Understanding LLMs Through 200 Lines of Python

Andrej Karpathy's MicroGPT is a 200-line pure Python script that trains and runs a GPT from scratch — no libraries, no dependencies. The script contains the same algorithm that powers large language models like ChatGPT. Developer growingSWE has now created an interactive, visual walkthrough designed to make this code accessible to beginners.

What You Will Learn

Tokenizer: Converting text to integer sequences. Type any name and watch it get tokenized in real time.
Softmax: How raw logit scores get converted into a probability distribution over possible next tokens.
Backpropagation: Step through gradient flow on a computation graph to understand how the model learns from its mistakes.
Attention heatmaps: Visualize which tokens the self-attention mechanism focuses on during generation.

From Names to ChatGPT

The model trains on 32,000 human names and learns to generate plausible new ones like 'kamon', 'karai', 'anna', and 'anton'. The key insight: from ChatGPT's perspective, your entire conversation is just a document. The model's response is a statistical document completion — the same principle at work in this 200-line script.

The tutorial earned 182 points on Hacker News, complementing Karpathy's original MicroGPT post which scored 1,678 points. Together, they represent one of the most accessible entry points into understanding how modern LLMs work under the hood.

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MicroGPT Explained Interactively

MicroGPT Explained Interactively