Context Managers

Introduction

Context managers (the with protocol) guarantee setup/teardown. Implement via __enter__/__exit__ or the @contextmanager decorator.

Understanding the topic

Core concepts to understand:

• with calls __enter__ / __exit__.
• @contextmanager turns a generator into one.
• Used for files, locks, DB sessions, timers.
• __exit__ can suppress exceptions.

Syntax reference

Visual flow / code:

from contextlib import contextmanager
import time

@contextmanager
def timer(label: str):
    start = time.perf_counter()
    try:
        yield
    finally:
        print(f"{label}: {time.perf_counter()-start:.3f}s")

with timer("db query"):
    run_query()

# Class-based
class Connection:
    def __enter__(self):
        self.conn = connect()
        return self.conn
    def __exit__(self, *exc):
        self.conn.close()

with Connection() as conn:
    conn.execute(...)

Execution workflow

Real-world use

Database sessions, file locks, distributed transactions — all use context managers to guarantee cleanup. Critical for resource safety.

Best practices

• Prefer @contextmanager for simple cases.
• Always release resources in __exit__.
• Use ExitStack for dynamic stacks.

Hands-on exercise

Interview preparation — practice these questions:

• Walk through __enter__/__exit__.
• @contextmanager — how does it work?
• What does ExitStack do?

Summary

In summary: with = guaranteed cleanup. @contextmanager is the easy way.