expect_s3_class

What it does

Checks for usage of expect_equal(class(x), "y") and expect_identical(class(x), "y").

Why is this bad?

expect_equal(class(x), "y") will fail if x gets more classes in the future, even if "y" is still one of those classes. expect_s3_class(x, "y") is more robust because the test success doesn’t depend on the number or on the order of classes of x. This function also gives clearer error messages in case of failure.

To test that x only has the class "y", then one can use expect_s3_class(x, "y", exact = TRUE).

This rule is disabled by default. Select it either with the rule name "expect_s3_class" or with the rule group "TESTTHAT".

This rule has a safe automatic fix but doesn’t report cases where:

  • expect_s3_class() would fail, such as:

    testthat::expect_s3_class(list(1), "list")
testthat::expect_s3_class(1L, "integer")

    For those cases, it is recommended to use expect_type() instead.

  • the expected object could have multiple values, such as:

    testthat::expect_equal(class(x), c("foo", "bar"))
testthat::expect_equal(class(x), vec_of_classes)

Finally, the intent of the test cannot be inferred with the code only, so the user will have to add exact = TRUE if necessary.

Example

expect_equal(class(x), "data.frame")
expect_identical(class(x), "Date")

Use instead:

expect_s3_class(x, "data.frame")
expect_s3_class(x, "Date")