struct.pack bug?

Fredrik Lundh fredrik at pythonware.com
Fri Oct 27 13:31:22 CEST 2006


"Jansson Christer" wrote:

> I have discovered that in my Python 2.4.1 installation (on Solaris 8),
> struct.pack handles things in a way that seems inconsistent to me.
>
> I haven't found any comprehensible documentation over known issues with
> Python 2.4.1 so I try this...
>
> Here's the thing:
>
> >>> from struct import pack
> >>> pack('B', -1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> struct.error: ubyte format requires 0<=number<=255
> >>> pack('H', -1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> struct.error: short format requires 0<=number<=USHRT_MAX
> >>> pack('L', -1)
> '\xff\xff\xff\xff'
> >>>
>
> Shouldn't pack('L', -1) raise an exception like the others, rather than
> behaving like pack('l', -1)?

probably; the reason for the old behaviour is probably to simplify for code that
uses Python int's to represent 32-bit values.  (mixing longs and ints used to be a
lot more difficult than it is today).

> Is this fixed in later versions?

the "struct" module was rewritten in Python 2.5; the new module issues a
warning, and then appears to *clamp* the value to the allowed range, in-
stead of grabbing the low bits:

>>> import struct

>>> struct.pack("B", -1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python25\Lib\struct.py", line 63, in pack
    return o.pack(*args)
struct.error: ubyte format requires 0 <= number <= 255

>>> struct.pack("H", -1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python25\Lib\struct.py", line 63, in pack
    return o.pack(*args)
struct.error: short format requires 0 <= number <= USHRT_MAX

>>> struct.pack("I", -1)
__main__:1: DeprecationWarning: 'I' format requires 0 <= number <= 4294967295
'\x00\x00\x00\x00'

>>> struct.pack("L", -1)
__main__:1: DeprecationWarning: 'L' format requires 0 <= number <= 4294967295
'\x00\x00\x00\x00'

</F> 






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