[Python-ideas] Grammar for plus and minus unary ops

Jared Grubb jared.grubb at gmail.com
Sat Mar 28 20:26:33 CET 2009

On 27 Mar 2009, at 21:13, Steven D'Aprano wrote:
>> I was a bit surprised that this was syntactically valid,
> You shouldn't be. Unary operators are inspired by the equivalent
> mathematical unary operators.
> .....
> Why would we want to do this? I'm sure there are plenty of other  
> syntax
> constructions in Python which just happen to look like something from
> other languages, but have a different meaning. Do we have to chase our
> tails removing every possible syntactically valid string in Python  
> that
> has a different meaning in some other language? Or is C++ somehow
> special that we treat it differently from all the other languages?
> Not only is this a self-inflicted error (writing C++ code in a Python
> program is a PEBCAK error), but it's rare: it only affects a minority
> of C++ programmers, and they are only a minority of Python  
> programmers.
> There's no need to complicate the grammar to prevent this sort of
> error. Keep it simple. ---1 on the proposal (*grin*).

It *was* a surprise. Of the languages I've used in my life (BASIC, C, C 
++, Java, Javascript, Perl, PHP, and Python), only two would treat  
prefix ++ as double unary plus (and I try to forget BASIC as best I  
can :) ). I remember when I first picked up Python, I wrote "i++" once  
(I think many beginning Python programmers do), and I was grateful  
that a syntax error popped up (rather than silently doing nothing) and  
I never did it again... So, now, a few years later I was reviewing  
code that had "++i" in it (from a new Python developer), and did a  
double-take on the code and had a moment of surprise that it had even  
run at all.

As a devil's advocate: any code that requires double-unary plus is  
probably either abusing operator overloading or is abusing the eval  
keyword. It seems that adding a restriction to the grammar would  
probably be more helpful than harmful (the workaround for the alien  
case, if there is one, of needing double-unary plus would be to use  
parens: "+(+x)").

In any case, I understand that dynamic languages are going to allow  
for side effects to occur anywhere, so it's tough to remove it. I'm  
actually only +0 on it as it is... Just a "nice" feature I thought I'd  
throw out there.... :)


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