# funcdriven: use function driven tests instead of table driven tests

i would like to give my wholehearted endorsement to this article: https://itnext.io/f-tests-as-a-replacement-for-table-driven-tests-in-go-8814a8b19e9e.

it advocates to replace the table driven tests like

  func TestStringsIndex(t *testing.T) {
    tests := []struct {
      name   string
      s      string
      substr string
      want   int
    }{
      {
        name:   "firstCharMatch",
        s:      "foobar",
        substr: "foo",
        want:   0,
      },
      {
        name:   "middleCharMatch",
        s:      "foobar",
        substr: "bar",
        want:   4,
      },
      {
        name:   "mismatch",
        s:      "foobar",
        substr: "baz",
        want:   -1,
      },
    }

    for _, tc := range tests {
      t.Run(tc.name, func(t *testing.T) {
        got := strings.Index(tc.s, tc.substr)
        if got != tc.want {
          t.Fatalf("unexpected n; got %d; want %d", got, tc.want) // line 32
        }
      })
    }
  }

with function driven tests like

  func TestStringsIndex(t *testing.T) {
    f := func(s, substr string, nExpected int) {
      t.Helper()

      n := strings.Index(s, substr)
      if n != nExpected {
        t.Fatalf("unexpected n; got %d; want %d", n, nExpected)
      }
    }

    // first char match
    f("foobar", "foo", 0)

    // middle char match
    f("foobar", "bar", 4) // line 15

    // mismatch
    f("foobar", "baz", -1)
  }

in case of error this is what you see in the former case:

  > t.Fatalf("unexpected n; got %d; want %d", got, tc.want)
  funcdriven_test.go:32: unexpected n; got 3; want 4

in the latter case this is what you see in your editor:

  > // middle char match
  > f("foobar", "bar", 3)
  funcdriven_test.go:15: unexpected n; got 3; want 4

basically the error message points directly to the place where the erroneous data is. makes working with tests super convenient.

i used table driven tests for a long time but i now switched over tho this. i confirm from experience that i find these much easier and more natural to work with.

and when ready for an even bigger leap of faith then use https://pkg.go.dev/github.com/ypsu/efftesting to automate away the manual maintenance of the "want" argument.

i am starting to like writing tests, yay.

published on 2024-11-18

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