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# Typed AST
[](https://travis-ci.org/python/typed_ast)
[](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` up to Python 3.7, the parsers in
`typed_ast` include [PEP 484](https://www.python.org/dev/peps/pep-0484/) 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 3.6-3.10 on Linux, OS X and Windows.
**Note:** Starting with Python 3.8, we recommend to use the native `ast` parser
(see below).
## 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.
Due to reliance on certain C APIs, this library does not build on and there
are [no plans to support PyPy](https://github.com/python/typed_ast/issues/111).
### 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 Python 3 code, up to Python 3.7.
(For rationale and technical
details, see [here](update_process.md).) The AST it currently produces is described in
[ast3/Parser/Python.asdl](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](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](ast27/Parser/Python.asdl). For more information,
see the module docstring in [typed\_ast/ast27.py](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](typed_ast/conversions.py).
Note: as these parsers consider type comments part of the grammar, incorrectly
placed type comments are considered syntax errors.
## Releases
To make a new `typed_ast` release, see [`release_process.md`](release_process.md).
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