fix(formatter): wrap cells in function scope before ruff formatting by VishakBaddur · Pull Request #9889 · marimo-team/marimo

VishakBaddur · 2026-06-15T16:55:56Z

📝 Summary

RuffFormatter.format() was passing raw cell code directly to ruff format via stdin. Ruff treats stdin as file scope and applies E302 (2 blank lines around top-level function definitions). But when the user runs ruff on the saved .py file, cells are already wrapped in def _(): functions, so ruff applies E301 (1 blank line around nested defs). This creates a conflict - marimo's in-editor formatting and ruff on the saved file disagree on blank lines around nested functions.

Fix: Before sending to ruff, wrap each non-empty cell in def _():\n + indented body. After formatting, drop the first line, dedent, and strip. This makes marimo match what ruff produces on the saved file. Whitespace-only cells are left unwrapped (empty function body is invalid Python). If wrapping causes ruff to reject a cell, it falls back to formatting unwrapped.

📋 Pre-Review Checklist

For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions. (Issue: Extra lines around nested functions conflict with Ruff formatting #9848)
Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
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✅ Merge Checklist

I have read the contributor guidelines.
Documentation has been updated where applicable, including docstrings for API changes. (No public API changes; internal formatter only.)
Tests have been added for the changes made.

…arimo-team#9848)

vercel · 2026-06-15T16:56:04Z

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dmadisetti · 2026-06-15T20:53:26Z

+                # Wrapping caused ruff to reject this cell; retry unwrapped.
+                fallback_keys.append(key)
+
+        if fallback_keys:


Any known code paths trigger this or is this just defensive?

Couldn't find a real code path that triggers it(any valid cell body is valid inside def _():), so removed it.

dmadisetti

Thanks again @VishakBaddur !

Test looks great. I wonder if we can trigger that "failed" branch though
Think you can give a shot at using marimo/ast/_parse.py?

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VishakBaddur · 2026-06-15T22:20:41Z

Switched the unwrap to use marimo/_ast/parse.py and removed the fallback since I couldn't find a case that actually hits it. Let me know if you think there's an edge case I'm missing!

dmadisetti

Another comment, sorry. Also realizing- can you add a top level function test? e.g.

@app.function
def foo():
    """Something here"""

…e.py

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dmadisetti

Sorry found a bug- but clean!

dmadisetti · 2026-06-16T16:50:35Z

+        # left unwrapped because an empty function body is invalid Python.
+        wrapped: CellCodes = {}
+        for key, code in codes.items():
+            if code.strip():


This may fail for pure comments too just realizing- can we add a test?

def _(): # just a comment

is also a syntax error

fixed by precomputing skip_wrap, a set of keys whose code has no executable statements (all lines are whitespace or comments). Both the wrap and unwrap loops key off this set, so comment-only cells are never wrapped. I've also added a test covering this case.

dmadisetti · 2026-06-16T16:50:42Z

+    assert isinstance(fn, ast.FunctionDef)
+    assert fn.end_lineno is not None


We should throw a value error instead of assert here I think.
Assertions are a little obtuse, and although this path should never be hit (type safety sanity check)

replaced both with raise ValueError

dmadisetti · 2026-06-16T16:55:14Z

+        wrapped: CellCodes = {}
+        for key, code in codes.items():
+            if code.strip():
+                wrapped[key] = "def _():\n" + textwrap.indent(code, "    ")


While we're making changes, fstring?

nvm, the \n will make that annoying

Suggested change

wrapped[key] = "def _():\n" + textwrap.indent(code, " ")

wrapped[key] = "\n".join("def _():", textwrap.indent(code, " "))

or reuse the helper, which will take care of the comment case (because there will be a return)

Used "\n".join(...) as suggested. Regarding the alternative to reuse the helper, if we wrapped comment-only cells and appended a pass to make the function body syntactically valid, the pass would end up preserved in the unwrapped output. Handling them via skip_wrap keeps the final output completely clean.

dmadisetti · 2026-06-16T17:01:59Z

+        # parse mechanism in marimo/_ast/parse.py (Extractor + fixed_dedent).
+        result: CellCodes = {}
+        for key, code in codes.items():
+            if not code.strip():


If we just use the helper, then we won't need the conditional (all code contents should be handled)

The skip_wrap set removes the need for the inline condition in both loops. wrap loop uses key not in skip_wrap, unwrap loop uses key in skip_wrap. No duplication, no condition repeated inline

…lueError

dmadisetti

Nice!

github-actions · 2026-06-25T00:45:27Z

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.23.11-dev26

fix(formatter): wrap cells in function scope before ruff formatting (m…

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VishakBaddur and others added 2 commits June 15, 2026 17:09

fix(formatter): use AST-based unwrap via marimo/_ast/parse.py

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fix(formatter): extract unwrap_cell_body helper into marimo/_ast/pars…

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fix(formatter): handle comment-only cells and replace asserts with Va…

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dmadisetti merged commit 7ae012e into marimo-team:main Jun 25, 2026
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dmadisetti mentioned this pull request Jun 29, 2026

Format Ruff-backed cells as function bodies #9910

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fix(formatter): wrap cells in function scope before ruff formatting#9889

fix(formatter): wrap cells in function scope before ruff formatting#9889
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		assert isinstance(fn, ast.FunctionDef)
		assert fn.end_lineno is not None

	wrapped[key] = "def _():\n" + textwrap.indent(code, " ")
	wrapped[key] = "\n".join("def _():", textwrap.indent(code, " "))

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