Testing errors in Go

Assert on the sentinel or the type, never on the message text.

var ErrBadStars = errors.New("stars out of range")

func Validate(stars int) error {
    if stars < 0 || stars > 3 {
        return fmt.Errorf("validate %d: %w", stars, ErrBadStars)
    }
    return nil
}

func TestValidate(t *testing.T) {
    cases := []struct {
        stars   int
        wantErr error
    }{
        {2, nil},
        {9, ErrBadStars},
    }

    for _, c := range cases {
        err := Validate(c.stars)
        if !errors.Is(err, c.wantErr) {
            t.Errorf("Validate(%d) = %v, want %v", c.stars, err, c.wantErr)
        }
    }
}

How it works

1. errors.Is is the right comparison for a wrapped error.
2. A wantErr field in the table covers both paths.
3. Comparing err.Error() breaks the moment you add context.

Keywords and builtins used here

• error
• for
• func
• if
• int
• range
• return
• struct
• var

The run, in numbers

Lines

25

Characters to type

452

Tokens

124

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 4 of 4 in Writing tests, step 4 of 13 in Testing.

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