random-code Day 4: Littering - Building


I got inspired by Zac Hatfield-Dodds's blog post Sufficiently Advanced Testing to pursue the challenge of construction random programs. To get the intro of what I've worked on so far check out the Day 1 post and the latest Day 3 post. Check out the random-python project on Github.

Schrödinger's cat

The question I've been pondering today is: "How do you test a Transformer and if scoping is working?"

Scoping is the most important thing I've been focusing on so far with the project. On one side, you have name discovery to figure out which names we need to check against the scope. On the other side, you have the book-keeping for keeping track of which names are in scope at each step in the code.

Testing this logic feels a little bit like Schrödinger's cat. The scope is almost always changing as the Transformer is visiting different nodes. This makes it tricky to write a test to run the Transformer and then inspect if the scope is correct because the scope can be correct to start, correct at the end, but incorrect somewhere in the middle of exploring the AST.

Transformer you say?

NodeTransformer is a pattern for viewing and modifying the AST.

The general pattern is the Visitor, which makes it easy to view data in a tree structure by "visiting" each node in the tree and calling a specific function based on its type. This can be applied to lots of tree structures, but is useful for the AST because we're dealing with many (but well documented) types of nodes that we might want to view.

Taking it one step more specific, the Transformer is a Visitor that can help you swap out the node that you've visited. You can completely change the node that's in the tree, but for this use case we can stick to the simpler task of swapping out different instances of the same type of node (e.g. one function definition for another).

Solution: Constantly Observe the "Cat"

The solution I've come up to is to put the Transformer's common structure of visiting and modifying each node to use. At each step of the way, we can annotate the node with the scope that's available. I call this process littering. Afterwards, when observing the tree in a test, we can check each node and use that to understand if it's working properly. To stretch the Schrödinger's cat analogy one last time, we want to repeatedly observe the "cat" so we know its state.

Specifically for the random-python project, each AST node that gets processed (excepting strings) gets tagged with a new property _ending_scope and a copy of the scope is assigned. This is useful for asserting both that certain names are in scope (e.g. the name of a function is in scope in its body for recursive calls) and for asserting a name is not in scope (e.g. a local variable defined in a function definition body isn't present in the code after the function definition).

Testing with Littering

Testing with littering enabled are fairly straightforward. In about 10 lines of code, we set up a small Python snippet with the relevant AST node, parse it to the AST, then run it through the transformer. We can then assert on the scope. The tests look something like this:

def test_FunctionDef_annotation():
    input_text = """
def main(i: i):
    return 1
    ast = str_to_ast(input_text)
    transformer = build_transformer(ast)
    result = transformer.visit(ast)

    assert isinstance(result, FunctionDef)
    args = result.args
    assert isinstance(args, arguments)
    assert "i" not in args._ending_scope

Easy Littering

Remembering to assign these scopes in all of the relevant places can be easy to forget for functions with multiple returns or otherwise require copying around a lot of code. To make the littering process easier, the project defines a littering decorator that takes in the name of the variable its copying and the variable its writing to. The decorator calls into the main function, but after the main function returns and before the decorator returns it performs an assignment to "litter" the AST node being returned. This ensures that regardless of how the function exits we'll always get the littering for existing code and as I add new functionality.

A secondary benefit of moving to the decorator is that I had to standardize the interface to the visitor functions. Previously, I'd mixed functions that looked like free functions (no self) with functions that looked like member functions (that included self) and now they're all standardized as functions, both hand-written and generated, that accept self as members of the class.


All together, we've been able to go from uncertainty about scopes to easily writing tests for scoping by applying littering. This implementation pattern makes it easier to expand tests with simple or complicated examples and the decorator makes it easy to ensure that we get the same behavior across the AST as we add functionality.