How do I save a CSV that opens correctly on the other end?

Until now you were the one receiving. Now it's your turn to send: the September and October sales report goes to two people. Your boss will open it with a double click in Excel. And the Valencia supplier, who asked you for the same figures to plan their shipments, will load it into their system, which expects the same format it sends: semicolons and a decimal comma.

Everything you learned about reading works in reverse. A CSV doesn't state its encoding, doesn't state its separator, doesn't state which columns are text. So the question when writing isn't "what's the right format?" but "who will read it, and what will they assume?"

• Someone double-clicking in Excel. Microsoft documents that "You can open a CSV file encoded with UTF-8 normally if it was saved with BOM (Byte Order Mark)". That's your chance to make the BOM work in your favor: write it and the accents in Mérida arrive intact.
• A system that expects semicolons. Give it exactly what it expects: ; between columns and , as the decimal mark. If you don't know what it expects, ask before sending.
• Another program or a script. UTF-8, comma, decimal point and dates like 2026-09-02. DuckDB, for example, documents that its COPY always writes UTF-8.

From Excel

If the file comes out of an Excel sheet, Microsoft gives two ways to save a CSV, depending on the format you choose:

Three notes from Microsoft. First, the save-as-CSV page names the two formats, but it doesn't say whether the UTF-8 one carries a BOM: don't take it for granted, and if the file is going to someone who opens it with a double click, check its first three bytes, as revisar.py does in the exercise below. Second, "When you save a workbook in any of the text formats, all formatting will be removed": colors, column widths and other formatting don't travel in a CSV. Excel also reminds you, with a dialog box, that only the current sheet is saved. Third, Excel for the web can't save a CSV today; from there, use Editing > Open in Desktop App.

And remember module 3: if the sheet already lost zeros or digits, the CSV you save will have lost them too.

Try it

The project has the report in datos/ventas.csv and a script, exporta.py, that writes both files with the default options. At the end it calls revisar, which prints whether each file starts with a BOM and its contents exactly as they are. Run uv run exporta.py and see what each person would receive. Then change the two to_csv calls: you saw the first one solved above; the second is for you. If you don't know which option a file is missing, ask your Maestro.