[Tutor] Need help with structure unpacking module

[Luke Paireepinart]

Nirmal Sakthi wrote:
> I am using module struct.unpack() to decode data from a binary file, 
> so that I can use the value in a calculation.
>  
> I have been able to extract an integer value.
>  
>      >>>length = struct.unpack('i', '\x9c\x00\x00\x00')
>      >>>length = int(length[0])
>      >>>print length
>      156
>  
> I want to be able to extract a string.
>  
> I have tried,
>  
>     >>>first = struct.unpack('s', '\x02\x00')
>     >>>first = str(first[0])
>     >>>print first
>     Traceback (most recent call last):
> ......
>     error: unpack requires a string argument of length 1
I believe you have to provide the string length for this, like 
struct.unpack('2s', '\x02\x00') or something.
> and,
>  
>     >>>first = struct.unpack('cccc', '\x02\x00')
>     >>>first = str(first[0])
>     >>>print first
>      Traceback (most recent call last):
> ......
>          return o.unpack(s)
>      error: unpack requires a string argument of length 4
That's because \x02\x00 is only 2 characters long.  You don't get direct 
access to the hex, just to the characters.
vals = struct.unpack('cc','\x02\x00') #unpack values
>  
> My desired result would be the string  '0200'.  Actually, I would like 
> to be able to invert the bytes to get '0002'.
 >>> x = ['a','b']
 >>> x.reverse()
 >>> x
['b', 'a']


Sorry I don't have more time to explain, I have to run to class.
HTH,
-Luke