How to show custom failure messages in ScalaTest?
You can also use withClue
without importing anything or adding it to the test class:
withClue(s"Expecting distinct elements: ${elements.toList}") { elements.length shouldBe 3 }
This is imported from Assertions
class: org.scalatest.Assertions#withClue
New way since 2011: Matchers
and AppendedClue
1 traits. Also, for collection sizes, there are some default messages.
import org.scalatest.{AppendedClues, Matchers, WordSpec}
class SomeTest extends WordSpec with Matchers with AppendedClues {
"Clues" should {
"not be appended" when {
"assertions pass" in {
"hi" should equal ("hi") withClue "Greetings scala tester!"
}
}
"be appended" when {
"assertions fail" in {
1 + 1 should equal (3) withClue ", not even for large values of 1!"
}
}
"not be needed" when {
"looking at collection sizes" in {
val list = List(1, 2, 3)
list should have size 5
}
}
}
}
Output looks like this:
SomeTest:
Clues
should not be appended
- when assertions pass
should be appended
- when assertions fail *** FAILED ***
2 did not equal 3, not even for large values of 1! (SomeTest.scala:15)
should not be needed
- when looking at collection sizes *** FAILED ***
List(1, 2, 3) had size 3 instead of expected size 5 (SomeTest.scala:21)
Note that the List
size message isn't great for lists with long .toString
output.
See the scaladoc for more information.
1 I'm guessing the AppendedClues
trait was inspired by this question, Bill Venners of the accepted answer is the author of this trait.
You're the first to ask for such a feature. One way to achieve this is with withClue. Something like:
withClue("NumberOfElements: ") { NumberOfElements() should be (5) }
That should get you this error message:
NumberOfElements: 10 was not equal to 5
If you want to control the message completely you can write a custom matcher. Or you could use an assertion, like this:
assert(NumberOfElements() == 5, "NumberOfElements should be 5")
Can you elaborate on what your use case is? Why is it that 10 did not equal 5 is not up to snuff, and how often have you had this need?
Here's the kind of thing you're requesting:
scala> import org.scalatest.matchers.ShouldMatchers._
import org.scalatest.matchers.ShouldMatchers._
scala> withClue ("Hi:") { 1 + 1 should equal (3) }
org.scalatest.TestFailedException: Hi: 2 did not equal 3
at org.scalatest.matchers.Matchers$class.newTestFailedException(Matchers.scala:150)
at org.scalatest.matchers.ShouldMatchers$.newTestFailedException(ShouldMatchers.scala:2331)
scala> class AssertionHolder(f: => Any) {
| def withMessage(s: String) {
| withClue(s) { f }
| }
| }
defined class AssertionHolder
scala> implicit def convertAssertion(f: => Any) = new AssertionHolder(f)
convertAssertion: (f: => Any)AssertionHolder
scala> { 1 + 1 should equal (3) } withMessage ("Ho:")
org.scalatest.TestFailedException: Ho: 2 did not equal 3
at org.scalatest.matchers.Matchers$class.newTestFailedException(Matchers.scala:150)
at org.scalatest.matchers.ShouldMatchers$.newTestFailedException(ShouldMatchers.scala:2331)
So this way you can write:
{ NumberOfElements() should be (5) } withMessage ("NumberOfElements:")