Double sided scanning from a single sided scanner

#! /usr/bin/python
# A handy python script to reorder the pages of a PDF for duplex scanning
# from a simplex automatic document feed scanner.
# When using a single-sided (simplex) scanner to scan a double-sided document,
# you can first scan the front sides, then the back sides using the ADF.
# The problem is that the pages are not in the correct order. For a 10 page
# document, the ordering will be: [1, 3, 5, 7, 9, 10, 8, 6, 4, 2].
# This script reorders the pages to a new file with the correct ordering.
#
# Written by Kareem Shehata, March 2014.
# This is free and unencumbered software released into the public domain.
# see http://unlicense.org for details

# This script requires PyPDF. Be sure to install this package if you don't
# already have it. "pip install pypdf" should do the trick.
from pyPdf import PdfFileWriter, PdfFileReader

# specify the input and output filenames
# feel free to change the names.
# Exercise for the reader: change this to use command line parameters or
# drag and dropping the file onto the script.
# NB: You can't write to the same filename. I'd suggest renaming after writing
# the output if you want it to produce the same file on output.
input_filename = "Scan.pdf"
output_filename = "Scan_reordered.pdf"

# create a PDF writer to hold the output
output_pdf = PdfFileWriter()

# open up the input file and start reading the PDF
with open(input_filename, 'rb') as input_file:
    input_pdf = PdfFileReader(input_file)

    # check that we have an even number of pages
    total_pages = input_pdf.getNumPages()
    if (total_pages % 2) != 0:
        print "Cannot de-duplex an odd number of pages!"
        exit(1)

    # Get the pages in pairs, starting with the first and last, and then
    # proceeding to the second and second last, and so on
    for i in range(0, total_pages/2):
        print "Adding page " + str(i)
        output_pdf.addPage(input_pdf.getPage(i))
        print "Adding page " + str(total_pages - i - 1)
        output_pdf.addPage(input_pdf.getPage(total_pages - i - 1))

    # Write the output file. NB: must be done before closing the input file!
    with open(output_filename, 'wb') as output_file:
        output_pdf.write(output_file)