Change page sequence in PDF to document as a booklet (2-sided, 2 page per sheet)

booklet.py

# Change page sequence in PDF to document as a booklet
# Booklet is 2-sided, 2 page per sheet 
import sys
import os
import PyPDF3 as pdf

fin = pdf.PdfFileReader(open(sys.argv[1], 'rb'))
fout = pdf.PdfFileWriter()

# create a list of page indices
pages = list(range(fin.getNumPages()))

# pad page index list until its len is a multiple of four
while len(pages) % 4: pages.append(None)

# index of the last page
n = len(pages) - 1

# get document dimensions
w = fin.getPage(0).mediaBox.getWidth()
h = fin.getPage(0).mediaBox.getHeight()

# we loop in pairs
for x in range(len(pages) // 2):
    # alternately count from the front and back
    for index in (x, n - x) if x % 2 else (n - x, x):
        if pages[index] is None: fout.addBlankPage(w, h)  # pad
        else: fout.addPage(fin.getPage(index))

# write the output file
newName = "Booklet-" + os.path.basename(sys.argv[1])
fout.write((open(newName, 'wb')))