Context Managers with `with` statement in python

What is a Context Manager?

It's an object, that is notified when a context starts and ends, of a class that implements the 2 magic methods __enter__ and __exit__ and any other methods if needed.

For example: when a context manager ends, the object file is closed.

with open('test_ch30.txt' , 'w+') as file:
    file.write('Blew the lid of ur life')

 # the open file has automaticlly been clossed

Context manager is commonly used with reading and writing files to assist in conserving system memory and improve resource management by ensuring that each process holds its resources while executing.

Opening a file using with statement ensures that File descriptor are closed automatically.

How with statment implements a context manager?

1- When an object is called with a with statment, the
interpreter invokes the __eneter__ method ,which is used to
call and performe the setup code required and needed for the
process yo be excuted, before the with statment.

2- When the with statments ends, the interpreter invokes the __exit__ method .

Illustartion :

What's required to implement a contect manager class?

class ContextManager(object):
    """docstring for ContextManager"""
    def __init__(self):
        pass

    def __enter__(self):
        print("__enter__()")
        # optionaly return an obj
        return "an instance"

    def __exit__(self , exc_type , exc_val , traceback):
        print("__exit__ with " , exc_val if exc_val else ' nope')


with ContextManager() as e:
    print("is a " , e)

Multiple context managers

with open('test_ch30.txt') as input_file, open('test_ch77.txt', 'w') as output_file:
    # do something with both files.
    # e.g. copy the contents of input_file into output_file
    for line in input_file:
        output_file.write(line + '\n')

# It has the same effect as nesting context managers:
with open('test_ch30.txt') as input_file:
    with open('test_ch77.txt', 'w') as output_file:
        for line in input_file:
            output_file.write(line + '\n')

Manage Resources

class File():
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
    def __enter__(self):
        self.open_file = open(self.filename, self.mode)
        return self.open_file
    def __exit__(self, *args):
        self.open_file.close()


for _ in range(10000):
    with File('test_ch30.txt', 'w') as f:
        f.write('foo')