Context Managers (`with`)

Why this matters

Every open(), requests.Session(), SQLAlchemy session.begin(), and threading.Lock is a context manager. Skipping with blocks is why production servers leak file handles and DB connections. contextlib.contextmanager and ExitStack are quiet superpowers.

Common pitfalls

• Opening files without with — on CPython it works via refcount GC, but breaks on PyPy or under exceptions.
• Forgetting to yield inside a @contextmanager function — the with block silently gets None.
• Swallowing exceptions in __exit__ by returning truthy — only return True when you truly want to suppress.