(This is a reply to Joe's post on python-dev) That looks like a good solution. The downside I see with your rules is that combinations like "~+~-~ +~-" would still be valid, but if people want to write obfuscated code, there are always ways to do it. Forbidding the examples that you gave (and the ones I gave) is still a positive move, in my opinion. Jared On 27 Mar 2009, at 12:15, Joe Smith wrote:

Jared Grubb wrote:

...

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...)

So you want something like u_expr :: = power | "-" xyzzy_expr | "+" xyzzy_expr | "\~" u_expr xyzzy_expr :: = power | "\~" u_expr

Such that: 5 # valid u_expr +5 # valid u_expr -5 # valid u_expr ~5 # valid u_expr ~~5 # valid u_expr ~+5 # valid u_expr +~5 # valid u_expr ~-5 # valid u_expr -~5 # valid u_expr +~-5# valid u_expr

++5 # not valid u_expr +-5 # not valid u_expr -+5 # not valid u_expr --5 # not valid u_expr

While, I'm not a python developer, (just a python user) that sounds reasonable to me, as long as this does not silently change the meaning of any expression, but only noisily breaks programs, and that the broken constructs are not used frequently.

Can anybody come up with any expressions that would silently change in meaning if the above were applied?

Obviously a sane name would need to be chosen to replace xyzzy_expr.