struct unpack issue

[Peter Otten]

Ping Zhao wrote:

> I am writing a small program to decode MS bitmap image. When I use
> statements as follow, it works fine:
> 
> header['sig'] =  str(struct.unpack('2s', self.__read(src, 2))[0])
> header['len'] =  int(struct.unpack('1i', self.__read(src, 4))[0])
> 
> However, when I tried to rewrite them in short:
> 
> header = struct.unpack('2s1i', self.__read(src, 6))
> 
> The Python interpreter in my Linux box came up with an error:
> 
> ...
> header = struct.unpack('2s1i', self.__read(src, 6))
> File "/usr/lib/python2.5/struct.py", line 87, in unpack
> return o.unpack(s)
> struct.error: unpack requires a string argument of length 8
> 
> It was weired that the required argument length increased to 8. Any idea
> on this? I am using a 32bit pentium-m and the picture file was stored in
> little-edian format.

Try specifying byte order and alignment:

>>> struct.unpack("2s1i", "123456")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/struct.py", line 87, in unpack
    return o.unpack(s)
struct.error: unpack requires a string argument of length 8

>>> struct.unpack("=2s1i", "123456")
('12', 909456435)

See the second table on http://docs.python.org/lib/module-struct.html for
the options you have.

Peter