Why is this important?

The usage of an 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 clumsy 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 ast.py 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 realistic task that is worth the price (I’m not saying a simple task, but a realistic task, it’s still a complex task).

Other usages

Having an 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 ast.py and a templating engine for example).