Function encode 

pub fn encode<W: Write>(
    df: &mut DataFrame,
    format: ExportFormat,
    options: &ExportOptions,
    sink: W,
) -> Result<()>

Encode df into sink and finish: every buffer is drained, the encoder’s last block and trailer written and sink flushed before this returns, so a failure in any of them is an error here rather than one lost when a writer is dropped.