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__exit__ in Python

 4 years ago
source link: https://www.geeksforgeeks.org/__exit__-in-python/
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Context manager is used for managing resources used by the program. After completion of usage, we have to release memory and terminate connections between files. If they are not released then it will lead to resource leakage and may cause the system to either slow down or crash. Even if we do not release resources, context managers implicitly performs this task.

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

Context Manager

__exit__() method

This is a method of ContextManager class. The __exit__ method takes care of releasing the resources occupied with the current code snippet. This method must be executed no matter what after we are done with the resources. This method contains instructions for properly closing the resource handler so that the resource is freed for further use by other programs in the OS.

If an exception is raised; its type, value, and traceback are passed as arguments to __exit__(). Otherwise, three None arguments are supplied. If the exception is suppressed, then the return value from the __exit__() method will be True, otherwise, 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.

# Example 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')

Output :

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

# Example 2: Understanding parameters of __exit__(). We will create a context manager that will be used to divide two numbers. If the

# 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()

Output:

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

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