LibCST

A concrete syntax tree parser and serializer library for Python that preserves many aspects of Python's abstract syntax tree

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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 -> 3.12 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

You’ll need a recent Rust toolchain <https://rustup.rs>_ for developing.

We recommend using hatch <https://hatch.pypa.io/> for running tests, linters,
etc.

Then, start by setting up and building the project:

… code-block:: shell

git clone [email protected]:Instagram/LibCST.git libcst
cd libcst
hatch env create

To run the project’s test suite, you can:

… code-block:: shell

hatch run test

You can also run individual tests by using unittest and specifying a module like
this:

… code-block:: shell

hatch run python -m unittest libcst.tests.test_batched_visitor

See the unittest documentation <https://docs.python.org/3/library/unittest.html>_
for more examples of how to run tests.

We have multiple linters, including copyright checks and
slotscheck <https://slotscheck.rtfd.io>_ to check the correctness of class
__slots__. To run all of the linters:

… code-block:: shell

hatch run lint

We use ufmt <https://ufmt.omnilib.dev/en/stable/>_ to format code. To format
changes to be conformant, run the following in the root:

… code-block:: shell

hatch run format

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

To rebuild the ``libcst.native`` module, from the repo root:

.. code-block:: shell

    hatch env prune && hatch env create

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

hatch run typecheck

Generating Documents


To generate documents, do the following in the root:

.. code-block:: shell

    hatch run 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.

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

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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.