How do you use multiple context managers at once?

Question presented to candidate: "Say you need to read from one file and write to another, and hold a lock while you do it. How do you manage all three with with? And what if the number of files is only known at run time?"

What a strong answer should cover:

• with a() as x, b() as y: is exactly equivalent to nesting one with inside another; the parenthesised, one-per-line form is official since Python 3.10.
• Order: managers are entered left to right and exited right to left (last in, first out), so a later manager can use an earlier one.
• Failure part-way: if a later __enter__ raises, the managers already entered are still exited; the one that failed is not.
• Exceptions travel inside out: the innermost __exit__ sees the error first and can suppress it, and then outer managers see a clean exit.
• contextlib.ExitStack for a dynamic number of managers: enter_context(), callback(), and pop_all() for all-or-nothing acquisition; AsyncExitStack for async with.
• Look-alikes that are wrong: with (a, b) as t: makes a tuple, and with a and b: enters only one.

Clarifying questions expected:

• Is the number of resources fixed at write time, or does it depend on input?
• Must acquisition be all-or-nothing, and should the resources outlive the function that opens them?

Code / implementation expected: Yes — a comma-separated with showing enter/exit order, and an ExitStack loop over a list of resources.