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A concrete syntax tree with AST-like properties for Python 3.7 programs.

Project description

LibCST

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

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LibCST parses Python 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.

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.

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(),
  ],
)

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 with pip:

pip install libcst

Development

Start by setting up and activating a virtualenv:

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 and black to format code. To format changes to be conformant, run the following in the root:

isort -q -y && black libcst/

To run all tests, you’ll need to install tox and do the following in the root:

tox -e py37

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

python -m unitttest libcst.tests.test_batched_visitor

See the unittest documentation for more examples of how to run tests.

We use Pyre for type-checking. To verify types for the library, do the following in the root:

pyre check

To generate documents, do the following in the root:

tox -e docs

License

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

Project details


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