testthat (version 3.1.1)

expect_error: Does code throw an error, warning, message, or other condition?


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

In the 3rd edition, these functions match (at most) a single condition. All additional and non-matching (if regexp or class are used) conditions will bubble up outside the expectation. If these additional conditions are important you'll need to catch them with additional expect_message()/expect_warning() calls; if they're unimportant you can ignore with suppressMessages()/suppressWarnings().

It can be tricky to test for a combination of different conditions, such as a message followed by an error. expect_snapshot() is often an easier alternative for these more complex cases.


  regexp = NULL,
  class = NULL,
  inherit = TRUE,
  info = NULL,
  label = NULL

expect_warning( object, regexp = NULL, class = NULL, ..., inherit = TRUE, all = FALSE, info = NULL, label = NULL )

expect_message( object, regexp = NULL, class = NULL, ..., inherit = TRUE, all = FALSE, info = NULL, label = NULL )

expect_condition( object, regexp = NULL, class = NULL, ..., inherit = TRUE, 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


logical. Should Perl-compatible regexps be used?


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


Whether to match regexp and class across the ancestry of chained errors.


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.


DEPRECATED If you need to test multiple warnings/messages you now need to use multiple calls to expect_message()/ expect_warning()


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

Testing <code>message</code> vs <code>class</code>

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.

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 recommend using expect_snapshot() instead.

See Also

Other expectations: comparison-expectations, equality-expectations, expect_length(), expect_match(), expect_named(), expect_null(), expect_output(), expect_reference(), expect_silent(), inheritance-expectations, logical-expectations


Run this code
# Errors ------------------------------------------------------------------
f <- function() stop("My error!")
expect_error(f(), "My error!")

# You can use the arguments of grepl to control the matching
expect_error(f(), "my error!", ignore.case = TRUE)

# Note that `expect_error()` returns the error object so you can test
# its components if needed
err <- expect_error(rlang::abort("a", n = 10))
expect_equal(err$n, 10)

# Warnings ------------------------------------------------------------------
f <- function(x) {
  if (x < 0) {
    warning("*x* is already negative")
expect_warning(f(-1), "already negative")
expect_warning(f(1), NA)

# To test message and output, store results to a variable
expect_warning(out <- f(-1), "already negative")
expect_equal(out, -1)

# Messages ------------------------------------------------------------------
f <- function(x) {
  if (x < 0) {
    message("*x* is already negative")

expect_message(f(-1), "already negative")
expect_message(f(1), NA)
# }

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