JavaScript parser of Python
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JavaScript-based Python parser. Outputs an abstract syntax tree as specified by the Mozilla Parser API.

This parser is a work in progress, adapted from the JavaScript parser Acorn.

Check out the demo page to see what Filbert can do.

Want to contribute?

Thank you, we really appreciate you taking the time to help!

Filbert is under active development at Please submit pull requests or file GitHub issues to that repository. You can also email Matt.


npm install filbert


When run in a CommonJS (node.js) or AMD environment, exported values appear in the interfaces exposed by the individual files, as usual. When loaded in the browser without any kind of module management, a single global object filbert will be defined, and all the exported properties will be added to that.


This file contains the actual parser (and is what you get when you require("filbert") in node.js).

parse(input, options) is used to parse a Python program. The input parameter is a string, options can be undefined or an object setting some of the options listed below. The return value will be an abstract syntax tree object as specified by the Mozilla Parser API.

When encountering a syntax error, the parser will raise a SyntaxError object with a meaningful message. The error object will have a pos property that indicates the character offset at which the error occurred, and a loc object that contains a {line, column} object referring to that same position.

  • locations: When true, each node has a loc object attached with start and end subobjects, each of which contains the one-based line and zero-based column numbers in {line, column} form. Default is false.

  • ranges: To add a semi-standardized "range" property holding a [start, end] array with the same numbers, set the ranges option to true.

pythonRuntime holds an object required to execute JavaScript code that has been generated from the output AST. (e.g. the builtin len() function is not native to JavaScript). This object must be available if the AST output from parse will be used to generate and execute JavaScript code.

runtimeParamName name of the python runtime object that will be referenced in the output AST. (e.g. if set to '__pythonRuntime', AST nodes will have references to a '__pythonRuntime' object).

tokenize(input, options) exports a primitive interface to Filbert's tokenizer. The function takes an input string and options similar to parse, and returns a function that can be called repeatedly to read a single token, and returns a {start, end, type, value} object (with added startLoc and endLoc properties when the locations option is enabled). This object will be reused (updated) for each token, so you can't count on it staying stable.

tokTypes holds an object mapping names to the token type objects that end up in the type properties of tokens.


This file implements an error-tolerant parser. It exposes a single function.

parse_dammit(input, options) takes the same arguments and returns the same syntax tree as the parse function in filbert.js, but never raises an error, and will do its best to parse syntactically invalid code in as meaningful a way as it can. It'll insert identifier nodes with name "✖" as placeholders in places where it can't make sense of the input. Depends on filbert.js, because it uses the same tokenizer.

Language Support

Python3 is the target language. Much of it is working, and the remaining pieces are outlined below.



False None True and break class continue def elif else for if in is not or pass return while


abs() all() any() ascii() bool() chr() dict() enumerate() filter() float() hex() int() len() list() map() max() min() oct() ord() pow() print() range() repr() reversed() round() sorted() str() sum() tuple()

Coming Soon!


as assert del except finally from import global lambda nonlocal raise try with yield


bin() bytearray() bytes() callable() classmethod() compile() complex() delattr() dir() divmod() eval() exec() format() frozenset() getattr() globals() hasattr() hash() help() id() input() isinstance() issubclass() iter() locals() memoryview() next() object() open() property() set() setattr() slice() staticmethod() super() type() vars() zip() _import_()


grunt test

Get in touch

Please use the GitHub issues, or email Matt