@codemirror/lang-python

@codemirror/lang-python adds first-class Python editing to CodeMirror 6. It wraps the Lezer Python parser in a CodeMirror LanguageSupport, layering on syntax highlighting, context-aware indentation for blocks and continuation lines, code folding, and completion sources for both built-in globals and locally defined names.

Dropping the python() extension into an EditorView gives you a Python-aware editing surface without hand-rolling tokenizers or indentation heuristics. It is one of the official CodeMirror language packages, sharing the same architecture and release conventions as the rest of the ecosystem.

What You Get

• A ready-to-use python() LanguageSupport extension for CodeMirror 6.
• Syntax highlighting driven by the Lezer Python grammar.
• Context-aware indentation for blocks, brackets, and continuation lines, including match/case, else/elif, and except handling.
• Autocompletion for Python built-in globals, keywords, and locally defined names.
• Code folding for functions, classes, collections, and multiline strings.

Common Use Cases

• Building an in-browser Python code editor or REPL front-end.
• Adding a Python editing mode to a documentation site, notebook, or teaching tool.
• Powering code cells in a low-code or data-science web application.

Under The Hood

Architecture — The package is a thin, well-factored integration layer over the Lezer Python parser. src/python.ts defines pythonLanguage as an LRLanguage that configures the imported Lezer parser with indentNodeProp and foldNodeProp metadata; helper functions innerBody and indentBody walk the syntax tree to resolve Python’s whitespace-significant indentation, handling block bodies, match/case, and deindenting keywords. The exported python() factory returns a LanguageSupport bundling the language with two autocomplete data providers wired from src/complete.ts.

Tech Stack — Written entirely in TypeScript (100% of the source), it depends on @lezer/python and @lezer/common for parsing plus @codemirror/language, @codemirror/autocomplete, and @codemirror/state for editor integration. It builds to dual ESM/CJS output via the @codemirror/buildhelper toolchain (cm-buildhelper), ships type definitions, and is marked sideEffects: false for tree-shaking.

Code Quality — The codebase is compact (~12KB of source across python.ts and complete.ts) and idiomatic for the CodeMirror ecosystem, using a NodeWeakMap cache and a declarative gatherCompletions table keyed by node type to collect scoped definitions. Public APIs carry doc comments. Testing is present via a test-indent.ts suite run with cm-runtests, focused on the indentation logic that is the trickiest part of the package.

API Design — The public surface is minimal and ergonomic: importing python() and adding it to an editor’s extensions is the entire happy path, matching the uniform convention shared across all official CodeMirror language packages. Lower-level exports (pythonLanguage, globalCompletion, localCompletionSource) let advanced users compose custom behavior. The README documents each export with linked type references, keeping boilerplate near zero.