Reverse order of bit in repeating seqence of byte string

[Gary Herron]

imageguy wrote:
> I am looking for the most efficient method of replacing a repeating
> sequence in a byte string returned from a imaging .dll, connected via
>
> I receive the byte string with the following sequence 'bgrbgrbgrbgr'
> and I would like to convert this to 'rbgrbgrbgrbg'
> FWIW, the string is created using ctypes.create_string_buffer function
>
> The following code works but feels a bit clunk and is rather slow too.
>
> blist = list(buffer)
> for start in xrange(0,len(blist), 3):
>    try:
>         blue = blist[start]
>         red = blist[start+2]
>         blist[start] = red
>         blist[start+2] = blue
>    except IndexError:
>        pass
>
> new_buffer = ''.join(blist)
>
> new_buffer is then passed to a wx program to create and image.
>
> Any thoughts comments would be appreciated.
>
> geoff.
>
> PS:  I started this post earlier, but I think I hit the send button
> too soon.  My apologies to the group for sloppy typing.
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>   
I've not seen anyone mention numpy yet, but
numpy has a nice (efficient) way to do this:

>>> import numpy
>>> s = 'bgrBGRcbaCBA'
>>> a=numpy.array(s, 'c')  # Initialize an array
>>> a.shape = (4,3)            # Reinterpret as a ?? by 3 array
>>> b=a[...,::-1]                  # reverse the second dimension
>>> print b.tostring()          # Convert back to a string.
rgbRGBabcABC


Gary Herron