by_zero()
  File "gfg.py", line 21, in divide_by_zero
    print(self.num1 / self.num2)
ZeroDivisionError: division by zero