purelint

Excessive mutability checking for Python. This is a Pylint plugin.

I have been learning Haskell and Gleam, but those are not languages that I use in production. The motivation behind this project is to see how far I can get adding functional constraints to Python and see what I can actually build.

Features

• match exhaustiveness checker, ensuring that all match statements have a _ clause
  • It is recommended to use something like assert_never(value) so that type checkers like mypy pick up on any unmatched cases For example:

    from purelint import assert_never
    
    def taker(v: Animal | None | int):
        """Check animal lint"""
        match v:
            case None:
                pass
            case Animal():
                pass
            case int():
                pass
            case _:
                assert_never(v)

    Then run mypy on your code. It will give you an error if you're not checking all cases.
• Variable rebinding
• No augmented assignments
• No mutable methods
• No subscript assignment
• No deletes
• A pipe tool for immutable transformations on a data structure For example:

  from purelint import pipe
  
  sorted_after_deletes = pipe(
      build_tree(values),  # start with tree
      lambda t: delete(t, 2),  # delete 2
      lambda t: delete(t, 5),  # delete 5
      inorder,  # get sorted tuple
  )

Installing

With uv:

uv add --dev purelint

With pip:

pip install -U purelint

Running

pylint --load-plugins=purelint example.py

With uv:

uv run pylint --load-plugins=purelint example.py

Testing

uv run pytest