On Wed, Oct 12, 2016 at 12:38 PM, אלעזר <elaz...@gmail.com> wrote:
> What is the intuition behind [1, *x, 5]? The starred expression is
> replaced with a comma-separated sequence of its elements.
>
I've never actually used the `[1, *x, 5]` form. And therefore, of course,
I've never taught it either (I teach Python for a living nowadays). I
think that syntax already perhaps goes too far, actually; but I can
understand it relatively easily by analogy with:
a, *b, c = range(10)
But the way I think about or explain either of those is "gather the extra
items from the sequence." That works in both those contexts. In contrast:
>>> *b = range(10)
SyntaxError: starred assignment target must be in a list or tuple
Since nothing was assigned to a non-unpacked variable, nothing is "extra
items" in the same sense. So failure feels right to me. I understand that
"convert an iterable to a list" is conceptually available for that line,
but we already have `list(it)` around, so it would be redundant and
slightly confusing.
What seems to be wanted with `[*foo for foo in bar]` is basically just
`flatten(bar)`. The latter feels like a better spelling, and the recipes
in itertools docs give an implementation already (a one-liner).
We do have a possibility of writing this:
>>> [(*stuff,) for stuff in [range(-5,-1), range(5)]]
[(-5, -4, -3, -2), (0, 1, 2, 3, 4)]
That's not flattened, as it should not be. But it is very confusing to
have `[(*stuff) for stuff in ...]` behave differently than that. It's much
more natural—and much more explicit—to write:
>>> [item for seq in [range(-5,-1), range(5)] for item in seq]
[-5, -4, -3, -2, 0, 1, 2, 3, 4]
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