Pegen

Pegen is a parser generator used by the CPython project to build Python’s parser from a grammar defined in a Parsing Expression Grammar (PEG). It is implemented in Python and forms part of the tooling that developers use to evolve Python’s syntax.

Pegen works by taking grammar specifications written in a PEG-like notation and generating the C code that CPython uses to parse Python source. The output is integrated into the CPython build process, producing the parser component that reads tokens produced by the tokenizer and constructs the abstract syntax tree that the interpreter uses during execution.

The tool was introduced to streamline grammar maintenance and enable easier experimentation with Python’s syntax. By consolidating grammar rules in a high-level specification, Pegen aims to improve consistency, readability, and the ability to evolve the language while providing clearer error messages and better diagnostics when parsing source code.

Pegen is open source and maintained as part of the Python project. Its usage is primarily within the CPython development workflow; end users who install Python largely interact with the parser indirectly through the interpreter rather than with Pegen itself. Changes to the grammar definitions and the corresponding generated parser are typically coordinated as part of CPython releases.

In practice, developers modify the grammar files in the repository and run Pegen to regenerate the parser, ensuring that the interpreter remains aligned with the defined syntax. This approach supports ongoing evolution of Python’s syntax while keeping the parsing logic centralized and maintainable.