h2py.py bug?

[Gabriel Genellina]

En Wed, 11 Jun 2008 04:21:03 -0300, Gabriel Rossetti  
<gabriel.rossetti at arimaz.com> escribió:

> Gabriel Genellina wrote:
>> En Tue, 10 Jun 2008 09:44:13 -0300, Gabriel Rossetti  
>> <gabriel.rossetti at arimaz.com> escribió:
>>
>>> I wanted to use the h2py.py script (Tools/scripts/h2py.py) and it  
>>> didn't like char litterals :
>>>
>>> Skipping: PC_ERROR = ord()
>>>
>>> where my *.h file contained :
>>>
>>> #define PC_ERROR '0'
>>>
>>> I searched the web and found a post with the same error :
>>>
>>> http://mail.python.org/pipermail/python-list/2005-September/340608.html
>>>
>>> but it got no replies, I tried the fix and it works. I have the  
>>> following questions:
>>>
>>> 1) Why did it not get any attention, is something wrong with it?
>>> 2) If nothing is wrong, did the fix not get applied because a bug  
>>> report wasn't filed?
>>
>> Very probably - bug reports outside the tracker are likely to go  
>> unnoticed or forgotten.
>>
> Ok, I'll file one then.
>>> 3) Isn't turning a char literal into the ordinal value not contrary to  
>>> what a C programmer had in mind when he/she defined it? I mean if you  
>>> define a char literal then in python you would have used a string  
>>> value :
>>>
>>> #define PC_ERROR '0'
>>>
>>> would become :
>>>
>>> PC_ERROR = '0'
>>>
>>> in python, and if you intended to use the char type for an 8 bit  
>>> numerical value you would have done :
>>>
>>> #define PC_ERROR 0x30
>>>
>>> where 0x30 is the '0' ascii hex value, so shouldn'it the line in the  
>>> diff (see the post) be :
>>>
>>> body = p_char.sub("'\\1'", body)
>>>
>>> instead of :
>>>
>>> body = p_char.sub("ord('\\1')", body)
>>
>> It's not so clear what's the intended usage - chars are also integers  
>> in C. (I prefer the current behavior, but certainly it may be wrong in  
>> several places).
>>
> Yes, true, but if you intend to use it as an integer, wouldn't you use a  
> numeric value instead of a character literal?

Not always. Using characters as ordinal numbers is very common in that  
language. A small example:

#define FIRST 'A'
#define LAST  'Z'
#define index(letter) ((letter)-FIRST)

int accum[LAST-FIRST+1];
...
char x = some_word[0];
accum[index(x)]++;

accum is an array of 26 integers, element 0 corresponding to letter 'A'.  
If one has to reproduce the exact same structure, in Python it would be:

FIRST = ord('A')
LAST = ord('Z')
def index(letter):
   return ord(letter)-FIRST

accum = [0 for i in range(LAST-FIRST+1)]
x = some_word[0]
accum[index(x)] += 1

Of course, in other cases, character constants are intended to be used as  
character data, so ord(...) is not the appropiate thing to do when  
converting to Python. But I don't think my example above is very contrived  
or uncommon - chars *are* integers from the C point of view, and some  
people isn't worried too much about the distinction, specially those who  
use C as their main or only language.

-- 
Gabriel Genellina