psimodpy: A Python library for the PSI-MOD protein modification ontology

psimodpy is a typed Python package for parsing and querying the PSI-MOD protein modification ontology. It ships with the bundled PSI-MOD OBO data so it works offline, parses OBO source files into structured PsiModEntry objects covering terms, synonyms, cross-references, relationships, and formula/mass annotations, and exposes a queryable in-memory database with TSV/CSV export and a round-trip OBO writer. An optional FastAPI and Model Context Protocol server exposes the same database over HTTP for interactive lookups and integration with proteomics tooling. psimodpy is intended for proteomics software, annotation pipelines, and research scripts that need programmatic access to PSI-MOD terms. It requires Python 3.12 or later.