[Tutor] Dealing with bitfields in Python

[Luke Paireepinart]

You can just do a binary AND with your consts and your bitfield value to get
each bit.  The values will be the actual value (2, 4, 8, etc.) but if you
use these as a boolean it won't matter.I.E.
>>> bitfield = 119
>>> seek = bitfield & CAN_SEEK
>>> seek
16
>>> if seek:
print "Hello"

Hello
>>>

if you have to have these constants defined.
The way I'd actually do it is probably without defining constants.
#unpack bitfield values
go_next, go_prev, pause, play, seek, meta, tracklist = [(1<<i) & bitfield
for i in range(7)]

If you need them as booleans for some reason,
go_next, go_prev, pause, play, seek, meta, tracklist = [((1<<i) & bitfield >
0) for i in range(7)]

If this isn't what you're asking, just clarify what you meant and I'll try
to help.
HTH,
-Luke

On Sun, Aug 30, 2009 at 5:59 PM, Skipper Seabold <jsseabold at gmail.com>wrote:

> Hello all,
>
> Fair warning, I didn't know what a bitfield was a few hours ago.
>
> I am working with a program via the dbus module and I am wondering if
> there is built-in support to deal with bitfields in Python.  I query
> my application and it returns a bitfield 119.  The bitfield "key" is
>
> NONE                  = 0,
> CAN_GO_NEXT           = 1 << 0,
> CAN_GO_PREV           = 1 << 1,
> CAN_PAUSE             = 1 << 2,
> CAN_PLAY              = 1 << 3,
> CAN_SEEK              = 1 << 4,
> CAN_PROVIDE_METADATA  = 1 << 5,
> CAN_HAS_TRACKLIST     = 1 << 6
>
> And a call to the method returns 119.  I have gotten far enough to
> understand that 119 is
>
> >>> (1<<0)+(1<<1)+(1<<2)+(0<<3)+(1<<4)+(1<<5)+(1<<6)
> 119
>
> 119 is 01110111 as a binary byte (I'm reaching back to high school
> computer science here...)
>
> So I guess I understand the basics of what it's telling me, but I'd
> like to unpack 119 into binary, so I can read it and use the
> information in my program.  I've adapted a code snippet that I found
> online to do this, but I'm wondering if there is a better way in
> python maybe using binascii or struct?
>
> Here is the helper function I've adapated
>
> def int_2_binary(int):
>    const = 0x80000000
>    output = ""
>    ## for each bit
>    for i in range(1,33):
>        ## if the bit is set, print 1
>        if( int & const ):
>            output = output + "1"
>        else:
>            output = output + "0"
>        ## shift the constant using right shift
>        const = const >> 1
>    output = list(output)
>    output = "".join(output[-8:])
>    return output
>
> As you can see const is the smallest signed 32-bit integer, and it
> would return a length 32 string.  But I know that my bitfield will be
> 8-bit, I just don't know what this is in hexadecimal (?) to define
> const.  Any pointers to do this in a better way would be appreciated.
>
> Thanks,
>
> Skipper
> _______________________________________________
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> http://mail.python.org/mailman/listinfo/tutor
>
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