csv writer using buffer results in empty output

Question

I have created a function to get some data and write them to CSV and the output is stored on a buffer.

type OptIn struct {
    Email      string `json:"email"`
    LastUpdate string `json:"opt_in_last_update"`
}

func writeCSV(data []OptIn) ([]byte, error) {
    var buf bytes.Buffer
    writer := csv.NewWriter(&buf)

    defer writer.Flush()

    for _, obj := range data {
        var record []string
        record = append(record, obj.Email)
        record = append(record, obj.LastUpdate)
        err := writer.Write(record)
        if err != nil {
            panic(err.Error())
        }
    }

    return buf.Bytes(), nil
}

The problem is that the buf.Bytes() is always empty, even though the input is not empty and there are no errors thrown.

You are calling Bytes() before Flush().

Peter
– Peter

2018-08-24 15:33:26 +00:00
Commented Aug 24, 2018 at 15:33 — Peter
– Peter, Commented Aug 24, 2018 at 15:33

maerics · Accepted Answer · 2018-08-24 15:45:17Z

11

You need to call writer.Flush() before calling .Bytes() and check .Error() before returning:

  // TODO: remove `defer writer.Flush()`
  // ...

  writer.Flush()

  if err := writer.Error(); err != nil {
    return nil, err
  }

  return buf.Bytes(), nil
}

Your sample code does things in the following order:

call buf.Bytes()
return the generated byte slice and nil
call writer.Flush()

Clearly this is not the intended order since we need to flush (and check for any writer errors!) before accessing the generated byte slice.

edited Aug 24, 2018 at 15:45

answered Aug 24, 2018 at 15:34

maerics

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