Provide an example of using a context manager in Python.

# Opening a file:
# Using a context manager guarantees automatic file closure,
# even if an exception occurs.
with open('my_file.txt', 'w') as f:
    # Writing data to the file
    f.write('Hello, world!\n')
    f.write('This is a test.')

# Example of creating a custom context manager using a class:
class ManagedResource:
    def __enter__(self):
        print('Resource allocated')
        # Can return an object that will be assigned after 'as'
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('Resource released')
        # Can handle exceptions. Returning True suppresses the exception.
        return False

# Using the created context manager:
with ManagedResource() as res:
    print('Working with resource')
    # res is the object returned by __enter__

# Example of creating a custom context manager using the @contextmanager decorator:
from contextlib import contextmanager

@contextmanager
def efficient_resource():
    print('Efficient resource allocated')
    try:
        yield 'Efficient resource' # The object to be returned
    finally:
        print('Efficient resource released')

# Using the context manager with the decorator:
with efficient_resource() as eff_res:
    print(f'Using: {eff_res}')