Python | What is a context manager and how do you create one using the “enter” and “exit” methods?

A context manager is an object in Python that provides a convenient way to manage resources such as files, sockets, or database connections. It is typically used with the with statement to ensure that resources are properly acquired and released when needed. The primary use case for context managers is to simplify the code and reduce the chances of bugs related to resource management.

A context manager is defined by implementing two methods: __enter__() and __exit__().

__enter__() is called when entering the with block and is responsible for setting up and acquiring the resource. It usually returns the resource or self, depending on the use case.

__exit__() is called when exiting the with block, regardless of whether an exception occurred or not. It is responsible for releasing or cleaning up the resource. It takes three arguments: the exception type, exception value, and traceback. If there was no exception, these arguments will be None.

To create a context manager using __enter__ and __exit__ methods, you can define a class with these methods:

class TheContextManager:
    def __enter__(self):
        print("Entering the context")
        # Acquire the resource and return it or return self
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print("Exiting the context")
        # Release or clean up the resource
        # If you handle the exception and don't want to propagate it, return True

# Usage
with TheContextManager() as cm:
    print("Inside the context")

This would be the output:

Entering the context
Inside the context
Exiting the context

If you need to create a context manager for a simple use case, you can use the contextlib module which provides a decorator @contextlib.contextmanager for creating context managers using generator functions.