Build a Context Manager in Python with contextlib.contextmanager

Create a reusable context manager that safely opens and closes files using the contextlib contextmanager decorator.

Easy Python 3.9+ Aug 9, 2026 Functions & basics 14 views 0 copies

context manager contextlib file handling

Python code

18 lines

Python 3.9+

from contextlib import contextmanager

@contextmanager
def managed_file(filename, mode='r'):
    """Context manager that opens and closes a file safely."""
    file = open(filename, mode)
    yield file
    file.close()

if __name__ == "__main__":
    # Write a sample file
    with managed_file("sample.txt", "w") as f:
        f.write("Hello, context managers!")

    # Read the file back using the context manager
    with managed_file("sample.txt") as f:
        content = f.read()
        print(content)

Output

stdout

Hello, context managers!

How it works

The @contextmanager decorator lets you write a context manager as a generator function. The yield statement splits the function into two parts: code before it runs on __enter__, and code after it runs on __exit__. Here, the file is opened before the yield and closed after the block finishes. This ensures the resource is always released, even if an exception occurs. Using the decorator avoids writing a class with __enter__ and __exit__ methods for simple use cases.

Common mistakes

Forgetting to wrap the yield in a try/finally block if you want to handle exceptions or ensure cleanup on errors.
Using this pattern when you need a manager that supports re-entry or multiple times.
Not specifying the correct file mode, which can cause errors like reading a file opened without 'r'.

Variations

Write the same manager as a class with `__enter__` and `__exit__` methods for more control.
Use `open()` directly with the `with` statement for simpler file handling without a custom manager.

Real-world use cases

Managing database connections so they always close after a transaction block.
Temporarily changing working directory and restoring it afterwards.
Timing code execution blocks while ensuring the timer stops correctly.

Build a Context Manager in Python with contextlib.contextmanager

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

Keep learning

Tutorials

Quizzes