Day 1: Python Basics — Variables, Functions, Loops

What You'll Build Today

A tip calculator (takes a bill amount, calculates tip at different percentages) and a password generator (creates random strong passwords of a specified length). Both run in your terminal by the end of this lesson.

Setup

Installing Python

Go to python.org/downloads and download Python 3.11 or newer. Run the installer. On Windows, check "Add Python to PATH" before clicking Install.

Once installed, open a terminal (Terminal on Mac, Command Prompt or PowerShell on Windows) and run:

bash

$ python --version

You should see Python 3.11.x or similar. If you get an error, try python3 --version — on Mac, that's often the right command.

One more thing: Install VS Code from code.visualstudio.com. It's free, and it's what every professional Python developer uses. Install the Python extension once it's open.

Core Concepts

Variables and Data Types

A variable is a name that holds a value. Python has four basic types you'll use constantly:

python types.py

# String — text, wrapped in quotes
name = "Claude"
message = "Analyze this document"

# Integer — whole numbers
max_tokens = 1024
num_files = 47

# Float — decimal numbers
temperature = 0.7
bill_amount = 84.50

# Boolean — True or False
is_complete = True
has_error = False

# Print any variable
print(name)        # Claude
print(max_tokens)  # 1024

Variable names are lowercase with underscores (max_tokens, not MaxTokens). This convention is called snake_case and it's what all Python code uses.

Control Flow

if/else, for loops, while loops

These three control structures handle 90% of all decision-making and repetition in Python programs.

if/else

python

api_response = "success"
tokens_used = 850

if api_response == "success":
    print("Got a response!")
elif api_response == "error":
    print("Something went wrong")
else:
    print("Unknown status")

# Comparison operators
if tokens_used > 1000:
    print("Warning: high token usage")
else:
    print(f"Used {tokens_used} tokens — within budget")

for loops

python

documents = ["report.pdf", "notes.txt", "data.csv"]

for doc in documents:
    print(f"Processing: {doc}")

# range() generates numbers
for i in range(5):
    print(f"Step {i}")  # 0, 1, 2, 3, 4

Why this matters for AI: In Day 5, you'll use a for loop to process every file in a folder — iterating over documents exactly like this, sending each one to Claude.

Functions

Writing Reusable Code

A function is a reusable block of code. Define it once, call it anywhere. Every AI program you write will be organized into functions.

python functions.py

def calculate_tip(bill, tip_percent):
    """Calculate the tip amount and total."""
    tip = bill * (tip_percent / 100)
    total = bill + tip
    return tip, total

# Call the function
tip, total = calculate_tip(84.50, 20)
print(f"Tip: ${tip:.2f}")    # Tip: $16.90
print(f"Total: ${total:.2f}") # Total: $101.40

# Functions with default values
def build_prompt(task, tone="professional"):
    return f"Please {task}. Tone: {tone}."

print(build_prompt("summarize this report"))
print(build_prompt("rewrite this email", "casual"))

The def keyword defines a function. The function name is followed by parentheses containing the inputs (called parameters). return sends a value back to whoever called the function.

Today's Exercise

Build a Tip Calculator + Password Generator

Create a file called day1.py and build both tools using what you learned today.

Tip calculator: takes bill amount as input, prints tip and total at 15%, 18%, and 20%
Password generator: uses import random and import string, generates a password of a length you specify
Wrap both in functions and call them from a if __name__ == "__main__": block

python day1.py — starter

import random
import string

def tip_calculator(bill):
    """Print tip amounts at 15%, 18%, and 20%."""
    for pct in [15, 18, 20]:
        tip = bill * (pct / 100)
        total = bill + tip
        print(f"{pct}%: tip=${tip:.2f}, total=${total:.2f}")

def generate_password(length=16):
    """Generate a random strong password."""
    chars = string.ascii_letters + string.digits + "!@#$%"
    return "".join(random.choice(chars) for _ in range(length))

if __name__ == "__main__":
    tip_calculator(84.50)
    print(generate_password(20))

Run it: python day1.py

What You Learned Today

Installed Python and set up a working development environment
Variables and the four core data types: string, int, float, boolean
Control flow: if/elif/else, for loops, while loops
Functions: defining them, calling them, returning values, default parameters
Built two working programs: a tip calculator and a password generator

Course Progress

Day 1 of 5 — 20%

Day 1 Complete

Day 2: Lists, Dicts, and JSON

Tomorrow you'll learn the data structures that AI APIs use to talk to you — and how to read any JSON response.

Start Day 2

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