Introduction

This tutorial is part of a comprehensive project that guides you through building a Byte Pair Encoding (BPE) tokenizer from scratch using Test-Driven Development (TDD). The project is inspired by Sebastian Raschka’s blog post on BPE tokenizer implementation but has been redesigned with an educational focus and TDD approach.

Our implementation differs from the original in several ways:

  • It’s a character-level (rather than byte-level) BPE tokenizer for simplicity
  • The code is organized into progressive tutorial sections
  • Each section includes comprehensive documentation, tests, and implementation templates
  • We emphasize test-driven development practices throughout

This tutorial series will help you understand both the theoretical concepts of BPE tokenization and practical implementation details while following software engineering best practices.

Getting Started

Welcome to the first part of our series on building a Byte Pair Encoding (BPE) tokenizer using Test-Driven Development (TDD). In this tutorial, we’ll set up our project structure, create a virtual environment, and write our first test for the tokenizer’s initialization.

Source Code: The complete source code for this tutorial series is available on my GitHub repo at Aken-2019/bpe-tokenizer-tdd. Feel free to clone the repository and follow along with the implementation.

Prerequisites

  • Tested with Python 3.12 (should work with Python 3.7+)
  • pytest for testing
  • Basic knowledge of Python and unit testing

Project Structure

Let’s start by creating the following directory structure for our project:

bpe-tokenizer/
├── src/
│   └── __init__.py
│   └── BPETokenizer.py      # Our main tokenizer implementation
├── tests/
│   ├── __init__.py
│   └── test_bpe_tokenizer.py # Our test file
├── requirements.txt
└── README.md

Setting Up the Development Environment

  1. First, create and activate a virtual environment:
# Create a virtual environment
python -m venv venv

# Activate the virtual environment
# On Windows:
# .\venv\Scripts\activate
# On macOS/Linux:
source venv/bin/activate
  1. Install the required packages:
pip install -r requirements.txt

Writing Our First Test

Let’s start by writing a test for the basic initialization of our BPE tokenizer. We’ll use TDD, so we’ll write the test first and then implement the functionality.

Create a new file tests/test_tokenizer.py with the following content:

import pytest
from src.tokenizer import BPETokenizer


def test_tokenizer_initialization():
    """Test that the tokenizer initializes with empty vocabularies and merges."""
    # Arrange
    tokenizer = BPETokenizer()
    
    # Assert
    assert isinstance(tokenizer.vocab, dict), "Vocab should be a dictionary"
    assert isinstance(tokenizer.inverse_vocab, dict), "Inverse vocab should be a dictionary"
    assert isinstance(tokenizer.bpe_merges, dict), "BPE merges should be a dictionary"
    assert len(tokenizer.vocab) == 0, "Initial vocab should be empty"
    assert len(tokenizer.inverse_vocab) == 0, "Initial inverse vocab should be empty"
    assert len(tokenizer.bpe_merges) == 0, "Initial BPE merges should be empty"

Implementing the Basic Tokenizer Class

Now, let’s create the initial implementation of our tokenizer in src/tokenizer.py:

class BPETokenizer:
    """A simple implementation of Byte Pair Encoding (BPE) tokenizer."""
    
    def __init__(self):
        """Initialize the BPE Tokenizer with empty vocabularies and merges."""
        # Maps token_id to token_str (e.g., {11246: "some"})
        self.vocab = {}
        # Maps token_str to token_id (e.g., {"some": 11246})
        self.inverse_vocab = {}
        # Dictionary of BPE merges: {(token1, token2): merged_token_id}
        self.bpe_merges = {}

Running the Tests

Let’s run our test to make sure everything is working:

pytest tests/

You should see output indicating that the test passed. If you encounter any issues, make sure:

  1. Your virtual environment is activated
  2. You’ve installed pytest
  3. The file structure matches exactly what’s shown above

What We’ve Accomplished

In this first part of the series, we’ve:

  1. Set up our project structure
  2. Created a virtual environment
  3. Installed necessary dependencies
  4. Written our first test for tokenizer initialization
  5. Implemented the basic tokenizer class structure

Next Steps

In the next part of this series, we’ll:

  1. Add functionality to train the tokenizer on text data
  2. Implement the BPE algorithm
  3. Add methods to encode and decode text

In Part 2 we’ll dive deeper into implementing the BPE algorithm!

Resources

Happy coding!