Building a Neural Network from Scratch: Implementing Core Deep Learning Algorithms in Pure Python

This project aims to bridge the understanding gap caused by the convenience of deep learning frameworks. It implements core neural network components (matrix operations, activation functions, backpropagation, optimizers, etc.) using pure Python without relying on external libraries like NumPy or TensorFlow. Its core value lies in educational significance—allowing learners to understand the internal mechanisms of deep learning through a "white-box" experience, rather than just knowing how to call APIs.

In today's era of highly developed deep learning frameworks, practitioners often can call APIs to train models but have only a superficial understanding of internal mechanisms. This project is positioned as an educational tool: it strips away abstraction layers, exposes details of each component, allows learners to see the process of data flow, gradient calculation, and weight updates, and provides a "white-box" learning experience that frameworks cannot offer.

The project strictly adheres to the principle of no external dependencies, using only Python standard libraries (math, random, csv, json). The core component implementations include:

• Matrix operations: Manually implemented matrix multiplication, transposition, and element-wise operations;
• Activation functions: Forward and backward propagation for Sigmoid, ReLU, and Tanh;
• Loss functions: MSE, cross-entropy, and their gradients;
• Backpropagation: Calculation of parameter gradients via the chain rule;
• Optimizers: Batch gradient descent, SGD, and learning rate scheduling.

The project uses a modular design:

• File organization: main.py (entry point), network.py (core implementation), utils.py (tools), config.json (configuration), train_data.csv (data);
• Configuration-driven: Adjust network structure, activation functions, optimizers, etc., by modifying config.json;
• Training flow: Data loading → Initialization → Iterative training (forward propagation → loss calculation → backpropagation → weight update) → Monitoring → Save results.

This project helps learners deeply understand core concepts: weights and biases, forward/backward propagation, gradients and convergence, overfitting and generalization. It also serves as practice material for Python: data structures, file operations, modular programming, and debugging skills.

The pure Python implementation has limited performance (no vectorization/GPU acceleration), making it suitable for learning rather than large-scale applications. Improvement directions include: extending convolutional/recurrent layers, adding regularization/batch normalization, implementing more optimizers (e.g., Adam), and adding training visualization (loss curves, weight distributions).

This project complements other resources:

• Compared to framework tutorials: Teaches tool principles rather than usage;
• Compared to theoretical courses: Provides runnable code to concretize abstract theories;
• Compared to visualization tools: Allows modifying experiments and observing effect changes.

This project demonstrates core deep learning mechanisms in a plain way—no fancy features, only essential algorithm implementations. For learners who want to understand why and how neural networks work, simplicity is its greatest advantage. Hand-writing a neural network means mastering basic concepts like backpropagation and gradient descent; this ability is of great value when facing new models, making it an excellent teaching material.