A Bug By Any Other Name ...

Dave Angel davea at ieee.org
Mon Jul 6 12:54:35 EDT 2009

Rhodri James wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">On Mon, 
> 06 Jul 2009 10:58:21 +0100, Steven D'Aprano 
> <steve at remove-this-cybersource.com.au> wrote:
>> On Mon, 06 Jul 2009 02:19:51 -0300, Gabriel Genellina wrote:
>>> En Mon, 06 Jul 2009 00:28:43 -0300, Steven D'Aprano
>>> <steve at remove-this-cybersource.com.au> escribió:
>>>> On Mon, 06 Jul 2009 14:32:46 +1200, Lawrence D'Oliveiro wrote:
>>>>> I wonder how many people have been tripped up by the fact that
>>>>>     ++n
>>>>> and
>>>>>     --n
>>>>> fail silently for numeric-valued n.
>>>> What do you mean, "fail silently"? They do exactly what you should
>>>> expect:
>>>>>>> ++5  # positive of a positive number is positive
>>>> I'm not sure what "bug" you're seeing. Perhaps it's your expectations
>>>> that are buggy, not Python.
>>> Well, those expectations are taken seriously when new features are
>>> introduced into the language - and sometimes the feature is dismissed
>>> just because it would be confusing for some. If a += 1 works, expecting
>>> ++a to have the same meaning is very reasonable (for those coming from
>>> languages with a ++ operator, like C or Java) - more when ++a is a
>>> perfectly valid expression. If this issue isn't listed under the 
>>> various
>>> "Python gotchas" articles, it should...
>> The fact that it isn't suggests strongly to me that it isn't that common
>> a surprise even for Java and C programmers. This is the first time I've
>> seen anyone raise it as an issue.
> Indeed, arguably it's a bug for C compilers to fail to find the valid
> parsing of "++5" as "+(+5)".  All I can say is that I've never even
> accidentally typed that in twenty years of C programming.
But the C language specifically defines the tokenizer as doing a 
max-match, finding the longest legal token at any point.  That's how 
many things that would otherwise be ambiguous are well-defined.  For 
example, if you want to divide two integers, given pointers to them, you 
need a space between the slash and the start.
         *p1/*p2    begins a comment,  while   *p1/ *p2   does a division

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