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At least for the people who send me mail about a new language that they're designing, the general advice is: do it to learn about how to write a compiler. Don't have any expectations that anyone will use it, unless you hook up with some sort of organization in a position to push it hard. It's a lottery, and some can buy a lot of the tickets. There are plenty of beautiful languages (more beautiful than C) that didn't catch on. But someone does win the lottery, and doing a language at least teaches you something.

Dennis Ritchie (1941-2011) Creator of the C programming language and of Unix


TatSu is a tool that takes grammars in a variation of EBNF as input, and outputs memoizing (Packrat) PEG parsers in Python.

Why use a PEG parser? Because regular languages (those parsable with Python's re package) "cannot count". Any language with nested structures or with balancing of demarcations requires more than regular expressions to be parsed.

TatSu can compile a grammar stored in a string into a tatsu.grammars.Grammar object that can be used to parse any given input, much like the re module does with regular expressions, or it can generate a Python module that implements the parser.

TatSu supports left-recursive rules in PEG grammars using the algorithm by Laurent and Mens. The generated AST has the expected left associativity.

Starting with version 5.8.0 竜 TatSu requires Python 3.10 or later. While no code in 竜 TatSu yet depends on new language or standard library features, the authors don't want to be constrained by Python version comaptibility consideration when developing features that will be part future releases. Therefore, to simplify version pinning for users of the library, they decided to proactively bump the Python minimum required version to 3.10.

TatSu releases in the 5.7 series closely track releases in the 5.8 series while maintaining compatibility with Python 3.8 and later. Bug fixes are back-ported from 5.8 releases. Features are back-ported from the 5.8 releases unless they depend on Python features not available on the supported Python versions. Refer to the CHANGELOG for details.


$ pip install TatSu

Using the Tool

TatSu can be used as a library, much like Python's re, by embedding grammars as strings and generating grammar models instead of generating Python code.

  • tatsu.compile(grammar, name=None, **kwargs)

    Compiles the grammar and generates a model that can subsequently be used for parsing input with.

  • tatsu.parse(grammar, input, **kwargs)

    Compiles the grammar and parses the given input producing an AST as result. The result is equivalent to calling:

    model = compile(grammar)
    ast = model.parse(input)

    Compiled grammars are cached for efficiency.

  • tatsu.to_python_sourcecode(grammar, name=None, filename=None, **kwargs)

    Compiles the grammar to the Python sourcecode that implements the parser.

This is an example of how to use 竜 TatSu as a library:


    start = expression $ ;

        | expression '+' term
        | expression '-' term
        | term

        | term '*' factor
        | term '/' factor
        | factor

        | '(' expression ')'
        | number

    number = /\d+/ ;

if __name__ == '__main__':
    import json
    from tatsu import parse
    from tatsu.util import asjson

    ast = parse(GRAMMAR, '3 + 5 * ( 10 - 20 )')
    print(json.dumps(asjson(ast), indent=2))

TatSu will use the first rule defined in the grammar as the start rule.

This is the output:



For a detailed explanation of what 竜 TatSu is capable of, please see the documentation.


Please use the [tatsu] tag on StackOverflow for general Q&A, and limit Github issues to bugs, enhancement proposals, and feature requests.


See the CHANGELOG for details.


You may use 竜 TatSu under the terms of the BSD-style license described in the enclosed LICENSE.txt file. If your project requires different licensing please email.