Extracting 3-byte integers

[John Machin]

On 27/06/2006 9:36 AM, Bob Greschke wrote:
> I have some binary data read from a file that is arranged like
> 
>    <3-byte int> <3-byte int> <3-byte int> etc.
> 
> The "ints" are big-endian and there are 169 of them.  Is there any clever 
> way to convert these to regular Python ints other than (struct) unpack'ing 
> them one at a time and doing the math?
> 

I'd call that "arithmetic", not "math" :-)

Here's another way, not touted as "clever":

|>> guff = "\x00\x00\x01\x00\x02\x01\x03\x00\x00\x00\x00\xFF"
|>> expected = [1, 513, 3 * 65536, 255]
|>> expected
[1, 513, 196608, 255]

|>> import array
|>> b = array.array('B', guff)
|>> b
array('B', [0, 0, 1, 0, 2, 1, 3, 0, 0, 0, 0, 255])
|>> actual = [b[x]*65536 + b[x+1]*256 + b[x+2] for x in range(0, len(b), 3)]
|>> actual
[1, 513, 196608, 255]
|>> actual == expected
True

Cheers,
John