Increasing Types in Large Codebases with Pyrefly and AI Tooling

Learn how Meta approaches adding type annotations to massive Python codebases using Pyre, Pyrefly, and AI-powered tooling in this 24-minute conference talk from PyBay 2025. Discover the significant benefits of typed Python code and explore practical strategies for incrementally increasing type coverage in large-scale projects. Examine Meta's experience managing some of the world's largest typed Python codebases and understand the challenges of working with substantial amounts of untyped legacy code. Explore how Pyrefly, Meta's latest tooling innovation, enhances developer experience when working with extensive Python projects. Gain insights into scalable approaches for type annotation adoption, including automated tooling and AI-assisted methods that can help development teams efficiently modernize their codebases. Understand the technical considerations and best practices for implementing type checking systems in enterprise-level Python applications, drawing from real-world experience at one of the world's largest technology companies.