Nick Coghlan added the comment:

Making sure I'm following the issue correctly here:

1. wrapper is a normal function, so there's no change for 
set_couroutine_wrapper() to detect anything might be amiss

2. any "async def" statement will call the registered coroutine wrapper to wrap 
the created function and turn it into a coroutine

3. this means any coroutine wrapper that directly or indirectly includes an 
"async def" statement will fail with RecursionError, without the problem being 
at all obvious

4. Yury's proposed patch effectively detects the use of "async def" inside a 
coroutine wrapper definition

I like the idea in principle, I don't like the error message in the current 
patch (since it only makes sense if you already understand the chain of 
reasoning above).

While it's a little verbose, I suggest an error like: "Coroutine wrapper %r 
attempted to recursively wrap %r", passing in the currently registered 
coroutine wrapper, and the code object we're attempting to wrap, respectively.

The latter repr gives the name, filename and line number of the offending code 
object, while the former should give the qualname of the registered wrapper.

The docs for set_coroutine_wrapper() should also be tweaked to note the 
constraint that the wrapper function cannot itself define new asynchronous 
functions (neither directly nor indirectly).

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue24342>
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