Python __exit__用法及代码示例

上下文管理器用于管理程序使用的资源。使用完成后，我们必须释放内存并终止文件之间的连接。如果不释放它们，则将导致资源泄漏，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

这是一种方法ContextManager类。的__exit__ 方法负责释放当前代码段占用的资源。无论我们在处理完资源后如何执行此方法，都必须执行。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OS中的其他程序进一步使用。

如果引发异常；其类型，值和追溯作为参数传递给__exit__()。不然三None 提供了参数。如果抑制了异常，则从__exit__()方法将是True， 除此以外，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：了解参数__exit__()。我们将创建一个上下文管理器，用于将两个数相除。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