Reading Basic Python Scripts

What it is

Python is the main language for network automation, and the exam asks you to read short scripts and say what they do.

You are not asked to write long programs. You need to recognise the building blocks, follow the flow of a script and predict its output. The same few parts appear again and again.

How it works

A script is a sequence of statements, and the indentation shows the structure.

Variables and data types

A variable holds a value. Python works out the type from the value.

A list is read by position, and a dictionary is read by key.

devices = ["R1", "R2", "R3"]
router = {"name": "R1", "asn": 65001}
print(devices[0])        # R1
print(router["asn"])     # 65001

Conditions and loops

An if statement chooses a path. A for loop repeats a block for each item. The block is marked by a colon and by indentation, and the indentation is part of the syntax.

for name in devices:
    if name == "R2":
        print(name, "is the border router")
    else:
        print(name, "is internal")

This prints one line for each device. Note that == compares two values, and a single = assigns.

A while loop repeats while a condition is true. The range() function makes a sequence of numbers, so range(3) gives 0, 1 and 2.

Functions

A function is a named block that you can call. The def keyword defines it, and return sends a value back.

def is_up(status):
    return status == "up"

print(is_up("up"))      # True
print(is_up("down"))    # False

Strings and f-strings

An f-string puts values inside text, using braces.

host = "R1"
print(f"Connecting to {host}")   # Connecting to R1

Errors

A try block runs code that might fail. An except block handles the error.

try:
    value = int("abc")
except ValueError:
    print("Not a number")

Without the try, the script would stop with a traceback. Reading a traceback from the bottom up tells you the error type and the line.

Imports and libraries

The import statement brings in a library. The standard json library converts between text and Python objects. The requests library, which is installed separately, makes HTTP calls.

import json

text = '{"hostname": "R1", "interfaces": ["Gi1", "Gi2"]}'
data = json.loads(text)
print(data["hostname"])           # R1
print(len(data["interfaces"]))    # 2

The function json.loads turns a JSON string into a dictionary. The function json.dumps does the reverse. A JSON object becomes a dict, and a JSON array becomes a list.

Reading a REST call

This script reads a list of interfaces from a RESTCONF endpoint.

import requests
from urllib.parse import urlunsplit

HOST = "10.1.1.1"
path = "/restconf/data/ietf-interfaces:interfaces"
url = urlunsplit(("https", HOST, path, "", ""))
headers = {"Accept": "application/yang-data+json"}

response = requests.get(url, headers=headers, auth=("admin", "PASSWORD"), verify=False)

if response.status_code == 200:
    for item in response.json()["ietf-interfaces:interfaces"]["interface"]:
        print(item["name"])
else:
    print("Error", response.status_code)

Read it in steps. The script builds a URL, sets a header, and sends a GET request with a user name and password.

If the status code is 200, it turns the body into a dictionary and prints the name of each interface. Otherwise it prints the code.

The argument verify=False turns off certificate checks, which is acceptable only in a lab. A real script should keep them on.

Working with files

The with open pattern opens a file and closes it again automatically.

with open("devices.txt") as f:
    for line in f:
        print(line.strip())

Running a script

The line if __name__ == "__main__": marks code that runs only when the file is started directly, and not when another script imports it. Many scripts put their main logic under it.