Your first Python program

Small, complete, from-scratch: this lesson walks the whole write → save → run → fix cycle for a real program. If you follow along in Kali, by the end you'll have written and run five little programs of your own.

Step 1 — Where to write

Two options in Kali:

Option A: A terminal editor.

mkdir ~/python-labs
cd ~/python-labs
nano hello.py     # write in nano

Option B: A GUI editor. Kali ships with Mousepad. VS Code is nicer once you install it (sudo apt install code -y).

Either works. What matters is saving with a .py extension.

Step 2 — Hello world

Type this in your editor and save as hello.py:

# hello.py — the traditional first program
print("Hello, KashSec!")

Save (Ctrl+O, Enter, Ctrl+X in nano). Then run it from the terminal:

python3 hello.py

You should see:

Hello, KashSec!

Congratulations — you're a Python programmer. No, really. Everything from here is variations on this loop.

Step 3 — Input and output

Replace hello.py with:

name = input("What's your name? ")
print(f"Nice to meet you, {name}!")

Run it — the terminal now asks for a name.

Step 4 — Comments and doc

# This is a comment — Python ignores it. Use it to explain code.

"""
Triple-quoted strings can span
multiple lines. When placed at the
top of a file or function, they become
a 'docstring' — used for documentation.
"""

# Good comments explain WHY, not just WHAT:
timeout = 5   # 5 seconds is long enough for most local checks

Habits worth adopting:

• Every script starts with a docstring saying what it does and how to use it.
• Comments explain why a piece of code exists, not what a line already clearly does.

Step 5 — Errors and how to read them

When something breaks, Python tells you what and where. Save this as crash.py:

name = input("Name: ")
print("Age: " + age)   # oops — 'age' doesn't exist

Run it, enter a name, and Python shows you:

Traceback (most recent call last):
  File "crash.py", line 2, in <module>
    print("Age: " + age)
                   ^^^
NameError: name 'age' is not defined

Read a traceback bottom to top:

1. Type of error (NameError) — the category.
2. Message (name 'age' is not defined) — the specific thing.
3. Where — file and line.

The three most common early errors:

The oldest trick in programming: sprinkle print() calls to see what's happening at each step. Not glamorous. Extremely effective. Real professionals do this constantly.

Handling errors gracefully with try/except

Reading tracebacks helps you fix bugs while writing code — but a finished script often needs to anticipate errors that are expected to sometimes happen (a network request timing out, a file that doesn't exist) without crashing entirely. Python's try/except catches an error and lets your script recover instead of dying:

try:
    response = requests.get(url, timeout=5)
    print(response.status_code)
except requests.exceptions.Timeout:
    print(f"{url} timed out — skipping")
except requests.exceptions.ConnectionError:
    print(f"{url} refused connection — skipping")

This exact pattern is genuinely essential for any recon or scanning script (Path 04's automation lesson, Path 15's recon pipeline) — checking a list of 500 URLs where 10 happen to be offline should skip those 10 gracefully and keep going, not crash the entire run on the first failure.

Step 6 — Reading and writing files

Files are how your program remembers things between runs. This is the pattern:

# Write to a file:
with open("notes.txt", "w") as f:
    f.write("This is my first Python file write.\n")
    f.write("Second line here.\n")

# Read it back:
with open("notes.txt") as f:
    content = f.read()
print(content)

# Read line by line (memory-friendly for big files):
with open("notes.txt") as f:
    for line in f:
        print(line.strip())

The with block automatically closes the file, even if something goes wrong. Always use it.

File modes:

• "r" — read (default)
• "w" — write (overwrites!)
• "a" — append
• "rb" / "wb" — binary

Step 7 — Modules and imports

You don't have to write everything from scratch. Python's standard library ships with hundreds of modules.

import os
import sys
import hashlib
import socket
import datetime
import json
import random

# Use them:
print(os.getcwd())              # current directory
print(sys.version)              # Python version
print(hashlib.md5(b"hi").hexdigest())    # MD5 of "hi"
print(socket.gethostname())     # your hostname
print(datetime.datetime.now())  # right now
print(json.dumps({"a": 1}))     # dict → JSON string
print(random.randint(1, 100))   # random 1–100

Every one of these is available without installing anything. For libraries not in the standard library, use pip install.

Step 8 — A tiny password-hash lookup

Try this — save as hash-check.py:

# hash-check.py — hash a password and see what it looks like
import hashlib

pw = input("Password: ")

# Bad old ways:
md5 = hashlib.md5(pw.encode()).hexdigest()
sha1 = hashlib.sha1(pw.encode()).hexdigest()

# The right way (with a salt):
salt = "somethingRandomPerUser"
sha256_salted = hashlib.sha256((salt + pw).encode()).hexdigest()

print(f"\nMD5:            {md5}")
print(f"SHA-1:          {sha1}")
print(f"SHA-256+salt:   {sha256_salted}")
print("\n(Never store MD5/SHA-1 for passwords. Use bcrypt/argon2 — Path 10.)")

You just used a real security library. Path 10 explains why MD5 and SHA-1 are unfit for passwords in detail.

Common beginner tripwires

• Forgetting python3 filename.py (running python uses Python 2 on some systems).
• Mixing tabs and spaces in indentation.
• Adding () to input but forgetting them on print (or vice versa).
• Comparing with = instead of ==.
• Forgetting .encode() when you pass a string to a hash function (they take bytes).

🧪 Lab — Small challenges

Work through these one by one. Answers hidden — try first.

1. Personalised greeter. Ask for name and favourite tool. Print Hi <name>! Learn <tool> deeply this week.
2. Even or odd. Ask for a number, print "even" or "odd."
3. FizzBuzz. Print 1 to 30, but for multiples of 3 print Fizz, multiples of 5 Buzz, both FizzBuzz.
4. Line counter. Ask for a filename, print how many lines it has.
5. Palindrome check. Ask for a word, print whether it's a palindrome.

# 1
name = input("Name: ")
tool = input("Favourite tool: ")
print(f"Hi {name}! Learn {tool} deeply this week.")

# 2
n = int(input("Number: "))
print("even" if n % 2 == 0 else "odd")

# 3
for i in range(1, 31):
    if i % 15 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

# 4
path = input("File: ")
with open(path) as f:
    print(sum(1 for _ in f))

# 5
w = input("Word: ").lower()
print("palindrome" if w == w[::-1] else "not a palindrome")

Get the whole set working. That's your Python foundation cemented.

Which built-in Python function opens a file, and what's the special block that guarantees the file is closed?

Key terms

• .py file — a Python program on disk.
• Traceback — Python's error report, read bottom-up.
• Standard library — modules shipped with Python (os, sys, hashlib, ...).
• with block — auto-cleanup for files, sockets, etc.
• Docstring — triple-quoted docs at top of file/function.

Recap

• Write a .py file, run with python3 file.py.
• input() reads, print() writes; comments explain why.
• Read tracebacks bottom-up to find the error type and line.
• Use with open(...) as f: for files.
• Explore the standard library — no install needed.

References

• Python.org — Tutorial — official, thorough.
• Automate the Boring Stuff — Ch. 1 — free, gentle intro.
• Python standard library index
• Python Cheatsheet — pythoncheatsheet.org

Next up → String manipulation & regex