[Python-ideas] Generator unpacking

[Steven D'Aprano]

On Mon, Feb 15, 2016 at 02:55:26PM +0100, Sven R. Kunze wrote:
> On 15.02.2016 07:31, Andrew Barnert via Python-ideas wrote:

> >>    [] = []
> >
> >Yes, but that's completely different. The [] on the left isn't an 
> >expression, or even a target, but a target list with 0 targets in it. 

[..]

> Interestingly, doing the following results in an syntax error.
> 
> >>> [1,2]=[3,4]
> 
>   File "<input>", line 1
> SyntaxError: can't assign to literal
> 
> 
> So, if it's illegal to assign to the literal [1,2], I don't see why it 
> should be legal for []. But as you said, that's a highly theoretical 
> problem.


I believe you are misinterpreting the error. The error isn't that you 
are trying to assign to the literal [1,2]. The error is that you are 
trying to assign to the literal 1. It's a subtle difference, but 
important to understand that difference in order to understand why [] is 
a legal assignment target.

py> [a, b, c, 1] = "abcd"
  File "<stdin>", line 1
SyntaxError: can't assign to literal

Obviously [a, b, c, 1] is not a literal, but 1 is. If there is any 
doubt:

py> [a, b, c, None] = "abcd"
  File "<stdin>", line 1
SyntaxError: cannot assign to None


Since [1,2] is a *list of assignment targets*, not a single target, the 
assignment you attempted 

[1, 2] = [3, 4] 

is roughly equivalent to:

1 = 3
2 = 4

which obviously gives a syntax error.

[] is a valid assignment target so long as the right hand side is an 
empty iterable. If it is not empty, you get a TypeError:

py> [] = "abc"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack


There are three values on the right hand side, and zero targets.

This might even be useful. You can confirm that an iterable is empty 
(why you might want to do this, I can't imagine, but suppose you did) by 
assigning it to zero targets:

[] = iterable

succeeds only if iterable has zero items, otherwise it raises 
TypeError.



-- 
Steve