[Python-ideas] Grammar for plus and minus unary ops
bruce at leapyear.org
Sat Mar 28 04:16:05 CET 2009
If you want to make sure that you can't use ++ or --, then target that
directly: add ++ and -- as tokens to the language and make them always
illegal. While that might be a bit of a kludge, I think it's far better than
adding a complicated rule that you can't put two + or two - in a row.
And if you're worried about eval('+' + somecode), you've got three choices:
(1) leave out the '+' because it has no effect; (2) write eval('+ ' +
somecode) and (3) are you sure you really want to use eval?
On Fri, Mar 27, 2009 at 4:49 PM, Terry Reedy <tjreedy at udel.edu> 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():
>>> attempt = 0
>>> while attempt<MAX:
>>> ret = bar()
>>> if ret: break
>>> 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.
> A complete test suite would include such a case ;-).
> 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...)
> 1) This would be a petty, gratuitous restriction that would only complicate
> the language and make it harder to learn for no real gain.
> 2) It could break code. + ob maps to type(ob).__pos__(ob), which could do
> anything, and no necessary just return ob as you are assuming.
> 3) Consider eval('+' + somecode). Suppose somecode happens to start with
> '+'. Currently the redundancy is harmless if not meaningful.
> In summary, I think the following applies here:
> "Special cases aren't special enough to break the rules."
> Terry Jan Reedy
> Python-ideas mailing list
> Python-ideas at python.org
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