import pandas as pd
import io

def df_to_excel_bytes(df):

    """
    Convert a DataFrame to an in-memory XLSX file.

    Returns a BytesIO object suitable for st.download_button(data=...).
    """

    buffer = io.BytesIO()
    with pd.ExcelWriter(buffer, engine="openpyxl") as writer:
        df.to_excel(writer, index=False, sheet_name="Daten")
    buffer.seek(0)
    return buffer