go unit test for CSV is failing

I am wanting to use the csv package for a large project and I am starting with some basic tests. I cannot figure out why this unit test is failing, when it appears that the output matches the expected output.

FILE:

package csv

import (
	"bytes"
	"encoding/csv"
)

func GenerateCSV(records [][]string) (string, error) {
	buf := bytes.Buffer{}

	w := csv.NewWriter(&buf)

	for _, record := range records {
		if err := w.Write(record); err != nil {
			// handle the err
			}
	}
		w.Flush()

		if err := w.Error(); err != nil {
			// handle the error
		}

		return buf.String(), nil
	}

TESTFILE:

package csv

import "testing"

func TestGenerateCSV(t *testing.T) {

	records := [][]string{
		{"first_name","last_name","user_name"},
	}
	type args struct {
		records [][]string
	}
	tests := []struct {
		name    string
		args    args
		want    string
		wantErr bool
	}{
		// TODO: Add test cases.
		{"T1", args{records: records,}, "first_name,last_name,user_name", false},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			got, err := GenerateCSV(tt.args.records)
			if (err != nil) != tt.wantErr {
				t.Errorf("GenerateCSV() error = %v, wantErr %v", err, tt.wantErr)
				return
			}
			if got != tt.want {
				t.Errorf("GenerateCSV() = %v, want %v", got, tt.want)
			}
		})
	}

When the test runs, I get this output:

FAIL: TestGenerateCSV
GenerateCSV() = first_name,last_name,user_name
        , want first_name,last_name,user_name

It seems like the test output is exactly what I called for? Can anyone help me identify what I'm doing wrong?

To catch your error add this check in your test code.

	if len(got) != len(tt.want) {
			t.Errorf("GenerateCSV() string len = %v, want length %v", len(got), len(tt.want))
			return
		}

Reason : When you are writing your record using w.Flush, it adds a new line.

The test fails because your expected string doesn't contain a newline (\n) for the single-record test case:

{"T1", args{records: records}, "first_name,last_name,user_name", false},

It should be:

{"T1", args{records: records}, "first_name,last_name,user_name\n", false},

Every encoded record will have a newline.

Changing the %v to %q in your test code print-outs will make seeing the "quoted string" differences easier:

if got != tt.want {
	t.Errorf("GenerateCSV() = %q, want %q", got, tt.want)
}

Also, you if you don't need to do anything special to records as you are CSV-encoding them (you just want the quickest path to a CSV string), you can cut out the loop in GenerateCSV and instead use the Writer's WriteAll() method:

// GenerateCSV returns the CSV-encoded string representation of records.
func GenerateCSV(records [][]string) (string, error) {
	buf := bytes.Buffer{}

	w := csv.NewWriter(&buf)
	if err := w.WriteAll(records); err != nil {
		return "", err
	}

	w.Flush()
	if err := w.Error(); err != nil {
		return "", err
	}

	return buf.String(), nil
}

The expected output is first_name,last_name,user_name but the actual output is first_name,last_name,user_name, (notice the extra comma at the end). This is why the test is failing.