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Hi,
I find it weird that given this sample code:
def g():
 yield iter(None)
list(*g())
I get this traceback:
Traceback (most recent call last):
 File "a1.py", line 3, in <module>
 list(*g())
TypeError: type object argument after * must be a sequence, not generator
At a minimum the exception message looks awkward to me. With the
proposed patch, the new traceback would be:
Traceback (most recent call last):
 File "a1.py", line 3, in <module>
 list(*g())
 File "a1.py", line 2, in g
 yield iter(None)
TypeError: 'NoneType' object is not iterable

(Btw, the suggestion to check for tp_iter came from Antoine Pitrou. It
seemed fine to me.)

You must check tp_iter as well, not only Py_TPFLAGS_HAVE_ITER.
Py_TPFLAGS_HAVE_ITER only tells you that the tp_iter field exists, not
that it's non-NULL.
(see the code for PyObject_GetIter() in Objects/abstract.c for an example)

Thanks for taking a look, my bad for taking so long to reply.
I'm adding a new patch now. This one, besides the fix needed mentioned
above, disregards tp_iter from old class instances (since they are
always available and tell us nothing).

Is this ready to commit?

See also Issue 4806 

This is one of 4 duplicate issues, 3 with somewhat similar patches, with nearly disjoint nosy lists. Consolidating.