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

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Wed, 31 Oct 2001 12:39:56 -0800

Bugs item #476858, was opened at 2001-10-31 10:07 You can respond by visiting: http://sourceforge.net/tracker/?func=detail&atid=105470&aid=476858&group_id=5470

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?

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), argsa)

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

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

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.

You can respond by visiting: http://sourceforge.net/tracker/?func=detail&atid=105470&aid=476858&group_id=5470