[issue6924] struct.unpack weird behavior with bi (byte then integer)

[Emmanuel Bengio]

New submission from Emmanuel Bengio beng...@gmail.com:

Using the following command in Python 2.6.1:

 struct.unpack(BI,12345)
Traceback (most recent call last):
  File pyshell#1, line 1, in module
struct.unpack(BI,12345)
error: unpack requires a string argument of length 8

I get this error message. What confused me was that doing
 struct.unpack(IB,12345)
(875770417, 53)
Worked just fine.

I have found out that this only happens using the native byte
order(@), which is the default.
For Example:
 struct.unpack(!BI,12345)
(49, 842216501)
Works, and all other variants, =, ,  (native standard,little endian,
and small endian) also do.

I haven't found anything about that in the documentation.

Also, the requested 3 other bytes arent event used:
 struct.unpack(I,abcd)
(1684234849,) # see the big number starting with 16
 ord(x)
120
 struct.unpack(BI,xabcd) # we get the error
Traceback (most recent call last):
  File pyshell#7, line 1, in module
struct.unpack(BI,xabcd)
error: unpack requires a string argument of length 8
 struct.unpack(BI,xabcdefg)
(120, 1734763876) # not the same here
 struct.unpack(BI,xabcabcd)
(120, 1684234849) # same here
 struct.unpack(BI,x___abcd)
(120, 1684234849) # same again

--
components: Library (Lib)
messages: 92724
nosy: Manux
severity: normal
status: open
title: struct.unpack weird behavior with bi (byte then integer)
type: behavior
versions: Python 2.6

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[issue6924] struct.unpack weird behavior with bi (byte then integer)

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

I think this is expected behaviour:  the key point is that structs can 
include padding bytes.  From the documentation:

By default, C numbers are represented in the machine’s native format and 
byte order, and properly aligned by skipping pad bytes if necessary 
(according to the rules used by the C compiler).

'Native' struct formats include padding, while 'standard' formats don't.

So a native struct with format 'BI' has one byte for the 'B', followed by 
3 padding bytes, followed by four bytes for the 'I'.  This exactly matches 
the way a C struct of the form {char c; int x;} would be organized in 
memory on that machine.

--
assignee:  - marketdickinson
nosy: +marketdickinson
resolution:  - works for me
status: open - closed

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