A library for defining and manipulating ASTs

pip install iAST==0.2.1



(Supports Python 3.3 and 3.4)

This library provides a way of defining and transforming abstract syntax trees (ASTs) for custom languages. It can be used to help build a compiler or other program transformation system.

iAST reads your language's abstract syntax from an ASDL grammar, and automatically generates node classes. A standard visitor-style framework is provided for traversing, transforming, and pattern matching over trees. Nodes are hashable, have structural equality, and support optional type checking. (Parsing is not supported and should be handled by an external parser generator.)

Node definitions for the ASTs of Python 3.3 and Python 3.4 are provided out-of-the-box, along with tools for writing code templates and macros targeting Python code. However, the main framework works on ASTs for arbitrary languages.


See arith.py for basic usage and visitors/transformers. See toy.py for a comparison with Python's own ast module and the use of type checking. Both examples use abstract grammars from the corresponding ASDL files.


To install from pip/PyPI:

python -m pip install iast

To use a development version:

python -m pip install https://github.com/brandjon/iast/tree/tarball/develop

Python 3.3 and 3.4 are supported. The only dependency is simplestruct, which is used to define the node classes.


Tests can be run with python setup.py test, or by installing Tox and running python -m tox in the project root. Tox tests both Python 3.3 and 3.4 configurations. Building a source distribution (python setup.py sdist) requires the setuptools extension package setuptools-git.


[1]: Eli Bendersky's rewrite of the Python ASDL parser, which powers iAST's generation of nodes from ASDL.