How to convert bytearray into integer?

Thomas Jollans thomas at jollybox.de
Mon Aug 16 21:38:00 CEST 2010


On Monday 16 August 2010, it occurred to Jacky to exclaim:
> Hi Thomas,
> 
> Thanks for your comments!  Please check mine inline.
> 
> On Aug 17, 1:50 am, Thomas Jollans <tho... at jollybox.de> wrote:
> > On Monday 16 August 2010, it occurred to Jacky to exclaim:
> > > Hi there,
> > > 
> > > Recently I'm facing a problem to convert 4 bytes on an bytearray into
> > > an 32-bit integer.  So far as I can see, there're 3 ways:
> > > a) using struct module,
> > 
> > Yes, that's what it's for, and that's what you should be using.
> 
> 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?

The struct module is written in C, not in Python. It does have to parse a 
string, yes, so, if you wrote your own, limited, C function to do the job, it 
might be marginally faster.

> 
> > > b) using ctypes module, and
> > 
> > Yeeaah, that would work, but that's really not what it's for. from_buffer
> > wants a writable buffer interface, which is unlikely to be what you want.
> 
> Actually my buffer is writable --- it's an bytearray.  Turning it into
> a R/O one make me to do extra effort: wrapping the bytearray into
> buffer().
> 
> My question is, this operation seems like to be much simpler than the
> former one, and it's very straightforward as well.  Why is it slow?

Unlike struct, it constructs an object you're not actually interested in 
around your int.

> it's hard to image why socket object provides the interface:
> socket.recv_from(buf[, num_bytes[, flags]]) but forget the more
> generic one: socket.recv_from(buf[, offset[, num_bytes[, flags]]])

Well, that's what pointer arithmetic (in C) or slices (in Python) are for! 
There's an argument to be made for sticking close to the traditional 
(originally C) interface here - it's familiar.


 - Thomas



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