how many bytes in an int

["Martin v. Löwis"]

Reid Nichol wrote:
> I'm thinking of writing a movie file encoder (probably avi).  So, I need 
> to output DWORD (lookup revealed its a 4-byte int) to a binary file. 
> Therefore I need to know whether this can be done in python or not, 
> which will tell me whether I'll try to do it or not.

You looked up DWORD somewhat incorrectly. It is a four byte int in 
memory, but on disk, it is a little-endian byte string of four bytes.

So you *do* need the struct module, because only that will give you
byte strings (of course, Grant's formula also works)

> But, since the 64-bit archecture is out, short, long, etc may change 
> there meanings quite soon.  From what I've read in the struct module 
> docs I can only tell it that it's a short, long, etc. but not whether 
> it's exactly a 4-byte int.  Is there a way to do this?

As Grant says: use the struct module. Use struct.calcsize to find out
how large an int is. If the size is too large, try a short. If the size
is too small, try a long. If no type matches, take the next larger type,
and drop the extra bytes.

However, it does not actually need to be that difficult: "int" is 32-bit
on all current systems, including all 64-bit systems (only long is 
64-bits on some 64-bit systems).

Regards,
Martin