What is a context manager?

Question presented to candidate: "You write with open(path) as f: all the time. What is actually going on there? Explain what a context manager is, how you would write your own, and what happens when the code inside the block raises."

What a strong answer should cover:

• A context manager is an object that sets something up on entry and is guaranteed to tear it down on exit; the with statement drives it, and it is a structured replacement for try/finally.
• The protocol: __enter__() runs first and its return value is what as binds; __exit__(exc_type, exc_value, traceback) always runs, on normal exit, return, break or an exception.
• Exception handling: __exit__ receives the exception, and a truthy return value suppresses it; returning None/False lets it propagate.
• Two ways to write one: a class with __enter__/__exit__, or a generator decorated with contextlib.contextmanager (the code after yield must sit in finally to run on errors).
• Real uses and stdlib helpers: files, locks, transactions, temporary state; contextlib.suppress, nullcontext, closing, ExitStack, plus async with for async managers.
• A CPython detail: __enter__/__exit__ are looked up on the type, not the instance, and in 3.11 the bytecode is BEFORE_WITH plus WITH_EXCEPT_START.

Code / implementation expected: Yes — a small class-based manager and the same thing written with @contextmanager, showing what happens when the body raises.