Re: [Python-ideas] Grammar for plus and minus unary ops
Moved from python-dev to python-ideas. On Sat, 28 Mar 2009 04:19:46 am Jared Grubb wrote:
I was recently reviewing some Python code for a friend who is a C++ programmer, and he had code something like this:
def foo(): try = 0 while try<MAX: ret = bar() if ret: break ++try
I was a bit surprised that this was syntactically valid,
You shouldn't be. Unary operators are inspired by the equivalent mathematical unary operators. ...
It appears that the grammar treats the above example as the unary + op applied twice:
As it should. ...
I'm not a EBNF expert, but it seems that we could modify the grammar to be more restrictive so the above code would not be silently valid. E.g., "++5" and "1+++5" and "1+-+5" are syntax errors, but still keep "1++5", "1+-5", "1-+5" as valid. (Although, '~' throws in a kink... should '~-5' be legal? Seems so...)
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*). -- Steven D'Aprano
Does PyChecker check for uses of '--' and '++'? That would seem like the obvious place to have such a check. Mark
Mark Dickinson wrote:
Does PyChecker check for uses of '--' and '++'? That would seem like the obvious place to have such a check.
Yep, sounds like a pychecker/pylint kind of problem to me as well. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia ---------------------------------------------------------------
On Sat, Mar 28, 2009, Steven D'Aprano wrote:
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*).
In all fairness, "++" is valid in many C-derived languages, so it hits C and C++ programmers, plus Ruby and Perl programmers. I'm not in favor of this restriction, but I'm not opposed, either, and I think your thesis is invalid. -- Aahz (aahz@pythoncraft.com) <*> http://www.pythoncraft.com/ "At Resolver we've found it useful to short-circuit any doubt and just refer to comments in code as 'lies'. :-)" --Michael Foord paraphrases Christian Muirhead on python-dev, 2009-3-22
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.... :) Jared
participants (6)
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Aahz -
Jared Grubb -
Leif Walsh -
Mark Dickinson -
Nick Coghlan -
Steven D'Aprano