Unpack binary data with python

Question

I would like to unpack an array of binary data to uint16 data with Python.

Internet is full of examples using struct.unpack but only examples dealing with binary array of size 4.

Most of these examples are as follow (B is a binary array from a file)

U = struct.unpack("HH",B[0:4]);

So i tried to unpack an array of size 6:

U = struct.unpack("HHH",B[0:6]);

It works.

But how to do if I want to unpack an array of size L (L is pair)? I tried that:

U = struct.unpack("H"*(L/2),B[0:L]);

but it doesn't work, prompter gives me an error (for L=512 for example):

struct.error: unpack requires a string argument of length 512

This message is strange because if i want to unpack a binary array to uint16, I need a string "HHH...HHH" of half size of this array...

I would be very grateful if someone could provide me with some help.

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I progress a little bit... In fact, i tried:

U = struct.unpack("H"*8,B[0:8]); 

It works.

U = struct.unpack("H"*10,B[0:10]);

It works.

U = struct.unpack("H"*222,B[0:444]);

It still works

U = struct.unpack("H"*223,B[0:446]);

It doesn't work! and it never works for size bigger than 446

Hope it will help anyone to answer me.

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@MarkRansom I checked len(B) and in fact, the length is 444. I was so sure that B is an array of size 512 because B comes from : B = f.read(512) where F is a 8000-bytes-size file. So a problem with read()... Thanks for this answer! But if someone has a help to unpack binary array of size L, i would be grateful

Answer 1

Use array.fromstring or array.fromfile (see http://docs.python.org/2/library/array.html ):

import array
U = array.array("H")
U.fromstring(B)

Answer 2

Variable length version of the same thing:

n = 999
U = struct.unpack(str(n)+"H", B)