MultiObj has unnecessary special case where a single mapping input is given.

[andeaseme]

This special case is already handled by the named kwargs and is unclear what functionality this adds.

input = {'this': [1], 'that': [1, 2]}
MultiObj(input) == MultiObj(**input)

This prevents another special case where the input is a single unnamed obj of type mapping. The following example does not enter or exit.

class Store(dict):
    def __enter__(self):
        print('enter')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('exit')

store = Store()
with ContextFanout(store):
    pass

[thorwhalen]

The reason for this, as often it is, was to stay close to builtins' interface.
In this case, the dict:

input = {'this': [1], 'that': [1, 2]}
assert dict(input) == dict(**input)

But as you say, this fact stops us from being able to specify a single unnamed object.

I'll let @valentin-feron and @sylvainbonnot express their opinion on this, if they have one, and we can decide together.

In order not to reveal my vote, to not influence you, but give you the assurance that I'm being influenced by yours, here's the sha256 of my vote:

423323bee96b004d96deb0a76f6ba0556b3b29d2cfc38e20149435a30bc7101e

(Computed with https://emn178.github.io/online-tools/sha256.html)