How to read an integer value from a binary file?

[John Machin]

On Tue, 3 May 2005 19:46:17 +0200, Olivier Elbaz
<olivier.elbaz at gmail.com> wrote:

>Hi,
>
>I read a bmp file (binary file) and I want to extract an integer value
>from it (size value).
>The whole binary file is read inside a list "all_bmp_file".
>I can read the "size" value with the following code:
>for r in all_bmp_file[2:5]:
>   n = struct.unpack('B',r)#B for unsigned char
>   print n
>Result:
>(182,)
>(47,)
>(0,)
>The problem is that I would like to read the 4 bytes (integer) as one
>number like:
>struct.unpack('i','\xb6/\x00\x00')
>Result
>(12214,)
>Note that I can't use  struct.unpack('i',all_bmp_file[2:5]) inside my
>program since I got a syntax error.

You may well have got a syntax error -- we can't tell as you posted
neither the exact code you tried to run, nor the actual error message
that you got. After correcting any syntax error, one would have
expected you to get a run-time error, like the following:

>>> b3 = '\xb6/\x00'
>>> import struct
>>> struct.unpack('i', b3)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
struct.error: unpack str size does not match format
>>>

Luckily this time there was enough extraneous info for Jeff to
diagnose your problem. In future, please consider posting the code
that you ran, and the results that you got.

For the benefit of others who may fall into the same pit, you may like
to consider logging a feature request on sourceforge for the above
error message to be enhanced to show the sizes:

(e.g.) struct.error: unpack str size (3) does not match format (4)

This may make it easier for the perps to realise what has gone wrong
and save them from having to ask the newsgroup :-)

Cheers,
John