Adding Custom Formatters

To define a custom formatter, you can do so in the Jupyter notebook configuration (usually found ~/.jupyter/jupyter_notebook_config.py or something along those lines), the following example adds a rather useless formatter as a example.

from jupyterlab_code_formatter.formatters import BaseFormatter, handle_line_ending_and_magic, SERVER_FORMATTERS

class ExampleCustomFormatter(BaseFormatter):

    label = "Apply Example Custom Formatter"

    @property
    def importable(self) -> bool:
        return True

    @handle_line_ending_and_magic
    def format_code(self, code: str, notebook: bool, **options) -> str:
        return "42"

SERVER_FORMATTERS["example"] = ExampleCustomFormatter()

When implementing your custom formatter using third party library, you will likely use try... except in the importable block instead of always returning True.

The handle_line_ending_and_magic decorator hides notebook-specific syntax from the formatter. Most of it (shell commands with !, ? help lines, run scripts) only makes sense for Python, so formatters for other languages should declare which language they format:

class ExampleRustFormatter(BaseFormatter):
    language = "rust"
    ...

The language defaults to "python", so a formatter which does not declare one keeps receiving the full set of escapes. Only "python" and "r" escape anything today — cell magics and Quarto cell options are hidden from both, as neither is valid code in either language and # starts a comment in both. Any other language, "rust" above included, disables escaping altogether and gets the cell exactly as it was written.

Remember you are always welcomed to submit a pull request!