Better approach for error/non-error in table tests

[thaJeztah]

I notice I keep running into this, so let me open an issue;

When writing table-tests, I usually combine "happy" and "unhappy" tests, which means that for some tests, I'd be checking for an error, and for the happy tests, check that no error occurs.

For example;

func TestErrorOrNot(t *testing.T) {
	doSomething := func(withError bool) error {
		if withError {
			return errors.New("something went wrong")
		}
		return nil
	}
	for _, test := range []struct {
		withErr   bool
		errString string
	}{{withErr: true, errString: "something went wrong"}, {}} {
		err := doSomething(test.withErr)
		if test.errString != "" {
			assert.Error(t, err, test.errString)
		}
		assert.NilError(t, err)
	}
}

I'd like to get rid of the if test.errString != "" { in these tests;

Initially thought; let's use assert.Error(t, err, test.errString), which fails, because a nil error is not the same as an empty error-string. Using assert.ErrorContains(t, err, test.errString) has the same issue, because "" must be included in the error (which isn't the case if there's no error at all)

Wondering if there's a better approach, or if a new utility should be added that accepts "empty error string" as "no error is produced".

@vdemeester @dnephin

[dnephin]

I will generally do one of the following:

• write success cases in a separate table from error cases; or
• use an if in the test body, similar to what you have illustrated above, except I will generally return after assert because the test is in a t.Run(); or
• instead of expectedErr string in the testcase struct I will use a function that can be passed to https://godoc.org/gotest.tools/assert#ErrorType:

func noError(err error) bool {
    return err == nil
}

Maybe the last option fits your case?

I think a function that treats "" as "no error" is not very intuitive.