expect_error() and expect_condition() check that code throws an error or condition with a message that matches regexp, or a class that inherits from class. See below for more details.

expect_error(object, regexp = NULL, class = NULL, ..., info = NULL,
  label = NULL)

expect_condition(object, regexp = NULL, class = NULL, ...,
  info = NULL, label = NULL)



Object to test.

Supports limited unquoting to make it easier to generate readable failures within a function or for loop. See quasi_label for more details.


Regular expression to test against.

  • A character vector giving a regular expression that must match the error message.

  • If NULL, the default, asserts that there should be a error, but doesn't test for a specific value.

  • If NA, asserts that there should be no errors.


Instead of supplying a regular expression, you can also supply a class name. This is useful for "classed" conditions.


Arguments passed on to expect_match


Should all elements of actual value match regexp (TRUE), or does only one need to match (FALSE)


logical. Should Perl-compatible regexps be used?


logical. If TRUE, pattern is a string to be matched as is. Overrides all conflicting arguments.


Extra information to be included in the message. This argument is soft-deprecated and should not be used in new code. Instead see alternatives in quasi_label.


Used to customise failure messages. For expert use only.


If regexp = NA, the value of the first argument; otherwise the captured condition.

Testing message vs class

When checking that code generates an error, it's important to check that the error is the one you expect. There are two ways to do this. The first way is the simplest: you just provide a regexp that match some fragment of the error message. This is easy, but fragile, because the test will fail if the error message changes (even if its the same error).

A more robust way is to test for the class of the error, if it has one. You can learn more about custom conditions at https://adv-r.hadley.nz/conditions.html#custom-conditions, but in short, errors are S3 classes and you can generate a custom class and check for it using class instead of regexp. Because this is a more reliable check, you expect_error() will warn if the error has a custom class but you are testing the message. Eliminate the warning by using class instead of regexp. Alternatively, if you think the warning is a false positive, use class = "error" to suppress it for any input.

If you are using expect_error() to check that an error message is formatted in such a way that it makes sense to a human, we now recommend using verify_output() instead.

See also


f <- function() stop("My error!") expect_error(f()) expect_error(f(), "My error!") # You can use the arguments of grepl to control the matching expect_error(f(), "my error!", ignore.case = TRUE) # If you are working with classed conditions, it's better to test for # the class name, rather than the error message (which may change over time) custom_err <- function(var) { rlang::abort("A special error", var = var, .subclass = "testthat_special") } expect_error(custom_err("a"), class = "testthat_special") # Note that `expect_error()` returns the error object so you can test # its components if needed err <- expect_error(custom_err("a"), class = "testthat_special") expect_equal(err$var, "a")