How to convert bytearray into integer?

Jacky jacky.chao.wang at gmail.com
Tue Aug 17 03:05:26 CEST 2010


On Aug 17, 3:53 am, Mark Dickinson <dicki... at gmail.com> wrote:
> On Aug 16, 8:36 pm, Mark Dickinson <dicki... at gmail.com> wrote:
>
>
>
>
>
> > On Aug 16, 8:08 pm, Jacky <jacky.chao.w... at gmail.com> wrote:
> > > My concern is that struct may need to parse the format string,
> > > construct the list, and de-reference index=0 for this generated list
> > > to get the int out.
>
> > > There should be some way more efficient?
>
> > Well, you can improve on the struct solution by using the
> > struct.Struct class to avoid parsing the format string repeatedly:
>
> > >>> import struct
> > >>> S = struct.Struct('<I')
> > >>> S.unpack_from(buffer(bytearray([1,2,3,4,5])))
>
> > (67305985,)
>
> > This doesn't make a huge difference on my machine (OS X 10.6.4, 64-bit
> > build of Python 2.6) though;  it's probably more effective for long
> > format strings.
>
> Sorry, this was inaccurate:  this makes almost *no* significant
> difference on my machine for large test runs (10000 and up).  For
> small ones, though, it's faster.  The reason is that the struct module
> caches (up to 100, in the current implementation) previously used
> format strings, so with your tests you're only ever parsing the format
> string once anyway.  Internally, the struct module converts that
> format string to a Struct object, and squirrels that Struct object
> away into its cache, which is implemented as a dict from format
> strings to Struct objects.  So the next time that the format string is
> used it's simply looked up in the cache, and the Struct object
> retrieved.
>
> By the way, in Python 3.2 there's yet another fun way to do this,
> using int.from_bytes.
>
> >>> int.from_bytes(bytearray([1,2,3,4]), 'little')

Thanks!  It looks pretty like the ctypes way. ;)

>
> 67305985
>
> --
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