[Python-ideas] Grammar for plus and minus unary ops
steve at pearwood.info
Sat Mar 28 05:13:20 CET 2009
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
> 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*).
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