On Sat, Apr 18, 2020 at 12:19:30PM -0400, David Mertz wrote:
> On Sat, Apr 18, 2020, 8:15 AM Alex Hall <[email protected]> wrote:
>
> > Under your proposal, these two programs are both valid syntax with
> > different meanings:
> >
> > (1)
> >
> f(**{u, v})
> >
> > (2)
> >
> x = {u, v}
> > f(**x)
> >
>
> I don't understand what the meaning of (2) would be. Currently it is a
> TypeError... Is that "valid" because it's not a syntax error?!
Yes, as Andrew pointed out, it is valid syntax: it isn't rejected by the
compiler, it generates byte-code, and then when you run it, it has
behaves differently from (1).
So it has a meaning: "raise TypeError in an inefficient fashion". Not
*useful* meaning, to be sure, but still meaning.
I think a fundamental point is that `**{identifier}` looks like
you are applying `**` unpacking to a set, but you actually aren't, it is
a special syntactic form.
If that disturbs you, I'm not going to say you are wrong. I'm a little
sad about it myself, I just don't give it a very high weighting since we
already have plenty of other similar cases:
func(a, b, c) # Not a tuple.
import a, b, c # Also not a tuple.
del a, b, c # Still not a tuple.
mapping[key] # And not a single-item list either :-)
I think I would be more concerned by this if I wanted this keyword
argument shortcut to work everywhere, not just in function calls. E.g.
mydict = **{a, b c} # like {'a': a, 'b': b, 'c': c}
but I don't care much about that.
--
Steven
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