TatSu-LTS 5.16.0

竜 TatSu-LTS

竜 TatSu-LTS is a friendly fork of 竜 TatSu that guarantees compatibility with all supported versions of Python. Compatibility with older Python versions is maintained as long as that does not require additional changes. Only patches required to support Python version older than the ones supported by the upstream project are applied. 竜 TatSu-LTS releases use the same version number as the 竜 TatSu release on which they are based and should be published shortly after the corresponding upstream release.

Both the 竜 TatSu and 竜 TatSu-LTS distributions install a Python package and an executable named tatsu. Users can thus switch seamlessly between the two, depending on which Python version support guarantees they prefer. For project that desire to support a wide range of Python versions, there is no drawback in depending on 竜 TatSu-LTS other than the short delay in the release of new versions.

竜 TatSu

竜 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.

竜 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.

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

竜 TatSu expects a maintained version of Python (>=3.14 at the moment). 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 compatibility considerations when developing future releases. That said, currently all tests run in versions down to Python 3.12.

If you need support for previous versions of Python, please consider TatSu-LTS, a friendly fork of 竜 TatSu aimed at compatibility with other versions of Python still used by many projects. The developers of both projects work together to promote compatibility with most versions of Python.

$ pip install TatSu-LTS

parser = tatsu.compile(grammar)

ast = tatsu.parse(grammar, input)

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

parser_source = tatsu.to_python_sourcecode(grammar)

GRAMMAR = '''
    @@grammar::CALC


    start = expression $ ;


    expression
        =
        | expression '+' term
        | expression '-' term
        | term
        ;


    term
        =
        | term '*' factor
        | term '/' factor
        | 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))

[
  "3",
  "+",
  [
    "5",
    "*",
    [
      "10",
      "-",
      "20"
    ]
  ]
]