Mypy is a static type checker for Python, utilized at Nylas to enhance code quality and development efficiency by making Python code type-aware without compromising its dynamic nature. By using Python's type annotations, Mypy performs checks independently of code execution, thereby reducing runtime errors and facilitating easier code comprehension and maintenance. The integration of Mypy into Nylas' development workflow via custom linting tools and continuous integration pipelines has improved code scalability and reliability, helping developers catch errors early and streamline refactoring processes. Despite an initial learning curve, the adoption of Mypy has led to increased productivity and a better understanding of code structures, as type annotations simplify function interfaces and reduce the need for additional checks. The blog also highlights practical strategies for using Mypy, such as employing TypedDict for structured types and handling circular imports with TYPE_CHECKING, showcasing how Mypy has become a pivotal tool for managing large codebases and enhancing product quality at Nylas.