Python for network engineers: libraries and a first automation script

Why Python

Python is the most widely used language in network automation. It reads almost like English, runs on Windows, macOS and Linux, and has excellent libraries written specifically for network engineers. You don't need to become a software developer: a few hundred lines of Python can save hours every week.

Libraries worth knowing

A first script: backing up configurations

import datetime
import getpass
import yaml
from netmiko import ConnectHandler

with open("devices.yaml") as f:
    devices = yaml.safe_load(f)          # list of {name, host, type}

user = input("Username: ")
password = getpass.getpass("Password: ")  # never hard-code passwords
today = datetime.date.today().isoformat()

for d in devices:
    try:
        conn = ConnectHandler(device_type=d["type"], host=d["host"],
                              username=user, password=password)
        config = conn.send_command("show running-config")
        conn.disconnect()
        with open(f"{d['name']}_{today}.cfg", "w") as out:
            out.write(config)
        print("Saved", d["name"])
    except Exception as err:
        print("FAILED", d["name"], err)

The inventory file is plain YAML:

- name: BR1-SW1
  host: 192.168.99.11
  type: cisco_ios
- name: BR1-RTR
  host: 192.168.99.1
  type: cisco_ios

Making changes safely

commands = ["vlan 30", "name CAMERAS"]
output = conn.send_config_set(commands)   # enters config mode, applies, exits
conn.save_config()                         # write memory

• Test against a lab or one pilot device first.
• Back up before every change, and check the result afterwards with a show command.
• Keep credentials in environment variables or a vault, never in the script.
• Store scripts in Git so every change is tracked.

How to learn

1. Learn Python basics: variables, lists, dictionaries, loops, functions and files.
2. Run Python against a free lab (simulators or virtual routers).
3. Automate one boring task you really do, such as backups or a port report.
4. Learn Git, then try NAPALM, Nornir and APIs.

Real-world example: a technician's first script

An IT technician at a fictional international school looked after 35 switches and spent part of every Friday logging in to each one to save its configuration to a shared folder. Using a lab switch first, he wrote a Python script with Netmiko that read the device list from a YAML file, took the password from an environment variable rather than the code, connected to each switch in turn and saved its running configuration with the date in the file name. He stored the script and the backups in a private Git repository, so he could see exactly what had changed between weeks. The Friday task now runs automatically every night, and when a switch failed after a power cut its replacement was configured from the previous night's backup in minutes.