Python Context Managers

What Is A Context Manager?

A context manager controls setup and cleanup around a block of code. The most familiar example is opening a file.

with open("notes.txt", "r", encoding="utf-8") as file:
    content = file.read()

The file is automatically closed when the block ends. This works even if an exception happens inside the block.

The Context Manager Protocol

Objects used in a with statement implement two special methods: __enter__() and __exit__().

class ManagedResource:
    def __enter__(self):
        print("setup")
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print("cleanup")

__enter__() runs before the block. __exit__() runs after the block.

Handling Exceptions

__exit__() receives exception information if the block raises an error.

def __exit__(self, exc_type, exc_value, traceback):
    return False

Returning False means the exception should continue normally. Returning True suppresses the exception, which should be done carefully.

Using contextlib

The contextlib.contextmanager decorator lets you write a context manager as a generator.

from contextlib import contextmanager

@contextmanager
def section(name):
    print(f"enter {name}")
    try:
        yield
    finally:
        print(f"exit {name}")

The code before yield is setup. The code after yield is cleanup.

When Context Managers Help

Use context managers for files, locks, database sessions, temporary settings, timers, and any resource that must be cleaned up reliably.

They make cleanup visible at the call site and reduce forgotten cleanup bugs.

Common Mistakes

A common mistake is doing cleanup only after the with block manually. If an exception happens, manual cleanup may be skipped.

Another mistake is returning True from __exit__() without meaning to. That suppresses exceptions and can hide bugs.

Quick Summary

• A context manager wraps setup and cleanup around a block.
• with calls __enter__() and __exit__().
• The value returned by __enter__() can be bound with as.
• __exit__() receives exception details.
• Returning True from __exit__() suppresses exceptions.
• contextlib.contextmanager can create context managers from generator functions.

Practice Quiz

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