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

[Jared Grubb]

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, and because  
the timeout condition only occurred in exceptional cases, the error  
has not yet caused any problems.

It appears that the grammar treats the above example as the unary + op  
applied twice:

u_expr ::=
              power | "-" u_expr
               | "+" u_expr | "\~" u_expr

Playing in the interpreter, expressions like "1+++++++++5" and  "1+-+- 
+-+-+-+-5" evaluate to 6.

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...)

Jared