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   :alt: LibCST

A Concrete Syntax Tree (CST) parser and serializer library for Python

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.. intro-start

LibCST parses Python 3.0 -> 3.15 source code as a CST tree that keeps
all formatting details (comments, whitespaces, parentheses, etc). It's useful for
building automated refactoring (codemod) applications and linters.

.. intro-end

.. why-libcst-intro-start

LibCST creates a compromise between an Abstract Syntax Tree (AST) and a traditional
Concrete Syntax Tree (CST). By carefully reorganizing and naming node types and
fields, we've created a lossless CST that looks and feels like an AST.

.. why-libcst-intro-end

You can learn more about `the value that LibCST provides
<https://libcst.readthedocs.io/en/latest/why_libcst.html>`__ and `our
motivations for the project
<https://libcst.readthedocs.io/en/latest/motivation.html>`__
in `our documentation <https://libcst.readthedocs.io/en/latest/index.html>`__.
Try it out with `notebook examples <https://mybinder.org/v2/gh/Instagram/LibCST/main?filepath=docs%2Fsource%2Ftutorial.ipynb>`__.

Example expression::

    1 + 2

CST representation:

.. code-block:: python

    BinaryOperation(
        left=Integer(
            value='1',
            lpar=[],
            rpar=[],
        ),
        operator=Add(
            whitespace_before=SimpleWhitespace(
                value=' ',
            ),
            whitespace_after=SimpleWhitespace(
                value=' ',
            ),
        ),
        right=Integer(
            value='2',
            lpar=[],
            rpar=[],
        ),
        lpar=[],
        rpar=[],
    )

Getting Started
===============

Examining a sample tree
-----------------------

To examine the tree that is parsed from a particular file, do the following::

    python -m libcst.tool print <some_py_file.py>

Alternatively, you can import LibCST into a Python REPL and use the included parser
and pretty printing functions:

>>> import libcst as cst
>>> from libcst.tool import dump
>>> print(dump(cst.parse_expression("(1 + 2)")))
BinaryOperation(
  left=Integer(
    value='1',
  ),
  operator=Add(),
  right=Integer(
    value='2',
  ),
  lpar=[
    LeftParen(),
  ],
  rpar=[
    RightParen(),
  ],
)

For a more detailed usage example, `see our documentation
<https://libcst.readthedocs.io/en/latest/tutorial.html>`__.

Installation
------------

LibCST requires Python 3.9+ and can be easily installed using most common Python
packaging tools. We recommend installing the latest stable release from
`PyPI <https://pypi.org/project/libcst/>`_ with pip:

.. code-block:: shell

    pip install libcst

For parsing, LibCST ships with a native extension, so releases are distributed as binary
wheels as well as the source code. If a binary wheel is not available for your system
(Linux/Windows x86/x64 and Mac x64/arm are covered), you'll need a recent
`Rust toolchain <https://rustup.rs>`_ for installing.

Further Reading
---------------
- `Static Analysis at Scale: An Instagram Story. <https://instagram-engineering.com/static-analysis-at-scale-an-instagram-story-8f498ab71a0c>`_
- `Refactoring Python with LibCST. <https://chairnerd.seatgeek.com/refactoring-python-with-libcst/>`_

Development
-----------

See `CONTRIBUTING.md <CONTRIBUTING.md>`_ for more details.

Building
~~~~~~~~

In order to build LibCST, which includes a native parser module, you
will need to have the Rust build tool ``cargo`` on your path. You can
usually install ``cargo`` using your system package manager, but the
most popular way to install cargo is using
`rustup <https://rustup.rs/>`_.

To build just the native parser, do the following from the ``native``
directory:

.. code-block:: shell

    cargo build

The ``libcst.native`` module should be rebuilt automatically, but to force it:

.. code-block:: shell

    uv sync --reinstall-package libcst

Type Checking
~~~~~~~~~~~~~

We use `Pyre <https://github.com/facebook/pyre-check>`_ for type-checking.

To verify types for the library, do the following in the root:

.. code-block:: shell

    uv run poe typecheck

Generating Documents
~~~~~~~~~~~~~~~~~~~~

To generate documents, do the following in the root:

.. code-block:: shell

    uv run --group docs poe docs

Future
======

- Advanced full repository facts providers like fully qualified name and call graph.

License
=======

LibCST is `MIT licensed <LICENSE>`_, as found in the LICENSE file.

.. fb-docs-start

Privacy Policy and Terms of Use
===============================

- `Privacy Policy <https://opensource.facebook.com/legal/privacy>`_
- `Terms of Use <https://opensource.facebook.com/legal/terms>`_

.. fb-docs-end

Acknowledgements
================

- Guido van Rossum for creating the parser generator pgen2 (originally used in lib2to3 and forked into parso).
- David Halter for parso which provides the parser and tokenizer that LibCST sits on top of.
- Zac Hatfield-Dodds for hypothesis integration which continues to help us find bugs.
- Zach Hammer improved type annotation for Mypy compatibility.