The contextlib module in Python provides a number of utilities to help in creating and managing context managers.

Context managers offers a more convenient way to automate setup and teardown operations.

Typically, context manger objects are used with  the  with statements. They are marked by two methods i.e __enter__() and __exit__(). 

The __enter__() method is responsible for setting up and allocating the necessary resources for the object, while, the __exit__() method is responsible for cleaning up and freeing up all the resources used by the object.

The following example shows a simple example of a context manager object.

ExampleEdit & Run
class MyContext:
    def __init__(self):
        print('__int__() called')
    def __str__(self):
        return "MyObject()"

    #define the __enter__ method
    def __enter__(self):
        print('__enter__() called')

    #define the __exit__ method
    def __exit__(self, *args, **kwargs):
        print('__exit__() called')

with MyContext() as ctx:
     print('Do Something')
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Output:
__int__() called
__enter__() called
Do Something
__exit__() called
[Finished in 0.011232320219278336s]

As you can see above, when such an object is used with the with statement, the two methods __enter__() and __exit__() are called automatically. The __enter__() method is called when the control flow enters the with block while the __exit__() method is called immediately before the control flow leaves the with block.

A common example of context manager objects are the file objects created using the builtin  open() function. When we use the file objects with the with statement,  the file is automatically closed after the block of code within the with statement has been executed, this alleviates the need to call the file.close() method manually. 

#the file path
myfile = 'demo.txt'

#open the file using the with statement
with open(myfile) as file:
    #do something with the file
    for line in file.readlines():
         print(line) 

#the file is now closed
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Transform a generator function to a context manager

Generator functions are one of the common cases where setup and teardown operations maybe necessary. 

In situations where you have a generator function, it may be trivial to write a new context manager class in the traditional way by defining __enter__() and __exit__() methods. The @contextmanager decorator in the contextlib module allows you to transform a generator function into a context manager in a convenient way and without having to explicitly define the __enter__() and __exit__() methods.

ExampleEdit & Run
from contextlib import contextmanager

@contextmanager
def square(n):
    '''a generator function to get a list of squares of numbers from 0 to n'''
    try:
        data = []
        for i in range(n):
            data.append(i ** 2)
        yield data
    except RuntimeError as e:
          print('An erro occured')
    finally:
          print('Done')
     
with square(5) as sq:
     for s in sq:
         print(s)
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Output:
0
1
4
9
16
Done
[Finished in 0.013107263017445803s]

The generator function will have to yield exactly once, the value yielded will be assigned to the variable specified by the as clause. Any exception raised inside the with block are sent to the generator to be handled. 

Objects with a close() method

If an object needs to be closed after a task is done and has defined the close() method, we can use the closing() function to close the object automatically even if it does not have the __enter__() and __exit__() methods defined. 

ExampleEdit & Run
from contextlib import closing

class School:
    def __init__(self):
         print('__init__() called')
    def close(self):
        print('close() called')

with closing(School()) as sch:
     print('inside with block')
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Output:
A builtin module you are using is currently not supported.