random-code Day 5: Python Class Manipulation - 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 4 post. Check out the random-python project on Github.

Type Dispatching

In the AST visiting logic, there are a handful of functions that all need to dispatch to different logic based on the type of the node they're working with. They all look something like:

def interesting_node_logic(node):
    if isinstance(node, FunctionDef):
    elif isinstance(node, Lambda):
    elif isinstance(node, ListComp):

This continues on for quite a while because there are 40 or 50 different types of nodes we can encounter in the tree. Even if each only needs a couple of lines of logic, this can lead to long piles of code. If we can find more succinct ways to represent these functions, we can more easily group similar logic and compress the code into more readable groups of lines instead of having sprawling logic and repeated code.

Abstract Base Classes

I recently ran across Hillel Wayne's blog post "Crimes with Python's Pattern Matching" about pattern matching and Python's abstract base classes. It starts off with a bang:

One of my favorite little bits of python is __subclasshook__. Abstract Base Classes with __subclasshook__ can define what counts as a subclass of the ABC, even if the target doesn’t know about the ABC.

This first few sentences of the blog post provided me with all I needed to know to figure out how to simplify the type dispatching. For AST elements with similar properties, we can create groups to make them easier to work with.

The immediate example that comes to mind is called NotNameParent. When parsing an AST looking for names that need to be in scope, there are a handful of AST nodes that we know will not contain a name to check and will not have children with a name to check. These include: break, pass and constants. This means that we can create a simple abstract base class that collects these names into a single consistent idea. The class looks like:

class NotNameParent(ABC):
    _doesnt_contain_names = {

    def __subclasshook__(cls, C):
        name = C.__name__
        log.debug("NotNameParent.__subclasshook__ %s", name)
        return name in cls._doesnt_contain_names

And allows us to simplify the type dispatching logic to lines like:

if isinstance(element, NotNameParent):
    return []


In conclusion, there's Python mechanisms under the hood that can greatly simplify repetitive code if only you know where to look. Abstract Base Classes let you retroactively combine different classes across similar concepts to create an alternate tree for your use case.

Also, I highly recommend reading more than just the first paragraph of Hillel's post "Crimes with Python's Pattern Matching". It's well written and fascinating.