2 Python Programming Fundamentals

Chapter 2 has one job: help you turn a small idea into Python code that runs, saves data, handles errors, and can be explained.

See The Python Work Loop

Read the picture first. Most beginner Python programs are this loop:

input -> data structure -> function -> file/API/output

Python matters in AI because the same loop later becomes data cleaning, model training, RAG retrieval, API wrapping, and Agent tools.

Learning Order And Task List

Use this table as both the chapter guide and the task sheet.

Page	Follow-along action	Evidence to keep
2.1.1 Python Introduction to 2.1.5 Flow Control	Type small scripts with variables, input/output, conditions, and loops	5 changed scripts with expected output
2.1.6 Data Structures	Store the same task list with a list, dict, and JSON-shaped object	A note explaining why one structure fits best
2.1.7 Function Basics and 2.1.8 Modules and Packages	Split repeated logic into functions and a module	A script with clear inputs and return values
2.2.2 Exception Handling and 2.2.3 File Operations	Save data, read it back, and handle a missing or broken file	A JSON/text file plus one debug note
2.2.1 OOP, 2.2.5 Iterators, and 2.2.6 Type Hints	Skim first, then return when a project needs structure or clarity	One refactored function or class
2.3.1 Task Manager to 2.3.4 AI API Experience	Build small projects that save data, collect data, expose an API, and call an AI API	Project folders with README run commands
2.3.5 Follow-Along Workshop	Combine CLI commands, JSON persistence, stats, and report export	ch02_output/ plus terminal output

Key terms for this chapter:

Term	Meaning
CLI	Command-Line Interface: a program controlled by typed commands
I/O	Input/Output: data entering a program and results leaving it
JSON	A text format for nested data such as tasks and API responses
API	A doorway that lets one program call another program
SDK	A library that wraps an API into easier functions

First Runnable Loop

Run this in an empty practice folder. It creates a tiny JSON task manager without any third-party package.

import json
from pathlib import Path

DATA = Path("tasks.json")

def load_tasks():
    if not DATA.exists():
        return []
    try:
        return json.loads(DATA.read_text(encoding="utf-8"))
    except json.JSONDecodeError:
        return []

def save_tasks(tasks):
    DATA.write_text(json.dumps(tasks, ensure_ascii=False, indent=2), encoding="utf-8")

tasks = load_tasks()
tasks.append({"title": "Learn Python file I/O", "done": False})
save_tasks(tasks)
print(f"saved {len(tasks)} task(s)")

Expected output:

saved 1 task(s)

Run it twice. The second run should print saved 2 task(s). That proves the program can save state and read it back.

Depth Ladder

Level	What you can prove
Minimum pass	You can write expressions, conditions, loops, and functions that produce the expected output.
Project-ready	The program can persist data, handle one failure path, and explain its inputs and outputs in a README.
Deeper check	You can separate core logic from file/API boundaries, add type hints where they clarify intent, and test one edge case before changing the code.

Common Failures

Symptom	First thing to check	Usual fix
Syntax error	The reported line and the line above it	Check indentation, parentheses, quotes, and colons
File not found	The current working directory	Print Path.cwd() and move the file or change the path
JSON parsing failed	Whether the file is empty or malformed	Add try/except and fall back to an empty list
Function is confusing	Inputs, return value, and hidden global state	Split it into smaller functions with one responsibility
API call fails	Parameters, status code, and returned error body	Print the response safely and handle the error path

Pass Check

Move to Chapter 3 when you can answer these five questions:

• What data enters the program, and what result leaves it?
• When is a dictionary better than a list?
• What folder is a file path relative to?
• What is the difference between print and return?
• Can another person run your project from the README?

For a printable checklist, use 2.0 Study Guide and Task Sheet. The next chapter will use Python to process CSV files, analyze data, and connect databases.