Example on how to create an external Postfix script in python

postfix.py

#!/usr/bin/env python3

"""
An example on how to create a external Postfix script in python. By the end of this small tutorial,
"postmap -q 200 tcp:127.0.0.1:10444" should return "success".
This is not much 😅 but serves as an example on how to create Python script for Postfix.

I haven't found any snippet on this topic and ended up creating a toy script.
This is based on http://dozzie.jarowit.net/blog-archive/2013/11/13/postfix-sender-rewriting-scheme/

# Steps:

1) Add this to the end of /etc/postfix/master.cf

===
## Test Python external script
127.0.0.1:10444     inet  n       n       n       -       8       spawn
  user=ubuntu
  argv=/tmp/postfix.py
===

The command says is each time postfix receives a query at 127.0.0.1:10444, it will "spawn" the
script /tmp/postfix.py, running as "ubuntu" user. I need "ubuntu" as the script needs to be able to write to
"/tmp/postfix.log" and it seems "nobody" user cannot do that I think :).
"Spawn" is not efficient and not adapted for production.

2) Create in /tmp/postfix.py this script
3) Now "postmap -q 200 tcp:127.0.0.1:10444" should return "success"
4) (Future) If you decide to implement SRS using Python following this pattern, you can enable this by adding
===
sender_canonical_classes = envelope_sender
sender_canonical_maps = tcp:localhost:10444
====
into /etc/postfix/main.cf
"""
import logging
import sys

# as the script is launched by postfix master,
# we write the log to a file in order to know what's going on
logging.basicConfig(
    level=logging.DEBUG,
    filename="/tmp/postfix.log",
    filemode="a",
    format="%(asctime)s - %(name)s - %(levelname)s - %(process)d - %(module)s:%(lineno)d - %(funcName)s - %(message)s",
)


line = sys.stdin.readline()
if line:
    logging.debug("line %s.", line)

    # remove the trailing \n if any
    line = line.rstrip()
    logging.debug("line:%s %s", line, len(line))

    # postfix sends something like "get {data}" and expects to receive the the response
    # following the protocol on http://www.postfix.org/tcp_table.5.html
    data = line.replace("get ", "")
    logging.debug("email is %s %s %s.", data, len(data), type(data))

    if data == "500":
        logging.debug("handle 500")
        print("500 500error")
    elif data == "400":
        logging.debug("handle 400")
        print("400 400error")
    elif data == "200":
        logging.debug("handle 200")
        print("200 success")
    else:
        logging.error("Cannot handle %s", data)

logging.debug("End")