homepage

>>> set().union(*(None[k] for k in range(5)))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: union() argument after * must be a sequence, not generator
Clearly, exception in not relevant, since next line works:
>>> set().union(*([k] for k in range(5)))
{0, 1, 2, 3, 4}
Correct exception would be
>>> None[1]
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable
Problem is in python function call mechanics.
set().union can be replaced by any callable;
Generator can be replaced by any TypeError-raising iterable. Exceptions other then TypeError are handled correctly.
Python/ceval.c:4322
ext_do_call() converts stararg to tuple.
If any TypeError is raised, it is replaced with
TypeError("%s argument after * must be a sequence, not %s")
Proposed solution:
Probably, we can avoid replacing TypeError. Exceptions in the above cases would become relevant, and
>>> int(*None)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: type object argument after * must be a sequence, not NoneType
would become
>>> int(*None)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not iterable
so exception is still recognizable (and, may be, even more relevant, since we don't actually need _sequence_ as stararg, _iterable_ would be enough).

I unlinked to see if I could link it to 4806 instead. Did not work. Please nosy yourself there and upload your files to *that* issue.