I thank Chris and Serhiy for their helpful comments. I agree with both of you, that a lambda won't be able to give the desired result. However, although flawed the idea was a useful stepping stone for me. Perhaps as "the simplest thing that could possibly work".

I thank Serhiy for saying that

>>> with helper(map(open, ['a', 'b', 'c'])) as a, b, c:

>>> pass

can work.

Serhiy suggests

https://docs.python.org/3/library/contextlib.html#contextlib.ExitStack

>>> with ExitStack() as stack:
>>> files = [stack.enter_context(open(fname)) for fname in filenames]

A simpler alternative, for the OP's original post, could be:

>>> with ExitMap(open, ['a', 'b', 'c']) as a, b, c:

>>> pass

where ExitMap(*args) is equivalent to helper(map(*args)). Of course, using ExitStack is the easy way to code ExitMap.

In any case, I think that Chris, Serhiy and myself agree that the OP's problem is best solved using the capabilities that Python already has.

Jonathan