new_contents: #!/usr/bin/env python3
  import sys
  import os
  try:
      from PyPDF2 import PdfFileReader, PdfFileWriter
  except ImportError:
      from pyPdf import PdfFileReader, PdfFileWriter
  
  def pdf_cat(input_folder, output_file_name):
      input_streams = []
      try:
          # First open all the files, then produce the output file, and
          # finally close the input files. This is necessary because
          # the data isn't read from the input files until the write
          # operation. Thanks to
          # https://stackoverflow.com/questions/6773631/problem-with-closing-python-pypdf-writing-getting-a-valueerror-i-o-operation/6773733#6773733
          file_names = []
          for dirpath, _, filenames in os.walk(input_folder):
              for fname in filenames:
                  if not fname.endswith(".pdf"):
                      continue
                  full_name = os.path.join(dirpath, fname)
                  print("Appending %s" % full_name)
                  file_names.append("" + full_name)
  
          print(file_names)     
          file_names.sort()
          for fname in file_names:
              input_streams.append(open(fname, 'rb'))
  
          writer = PdfFileWriter()
          for reader in map(PdfFileReader, input_streams):
              for n in range(reader.getNumPages()):
                  writer.addPage(reader.getPage(n))
          writer.write(file(output_file_name, "wb"))
      finally:
          for f in input_streams:
              f.close()
  
  if __name__ == '__main__':
      if len(sys.argv) != 3:
          raise Exception('Expecting arguments: <folder with pdfs> <output file name>')
      input_folder = sys.argv[1]
      output_file_name = sys.argv[2]
      pdf_cat(input_folder, output_file_name)
  
subject: Create merge_pdfs.py