Modified fork of CPython's ast module that parses `# type:` comments
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Typed AST

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typed_ast is a Python 3 package that provides a Python 2.7 and Python 3 parser similar to the standard ast library. Unlike ast, the parsers in typed_ast include PEP 484 type comments and are independent of the version of Python under which they are run. The typed_ast parsers produce the standard Python AST (plus type comments), and are both fast and correct, as they are based on the CPython 2.7 and 3.6 parsers. typed_ast runs on Python 3.3-3.6 on Linux, OS X and Windows.

Development Philosophy

This project is a drop-in replacement for the builtin ast module. It is intended to be bug-for-bug compatible and behave identically, except for the presence of a few additional fields on the returned classes and a few additional optional arguments to the parse call. Therefore, typed_ast will not accept any bugfixes for bugs in ast -- they should be fixed upstream instead. To avoid feature bloat, any new features for typed_ast should have the potential to be broadly useful and not be built just for one niche usecase or in a manner such that only one project can use them.



The ast3 parser produces the AST from the latest version of Python 3 (currently Python 3.6). When new versions of Python 3 are released, it will be updated to match any changes in their AST. (For rationale and technical details, see here.) The AST it currently produces is described in ast3/Parser/Python.asdl. If you wish to limit parsing to older versions of Python 3, ast3 can be configured to to give a SyntaxError for new syntax features introduced beyond a given Python version. For more information, see the module docstring in typed_ast/


The ast27 parser tracks the standard Python 2.7 AST, which is expected to never receive further updates. The AST it produces is described in ast27/Parser/Python.asdl. For more information, see the module docstring in typed_ast/


typed_ast also provides a conversions module which converts ast27 ASTs into ast3 ASTs. This functionality is somewhat experimental, however. For more information, see the py2to3 docstring in typed_ast/conversions.

Note: as these parsers consider type comments part of the grammar, incorrectly placed type comments are considered syntax errors.

Updates and Releases

To update typed_ast for new major Python releases, see

To make a new typed_ast release, see