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README.md

Typed AST

Build Status Chat at https://gitter.im/python/typed_ast

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 3.7 parsers. typed_ast runs on CPython 3.5-3.8 on Linux, OS X and Windows.

Development Philosophy

This project is a (mostly) 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.

Incompatibilities

For the purposes of consuming syntax trees, this should be a drop-in replacement. It is not a drop-in replacement for users that wish to create or transform ASTs, as a number of syntax tree classes have additional fields that must be populated when constructing them.

Python 3.8

typed_ast will not be updated to support parsing Python 3.8 and newer. Instead, it is recommended to use the stdlib ast module there, which has been augmented to support extracting type comments and has limited support for parsing older versions of Python 3.

Submodules

ast3

The ast3 parser produces the AST from a recent 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/ast3.py.

ast27

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/ast27.py.

conversions

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 update_process.md.

To make a new typed_ast release, see release_process.md.

About

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

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