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array and struct 64-bit Linux change in behavior Python 3.7 and 2.7


On 12/2/19 9:26 AM, Chris Clark wrote:
> Test case:
> 
>                 import array
>                 array.array('L', [0])
> # x.itemsize == 8  rather than 4
> 
> This works fine (returns 4) under Windows Python 3.7.3 64-bit build.
> 
> Under Ubuntu; Python 2.7.15rc1, 3.6.5, 3.70b3 64-bit this returns 8. Documentation at https://docs.python.org/3/library/array.html explicitly states 'L' is for size 4.
> It impacts all uses types of array (e.g. reading from byte strings).
> 
> The struct module is a little different:
> 
> import struct
> x = struct.pack('L', 0)
> # len(x) ===8 rather than 4
> 
> This can be worked around by using '=L' - which is not well documented - so this maybe a doc issue.
> 
> Wanted to post here for comments before opening a bug at https://bugs.python.org/
> 
> Is anyone seeing this under Debian/Ubuntu?
> 
> 
> Chris
> 

I'd say not a bug, at least in array.  Reading that array documentation you 
linked, 4 is explicitly the MINIMUM size in bytes, not the guaranteed size.

The struct situation is, as you said, a bit different.  I believe that with the 
default native alignment @, you're seeing 4-byte data padded to an 8-byte 
alignment, not 8-byte data.  That does seem to go against what the struct 
documentation says, "Padding is only automatically added between successive 
structure members. No padding is added at the beginning or the end of the 
encoded struct."

= alignment is documented as having the platform native byte-order, but the size 
and alignment is standardized as having no padding, which is exactly the 
behavior you seem to want.  The documentation is a bit obtuse and scattered, but 
no more than any other.