feat: dataframe download separator

[vavavavavavavavava]

📝 Summary

Add configurable CSV separator support to dataframe downloads, with consistent behavior across table backends.
Closes #7353

🔍 Description of Changes

This PR adds download_csv_separator to mo.ui.dataframe(...) and threads it through the full CSV download path so user-provided separators are respected end-to-end.

What changed

• Added new dataframe option:
  • download_csv_separator: str | None = None
• Propagated separator through:
  • dataframe download handler
  • download_as(...)
  • TableManager.to_csv(...) / to_csv_str(...)
  • backend table managers (default / narwhals / pandas / polars)

Backend behavior

• pandas: passes separator to DataFrame.to_csv(sep=...)
• polars: passes separator to write_csv(separator=...) (including fallback conversion path)
• narwhals utility:
  • keeps native write_csv() for default comma
  • uses Python csv.writer for non-comma separators to avoid backend-specific inconsistency
• default table manager: forwards separator to delegated manager

Why

write_csv(separator=...) is not consistently reliable across all narwhals-backed dataframe combinations.
This change makes non-comma CSV output predictable while preserving existing default behavior.

Backward compatibility

• Default behavior is unchanged (comma-separated CSV).
• Existing callers continue to work without modification.
• New behavior is opt-in via download_csv_separator.

Example

import marimo as mo
import pandas as pd

df = pd.DataFrame({"name": ["Alice", "Bob"], "value": [1, 2]})

# Before: default download behavior
mo.ui.dataframe(df, show_download=True)

# After: configurable CSV encoding / separator and JSON escaping
mo.ui.dataframe(
    df,
    show_download=True,
    download_csv_separator=";",
)

Tests

Added/updated tests for:

• dataframe CSV download with custom separator
• dataframe_to_csv(..., separator=";") behavior across dataframe backends

📋 Checklist

• I have read the contributor guidelines.
• 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 (Please provide a link if applicable). (Issue: Feature request: Allow configuring encoding for mo.ui.dataframe() download button #7353)
• Tests have been added for the changes made.
• Documentation has been updated where applicable, including docstrings for API changes.
• Pull request title is a good summary of the changes - it will be used in the release notes.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Feb 14, 2026 2:47pm