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   :width: 600 px
   :alt: LibCST

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

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.. intro-start

LibCST parses Python 3.5, 3.6 or 3.7 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/master?filepath=docs%2Fsource%2Ftutorial.ipynb>`__.

::

    1 + 2

::

    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/usage.html>`__.

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

LibCST requires Python 3.6+ 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

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

Start by setting up and activating a virtualenv:

.. code-block:: shell

    git clone git@github.com:Instagram/LibCST.git libcst
    cd libcst
    python3 -m venv ../libcst-env/  # just an example, put this wherever you want
    source ../libcst-env/bin/activate
    pip install --upgrade pip  # optional, if you have an old system version of pip
    pip install -r requirements.txt -r requirements-dev.txt
    # If you're done with the virtualenv, you can leave it by running:
    deactivate

We use `isort <https://isort.readthedocs.io/en/stable/>`_ and `black <https://black.readthedocs.io/en/stable/>`_
to format code. To format changes to be conformant, run the following in the root:

.. code-block:: shell

    tox -e autofix

To run all tests, you'll need to install `tox <https://tox.readthedocs.io/en/latest/>`_
and do the following in the root:

.. code-block:: shell

    tox -e py37

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

.. code-block:: shell

    python -m unitttest 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 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

    pyre check

To generate documents, do the following in the root:

.. code-block:: shell

    tox -e docs

Future
======

- Extension to Matchers API which incorporates metadata on nodes.
- Addition to Matchers API which allows for operations such as extract/find/replaceall, similar to the RE module.
- Additional layer providing command-line frontend for executing refactors.
- More metadata providers from deep static analysis including variable fully qualified name, variable type annotation, etc.

License
=======

LibCST is MIT licensed, as found in the LICENSE file.

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.