Writing CSV

Writing CSV is the inverse: take rows of strings and emit valid CSV. The trick is quoting:

Decisions:

• Always quote vs quote when needed: always-quote produces consistent output but bigger files. Excel uses always-quote-strings.
• Line ending: \r\n (Windows-style) is RFC 4180 spec. \n is more common today.
• Encoding: UTF-8 with BOM for Excel compatibility (encoding='utf-8-sig').

Common bugs:

• Forgetting to escape quotes inside quoted fields → corrupted file
• Not quoting fields with newlines → row count mismatch
• Mixed line endings inside the file → some readers see different row counts