Writing text over a PDF in python3

I am trying to write some string to a PDF file at some position. I found a way to do this and implemented it like this:

from PyPDF2 import PdfFileWriter, PdfFileReader
import io
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import letter

packet = io.StringIO()
# create a new PDF with Reportlab
can = canvas.Canvas(packet, pagesize=letter)
can.drawString(10, 100, "Hello world")
can.save()

#move to the beginning of the StringIO buffer
packet.seek(0)
new_pdf = PdfFileReader(packet)
# read your existing PDF
existing_pdf = PdfFileReader(file("original.pdf", "rb"))
output = PdfFileWriter()
# add the "watermark" (which is the new pdf) on the existing page
page = existing_pdf.getPage(0)
page.mergePage(new_pdf.getPage(0))
output.addPage(page)
# finally, write "output" to a real file
outputStream = file("destination.pdf", "wb")
output.write(outputStream)
outputStream.close()

It throws me an error at can.save() line

The error :

File "/home/corleone/miniconda3/lib/python3.6/site-packages/reportlab/pdfgen/canvas.py", line 1237, in save
    self._doc.SaveToFile(self._filename, self)
  File "/home/corleone/miniconda3/lib/python3.6/site-packages/reportlab/pdfbase/pdfdoc.py", line 224, in SaveToFile
    f.write(data)
TypeError: string argument expected, got 'bytes'

Have read up at a lot of places on the internet. Found the same method everywhere. Is it the wrong way to do. Am I missing something?

Answer

Figured out I just had to use BytesIO instead of StringsIO.

Also open() instead of file() because I’m using Python3.

Here is the working script:

from PyPDF2 import PdfFileWriter, PdfFileReader
import io
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import letter

packet = io.BytesIO()
# Create a new PDF with Reportlab
can = canvas.Canvas(packet, pagesize=letter)
can.setFont('Helvetica-Bold', 24)
can.drawString(10, 100, "Hello world")
can.showPage()
can.save()

# Move to the beginning of the StringIO buffer
packet.seek(0)
new_pdf = PdfFileReader(packet)
# Read your existing PDF
existing_pdf = PdfFileReader(open("original.pdf", "rb"))
output = PdfFileWriter()
# Add the "watermark" (which is the new pdf) on the existing page
page = existing_pdf.getPage(0)
page.mergePage(new_pdf.getPage(0))
output.addPage(page)
# Finally, write "output" to a real file
outputStream = open("destination.pdf", "wb")
output.write(outputStream)
outputStream.close()