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Fri Mar 27 20:15:21 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-March/087826.html

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. 

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