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The new _asyncio module of Python 3.6 uses the _PyObject_CallMethodId() function to call functions. This function has a weird behaviour when using the format string "O": if the object is a tuple, the tuple is unpacked.
_PyObject_CallMethodId(obj, &PyId_meth, "O", tuple, NULL) calls obj.meth(*tuple) instead of obj.meth(tuple).
I only found one function which may have the bug: task_call_step(). But it seems like this function cannot be called with a tuple as "arg", only with an exception object.
But just in case, I would suggest to replace:
 _PyObject_CallMethodId(obj, nameid, "O", arg);
with
 _PyObject_CallMethodIdObjArgs(obj, nameid, arg, NULL);
Note: _PyObject_CallMethodId() is called with a NULL terminal in the argument list, but the NULL is useless. A terminator is only required by _PyObject_CallMethodIdObjArgs(). Yeah, Python has a wide choice of functions to call a callable object, with funny APIs... And I'm adding new ones to Python 3.7 ;-)

Previous similar bug in generators: issue #21209.

The issue was in fixed in Python 3.7 with a change made for optimization, not directly to fix this issue:
changeset: 105547:b29c719d5992
tag: tip
user: Victor Stinner <victor.stinner@gmail.com>
date: Fri Dec 09 14:24:02 2016 +0100
files: Modules/_asynciomodule.c
description:
Use _PyObject_CallMethodIdObjArgs() in _asyncio
Issue #28915: Replace _PyObject_CallMethodId() with
_PyObject_CallMethodIdObjArgs() when the format string was only made of "O"
formats, PyObject* arguments.
_PyObject_CallMethodIdObjArgs() avoids the creation of a temporary tuple and
doesn't have to parse a format string.

Minimum patch for Python 3.6: only modify calls using the "O" format.

New changeset f2745b64d8b7 by Victor Stinner in branch '3.6':
_asyncio uses _PyObject_CallMethodIdObjArgs()
https://hg.python.org/cpython/rev/f2745b64d8b7