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Full Syntax Tree for python to make writing refactoring code a realist task

Project description


Baron is a Full Syntax Tree (FST) library for Python. By opposition to an AST which drops some syntax information in the process of its creation (like empty lines, comments, formatting), a FST keeps everything and guarantees the operation fst_to_code(code_to_fst(source_code)) == source_code.


pip install baron

Basic Usage

from baron import parse, dumps

fst = parse(source_code_string)
source_code_string == dumps(fst)

There is a good chance that you’ll want to use RedBaron instead of using Baron directly. Think of Baron as the “bytecode of python source code” and RedBaron as some sort of usable layer on top of it.

If you don’t know what Baron is or don’t understand yet why it might be useful for you, read the « Why is this important? » section.


Baron documentation is available on Read The Docs.

Why is this important?

The usage of a FST might not be obvious at first sight so let’s consider a series of problems to illustrate it. Let’s say that you want to write a program that will:

  • rename a variable in a source file… without clashing with things that are not a variable (example: stuff inside a string)
  • inline a function/method
  • extract a function/method from a series of line of code
  • split a class into several classes
  • split a file into several modules
  • convert your whole code base from one ORM to another
  • do custom refactoring operation not implemented by IDE/rope
  • implement the class browser of smalltalk for python (the whole one where you can edit the code of the methods, not just showing code)

It is very likely that you will end up with the awkward feeling of writing clumpsy weak code that is very likely to break because you didn’t thought about all the annoying special cases and the formatting keeps bothering you. You may end up playing with until you realize that it removes too much information to be suitable for those situations. You will probably ditch this task as simple too complicated and really not worth the effort. You are missing a good abstraction that will take care of all of the code structure and formatting for you so you can concentrate on your task.

The FST tries to be this abstraction. With it you can now work on a tree which represents your code with its formatting. Moreover, since it is the exact representation of your code, modifying it and converting it back to a string will give you back your code only modified where you have modified the tree.

Said in another way, what I’m trying to achieve with Baron is a paradigm change in which writing code that will modify code is now a realist task that is worth the price (I’m not saying a simple task, but a realistic one: it’s still a complex task).


Having a FST (or at least a good abstraction build on it) also makes it easier to do code generation and code analysis while those two operations are already quite feasible (using and a templating engine for example).

Some technical details

Baron produces a FST in the form of JSON (and by JSON I mean Python lists and dicts that can be dumped into JSON) for maximum interoperability.

Baron FST is quite similar to Python AST with some modifications to be more intuitive to humans, since Python AST has been made for CPython interpreter.

Since playing directly with JSON is a bit raw I’m going to build an abstraction on top of it that will looks like BeautifulSoup/jQuery.

State of the project

Currently, Baron has been tested on the top 100 projects and the FST converts back exactly into the original source code. So, it can be considered quite stable, but it is far away from having been battle tested.

Since the project is very young and no one is already using it except my project, I’m open to changes of the FST nodes but I will quickly become conservative once it gets some adoption and will probably accept to modify it only once or twice in the future with clear indications on how to migrate.

Baron is targeting python 2.[67]. It has not been tested on python3 but should be working for most parts (except the new grammar like yield from, obviously). Baron runs under python 2 and python 3.


Run either py.test tests/ or nosetests in the baron directory.


You can reach us on


Old blog post announcing the project. Not that much up to date.


0.3 (2014-08-21)

  • path becomes a simple list and is easier to deal with
  • bounding box allows you to know the left most and right most position of a node see
  • redbaron is classified as supporting python3
  • ensure than when a key is a string, it’s empty value is an empty string and not None to avoid breaking libs that use introspection to guess the type of the key
  • key renaming in the FST: “delimiteur” -> “delimiter”
  • name_as_name and dotted_as_name node don’t have the “as” key anymore as it was useless (it can be deduce from the state of the “target” key)
  • dotted_name node doesn’t exist anymore, its existance was unjustified. In import, from_import and decorator node, it has been replaced from a key to a dict (with only a list inside of it) to a simple list.
  • dumps now accept a strict boolean argument to check the validity of the FST on dumping, but this isn’t that much a public feature and should probably be changed of API in the futur
  • name_as_name and dotted_as_name empty value for target is now an empty string and not None since this is a string type key
  • boundingbox now includes the newlines at the end of a node
  • all raised exceptions inherit from a common base exception to ease try/catch constructions
  • Position’s left and right functions become properties and thus attributes
  • Position objects can be compared to other Position objects or any iterables
  • make_position and make_bounding_box functions are deleted in favor of always using the corresponding class’ constructor

0.2 (2014-06-11)

0.1.3 (2014-04-13)

  • set sugar syntaxic notation wasn’t handled by the dumper (apparently no one use this on pypi top 100)

0.1.2 (2014-04-08)

  • baron.dumps now accept a single FST node, it was only working with a list of FST nodes
  • don’t add a endl node at the end if not present in the input string
  • de-uniformise call_arguments and function_arguments node, this is just creating more problems that anything else
  • fix
  • fix the fact that baron can’t parse “{1,}” (but “{1}” is working)

0.1.1 (2014-03-23)

  • It appears that I don’t know how to write correctly

0.1 (2014-03-22)

  • Init

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