[Python-bugs-list] [ python-Bugs-476858 ] Assignment to () should be legal

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Bugs item #476858, was opened at 2001-10-31 10:07
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Category: Parser/Compiler
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Guido van Rossum (gvanrossum)
Assigned to: Nobody/Anonymous (nobody)
Summary: Assignment to () should be legal

Initial Comment:
>From c.l.py:

Currently, 

    () = x

gives a compile-time error.

This should really be allowed (and require that x is an
empty sequence, of course) as an end case of

    (a,b,c) = x  # x must be a 3-sequence
    (a,b) = x   # x must be a 2-sequence
    (a,) = x    # x must be a 1-sequence
    () = x    # why can't x be z 0-sequence?



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>Comment By: Skip Montanaro (montanaro)
Date: 2001-10-31 12:39

Message:
Logged In: YES 
user_id=44345

I won't try and counter Tim's vote (besides, he's already got Guido's 51% vote to contend with), but just provide an example used in c.l.py.  You might have a function that can return a possibly-empty tuple as part of a larger sequence, e.g.:

def contrived(a, *args):
    return (len(args), args*a)

(foo, (bar,)) = contrived(1,2,3)
(foo, ()) = contrived(0,1,2)

Some folks view this as useful.  I reserve judgement on that. ;-)


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Comment By: Tim Peters (tim_one)
Date: 2001-10-31 10:37

Message:
Logged In: YES 
user_id=31435

-1.  "Assignment statements are used to (re)bind names to 
values and to modify attributes or items of mutable 
objects" (from the Ref Man).  Since the degenerate cases 
(don't forget "[] = x" too) don't do that, they're 
not "assignment statements" in a meanignful sense; they 
would just be a surprising way to spell

if tuple(x):
    raise ValueError

That isn't a frequent enough need to deserve special syntax.

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