Setup

You only need Python 3.10 or newer. Every example uses the standard library and runs offline.

python3 --version

Get the code

Each session’s Full source section has every file, ready to copy or download. Save a file and run it:

python3 1.singleton_new.py

The files are numbered in the order of the session: 0. is the code without the pattern, 1. is the same job with it, and the later numbers show variations and the AI example.

The AI examples

The AI examples (*llm*, *rag*, *agent*, 3.guardrails.py, …) use a tiny fake LLM so they run without an API key and without cost. They show the real provider code alongside it. To try a real model, install LangChain’s Groq package and set your key:

pip install langchain-groq

export GROQ_API_KEY=your-key-here

Then swap the fake for the real client, e.g. ChatGroq(model="openai/gpt-oss-120b"). The patterns work exactly the same.

Before you start

You should be comfortable with Python classes: __init__, methods, inheritance, and abc.ABC with @abstractmethod. If class Cat(Animal): and super().__init__() look new, revise classes and inheritance first.