To put it simple, unpacking raises ValueError:
x, = ()
Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: not enough values to unpack (expected 1, got 0)
x, = (1, 2)
Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: too many values to unpack (expected 1)
But if the iterator raises ValueError, there's no way to tell it apart from the unpacking:
def foo():
... yield None ... raise ValueError ...
foo()
<generator object foo at 0x7fa0e70e6430>
x = foo() x, = foo()
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in foo ValueError
And the workaround for this is a bit ugly. We already convert e.g. StopIteration into RuntimeError in many cases, why can't we do so here too?
For backwards compatibility, this should probably be an itertools utility tho.