Guido van Rossum added the comment:
So I think the test function here should be:
def f(*a, **k): print(list(a), list(k))
Then we can try things like:
f(x for x in ['ab', 'cd'])
which prints a generator object, because this is interpreted as an argument
that's a generator expression.
But now let's consider:
f(*x for x in ['ab', 'cd'])
I personally expected this to be equivalent to:
f(*'ab', *'cd')
IOW:
f('a', 'b', 'c', 'd')
However, it seems your current patch (#18) interprets it as still passing a
single argument which is the generator expression (*x for x in ['ab', 'cd'])
which in turn is equivalent to iter(['a', 'b', 'c', 'd']), IOW f() is called
with a single argument that is a generator.
The PEP doesn't give clarity on what to do here. The question now is, should
we interpret things like *x for x in ... as an extended form of generator
expression, or as an extended form of *arg? I somehow think the latter is more
useful and also the more logical extension.
My reasoning is that the PEP supports things like f(*a, *b) and it would be
fairly logical to interpret f(*x for x in xs) as doing the *x thing for each x
in the list xs.
I think this same interpretation works for [*x for x in xs] and {*x for x in
xs}, and we can also extend it to ** in {} and in calls (obviously ** has no
meaning in list comprehensions or generator expressions).
BTW I think I found another bug in patch #18:
>>> {**1}
1
>>>
That should be an error.
An edge case I'm not sure about: should {**x} accept an iterable of (key,
value) pairs, like dict(x)?
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