[Python-Dev] test_gzip/test_tarfile failure om AMD64

[Bob Ippolito]

On May 30, 2006, at 10:47 AM, Tim Peters wrote:

> [Bob Ippolito]
>> What should it be called instead of wrapping?
>
> I don't know -- I don't know what it's trying to _say_ that isn't
> already said by saying that the input is out of bounds for the format
> code.

The wrapping (now overflow masking) warning happens during conversion  
of PyObject* to long or unsigned long. It has no idea what the  
destination packing format is beyond whether it's signed or unsigned.  
If the packing format happens to be the same size as a long, it can't  
possibly trigger a range warning (unless range checks are moved up  
the stack and all of the function signatures and code get changed to  
accommodate that).

>> When it says it's wrapping, it means that it's doing x &= (2 ^ (8  
>> * n)) - 1 to force
>> a number into meeting the expected range.
>
> How is that different from what it does in this case?:
>
>>>> struct.pack('<B', 256L)
> /Users/bob/src/python/Lib/struct.py:63: DeprecationWarning: 'B' format
> requires 0 <= number <= 255
>  return o.pack(*args)
> '\x00'
>
> That looks like "wrapping" to me too (256 & (2**(8*1)-1)== 0x00), but
> in this case there is no deprecation warning about wrapping.  Because
> of that, I'm afraid you're drawing distinctions that can't make sense
> to users.

When it says "integer wrapping" it means that it's wrapping to fit in  
a long or unsigned long. n in this case is always 4 or 8 depending on  
the platform. The format-specific range check is separate. My  
description wasn't very good in the last email.

>> Reducing it to one warning instead of two is kinda difficult. Is it
>> worth the trouble?
>
> I don't understand.  Every example you gave that showed a wrapping
> warning also showed a "format requires i <= number <= j" warning.  Are
> there cases in which a wrapping warning is given but not a "format
> requires i <= number <= j" warning?  If so, I simply haven't seen one
> (but I haven't tried all possible inputs ;-)).
>
> Since the implementation appears (to judge from the examples) to
> "wrap" in every case in which any warning is given (or are there cases
> in which it doesn't?), I don't understand the point of distinguishing
> between wrapping warnings and  "format requires i <= number <= j"
> warnings either.  The latter are crystal clear.

A latter email in this thread enumerates exactly which circumstances  
should cause two warnings with the current implementation.

-bob