Python Morsels: How I made a dataclass remover

Table of contents

1. How does this undataclass.py script work?
2. Structural pattern matching
3. The sentinel object
4. Generating AST nodes for Python code
5. Building strings that represent Python code
6. Sliced assignment
7. Unparsing abstract syntax trees
8. This code makes sloppy assumptions
9. That was fun, now go appreciate dataclasses

How does this undataclass.py script work?

Essentially the undaclass.py script:

1. Parses the contents of a Python file into an abstract syntax tree (using the ast module)
2. Identifies dataclass-related AST nodes representing dataclasses, dataclass fields, and __post_init__ methods
3. Builds up strings representing Python code for the various equivalent non-dataclass methods
4. Parses those strings into AST nodes and injects them into the was-a-dataclass node
5. Converts all the AST nodes back into Python code

I used some tricks I don't usually get to use in Python. I used:

1. Many very hairy match-case blocks which replaced even hairier if-elif blocks
2. A sentinel object to keep track of a location that needed replacing
3. Python's textwrap.dedent utility, which I feel should be more widely known & used
4. slice assignment to inject one list into another
5. The ast module's unparse function to convert an abstract syntax tree into Python code

Let's take a look at some of the code.

Structural pattern matching

Python's structural pattern matching can …