How to Write a Context Manager Class in Python

Define a class with __enter__ and __exit__ to manage file resources safely using the with statement.

Easy Python 3.9+ Aug 9, 2026 Functions & basics 13 views 0 copies

context-manager with-statement resource-management

Python code

23 lines

Python 3.9+

class FileReader:
    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_val, exc_tb):
        if self.file:
            self.file.close()
        return False


if __name__ == "__main__":
    with FileReader("example.txt", "w") as f:
        f.write("Hello, context manager!")

    with FileReader("example.txt") as f:
        content = f.read()
        print(content)

Output

stdout

Hello, context manager!

How it works

The __enter__ method opens the file and returns the file object, which is bound to the as variable in the with statement. The __exit__ method is called automatically when the block exits, closing the file to prevent resource leaks. Returning False from __exit__ propagates any exceptions raised inside the block, allowing normal exception handling. This pattern encapsulates resource cleanup and makes code more readable.

Common mistakes

Forgetting to return the file object from __enter__.
Returning True from __exit__ to suppress exceptions unintentionally.
Not handling the case where __enter__ might fail midway.

Variations

Use the built-in open() function as a context manager instead of a custom class.
Use contextlib.contextmanager decorator to create a generator-based context manager.

Real-world use cases

Automatically releasing database connections or network sockets after a block of operations.
Managing file handles in data processing scripts to avoid corruption from unclosed files.
Implementing custom lock acquisition and release in multi-threaded applications.

How to Write 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 Functions & basics

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