struct problem

[Gordon McMillan]

Paul Stillwell writes:

> I'm not sure why the following code doesn't work.  I am reading a
> file and want to skip over the first 2 bytes of the file and read
> the next 2 bytes. So, I am doing this:
> 
>         from struct import *
> 
>         HeaderFormat = '2x2B'
>         HeaderLength = calcsize(HeaderFormat)
>         File = open('d:\starview', 'rb')
>         Header = File.read(HeaderLength)
>         (NumDomains, Version) = unpack(HeaderFormat, Header)
> 
> but when I run this code I get:
> 
>     (NumDomains, Version) = unpack(HeaderFormat, Header)
> error: unpack str size does not match format
> 
> So, what am I doing wrong?  I can read the first 2 bytes into don't
> care variables, but why should I have to?  I would think the pad
> byte format would skip the first 2 bytes and just return the 2 bytes
> I am interested in.

There's nothing wrong with your struct code. That leaves
  File = open('d:\starview', 'rb')
or
  Header = File.read(HeaderLength)

as the problem. Symptom indicates you didn't read 4 bytes. Perusal 
reveals a single backslash in the filename, (though you didn't get an 
IOError).

One or the other.

- Gordon