How to Build a Context Manager Class in Python

Create a reusable context manager class that opens and automatically closes resources using the with statement.

Easy Python 3.9+ Aug 9, 2026 OOP & classes 12 views 0 copies

context-manager with-statement resource-management

Python code

23 lines

Python 3.9+

class FileResource:
    def __init__(self, filename, mode='r'):
        self.filename = filename
        self.mode = mode
        self.file = None

    def __enter__(self):
        self.file = open(self.filename, self.mode)
        return self.file

    def __exit__(self, exc_type, exc_value, traceback):
        if self.file:
            self.file.close()
        return False


if __name__ == "__main__":
    with FileResource("sample.txt", "w") as f:
        f.write("Hello, resource management!")

    with FileResource("sample.txt", "r") as f:
        content = f.read()
        print(content)

Output

stdout

Hello, resource management!

How it works

The __enter__ method opens the file and returns it, allowing the with statement to bind it to the as f target. The __exit__ method is called when the block exits, closing the file regardless of whether an exception occurred. Returning False from __exit__ ensures any exceptions propagate, so you can handle them outside. This pattern guarantees cleanup, preventing resource leaks. The class can be reused for different files by instantiating it each time.

Common mistakes

Returning `True` from `__exit__` suppresses exceptions, which can hide bugs.
Forgetting to initialize `self.file` to `None` in `__init__`, causing AttributeError if __enter__ raises.
Assuming the same instance can be reused in multiple with statements; re-entrancy can break.

Variations

Use `@contextmanager` decorator from `contextlib` to define generator-based context manager.
Use `inspect` or `contextlib.ExitStack` for dynamically managing multiple resources.

Real-world use cases

Managing database connections that must be committed or rolled back after each session.
Acquiring and releasing file locks in concurrent applications to prevent data races.
Wrapping network socket connections to ensure they close after data exchange.

How to Build a Context Manager Class in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from OOP & classes

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