Syntactic sugar for assignment statements: one value to multiple targets?

gc gc1223 at gmail.com
Wed Aug 17 11:26:42 CEST 2011


On Aug 17, 3:13 am, Chris Angelico <ros... at gmail.com> wrote:

> Minor clarification: You don't want to initialize them to the same
> value, which you can do already:
>
> a=b=c=d=e=dict()

Right. Call the proposed syntax the "instantiate separately for each
target" operator.  (It can be precisely defined as a * on the RHS of a
one-into-many assignment statement--i.e. an assignment statement with
1 object on the RHS and more than 1 on the LHS).

It has only one very modest function, which is to unpack

a, b, c, d, e = *dict()

to

a, b, c, d, e = dict(), dict(), dict(), dict(), dict()

so that you have n separate objects instead of one. If you want the
same object duplicated five times, you'd best use a=b=c=d=e=dict().
(I'd guess that 90% of the people who try the a=b=c version actually
*want* separate objects and are surprised at what they get--I made
that mistake a few times!--but changing either behavior would be a
very bad idea. This proposed syntax would be the Right Way to get
separate objects.)

Maybe this is more visibly convenient with a complex class, like

x, y, z = *SuperComplexClass(param1, param2, kwparam = "3", ...)

where you need three separate objects but don't want to duplicate the
class call (for obvious copy-paste reasons) and where bundling it in a
list comprehension:

x, y, z = [SuperComplexClass(param1, etc, ...) for _ in range(3)]

layers gunk on top of something that's already complex.

> I think it's going to work out something very similar to a
> list comprehension (as has been mentioned).

Right; kind of a self-limiting generator[1], although that sounds MUCH
more complex than it needs to. It's really just sugar. Not that it
this is a suggestion :) but it could easily be done with a pre-
processor. It would also be perfectly amenable to automated code
conversion (i.e. 3to2).

On Aug 16, 10:11 pm, MRAB <pyt... at mrabarnett.plus.com> wrote:

> 1. Repeated evaluation of an expression: "dict()" would be evaluated as
> many times as necessary. In other words, it's an unlimited generator.

> 2. Lazy unpacking: unpacking normally continues until the source is
> exhausted, but here you want it to stop when the destination (the RHS)
> is satisfied.

Yes, this is a good way to think of it. (Although I think you meant to
type LHS, right?) * in this context would tell Python to do both of
these things at once: evaluate successively and unpack lazily to the
destination. Although it's still fully (and more simply) explainable
as sugar.

[1] Self-limiting is the key here. As above, the only proposed methods
which don't require manual tallying on the RHS are Tim's very creative
a,b,c,d,e,*scrap = (dict() for _ in range(9999)), which in this case
creates a list containing 9994 unnecessary dicts, or the purer but
Python-crashing a,b,c,d,e,*scrap = (dict() for _ in
itertools.count()). Tim's intuition, which I share, is that in both
cases *scrap should, since it's in final position, actually become the
generator-in-state-6, rather than slurping the rest into a list. One
argument for this is that the present behavior is (surprisingly,
counterintuitively) identical for list comprehensions and generator
expressions. Changing the outer parens to brackets yields the same
results. But that's a separate, more complex proposal which would need
its own use cases.



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