Why does my --[[== shouldThrowWithMessage **]] []] not match the expected message?

Introduction to Kotest and Test Assertions

What is Kotest?

Kotest is a popular testing framework for Kotlin that supports various types of tests, including unit tests, property-based tests, and integration tests. It provides a rich DSL (Domain Specific Language) for writing tests in a highly readable and expressive way, with support for advanced features such as assertions, matchers, and testing for exceptions.

In Kotest, you can define tests using several styles, such as:

Behavior-driven development (BDD): Focuses on the behavior of the code.

FunSpec: Functional style of writing tests.

ShouldSpec: Another BDD-like style.

WordSpec: Allows for more natural language-like structure.

The testing process typically includes assertions, where you check the correctness of the code being tested by comparing actual results to expected results.

Kotest Assertions and ShouldThrowWithMessage

In Kotest, assertions are commonly written using the should and shouldNot keywords. Some of the common assertion methods include:

shouldBe: To check for equality.

shouldContain: To check for inclusion.

shouldThrow: To test whether a block of code throws an exception.

shouldThrowWithMessage: A variation of shouldThrow that allows you to assert not only that an exception was thrown but also that the exception message matches the expected message.

The shouldThrowWithMessage function is particularly useful when you're writing tests for functions that are expected to throw exceptions, and you want to ensure that the exception includes a specific message. For example:

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val exception = shouldThrow { someFunctionThatThrows() } exception.message shouldBe "Expected error message"

The Purpose of withClue

withClue is a Kotest function that allows you to add custom clues to assertions. If an assertion fails, the clue helps clarify the context by providing a custom message or additional information.

It is typically used in scenarios where you want to provide more detailed information about why a test failed. The key idea is that withClue enriches the error message when an assertion fails, making it easier to diagnose the problem.

For example, let's say you have a test that checks whether an exception is thrown:

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shouldThrow { someFunctionThatThrows() } withClue "Failed when calling someFunctionThatThrows with argument xyz"

The custom clue will appear in the output if the assertion fails, making it clearer why the failure occurred.

Forcing Assertion Errors

In some cases, you may want to explicitly force an assertion error in your tests, particularly when testing how your code handles failures. Kotest provides an easy way to do this using assertions that are guaranteed to fail, like:

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val actual = "some value" actual shouldBe "unexpected value" // This will force a failure

By writing tests in this way, you can simulate failures and validate that your error handling logic behaves as expected.

Common Pitfalls in Kotest and the Unexpected Test Results

Scenario: Forcing an AssertionError and Wrapping It with withClue

Let’s break down the specific scenario you mentioned: forcing an AssertionError and wrapping it with withClue. You’re using the shouldThrowWithMessage assertion, but the result is not as expected.

Here’s a typical setup for forcing an assertion error:

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import io.kotest.core.spec.style.StringSpec import io.kotest.matchers.shouldBe import io.kotest.assertions.throwables.shouldThrow class MyTest : StringSpec({ "test forcing assertion error and using withClue" { // Code that will throw an assertion error val result = "some value" // Force assertion error result shouldBe "unexpected value" withClue "Custom error message" } })

This test will fail with an assertion error and output the custom clue message. The error message would look something like:

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AssertionError: "some value" should be "unexpected value" Custom error message

Now, suppose you try to assert that a specific exception was thrown with a particular message using shouldThrowWithMessage:

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import io.kotest.matchers.shouldThrowWithMessage class MyTest : StringSpec({ "test shouldThrowWithMessage with forced assertion error" { val exception = shouldThrowWithMessage { val result = "some value" result shouldBe "unexpected value" withClue "Custom error message" } exception.message shouldBe "Custom error message" } })

The Unexpected Result

If the test result is unexpected, here are some possible reasons:

Mismatch in Exception Type: The test may not throw the expected type of exception. If the forced assertion error is not wrapped correctly, or if the exception thrown is not of type AssertionError, shouldThrowWithMessage may fail to match the expected exception type.

Message Mismatch: Kotest's shouldThrowWithMessage works by matching the exception type and the message. If the message produced by the assertion error differs from the expected message, the test will fail. This can happen if there’s a discrepancy in how the clue message is included in the error.

Exception Propagation: If the exception is not properly propagated, or if the code inside the block has a return or early exit that prevents the exception from being thrown, the assertion will fail.

Debugging Unexpected Test Results

Understanding the Failure

If your test isn’t behaving as expected, you can debug it by following these steps:

Check the Stack Trace: Kotest provides detailed stack traces for failed tests. Examine the stack trace to see if the exception thrown matches what you expected. If the exception type or message differs, adjust your test accordingly.

Ensure Proper Exception Wrapping: If you’re using withClue, make sure it is being used in the correct context. It should wrap the failing assertion, but the custom message may not appear in the exception message if it isn’t properly attached.

Assert the Exception Type: Sometimes, Kotest may throw a TestFailedException (which is Kotest’s own exception type for failed assertions), not an AssertionError. In that case, adjust the test to expect TestFailedException instead.

Custom Assertion Messages: If the custom message in withClue isn't appearing as expected, make sure you are calling the assertion correctly. Kotest may combine the clue message with the default assertion error message, which might cause confusion.

Explicitly Catch and Re-Throw Exceptions: If you're testing specific code paths, consider catching exceptions explicitly within the test and re-throwing them as AssertionError or another expected exception type. This ensures that the test can capture the specific failure state.

Example Debugged Test

Let’s rewrite the previous test with more debug information:

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import io.kotest.core.spec.style.StringSpec import io.kotest.matchers.shouldBe import io.kotest.assertions.throwables.shouldThrow class MyTest : StringSpec({ "test shouldThrowWithMessage with forced assertion error" { val exception = shouldThrow { val result = "some value" result shouldBe "unexpected value" withClue "Custom error message" } exception.message shouldBe "Custom error message" } })

If this still doesn’t work as expected, try explicitly checking what the actual exception message is:

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println("Actual exception message: ${exception.message}")

This will give you insight into what the actual message is, and whether Kotest is combining the clue message with the default failure message.

Conclusion

To sum up, when writing tests in Kotest, especially when dealing with forced assertion errors and withClue, it’s important to ensure that:

The exception type matches what you're expecting in shouldThrowWithMessage.

The message being generated by the assertion includes your custom clue message.

Kotest is configured properly and that no subtle misconfigurations cause the unexpected results.

By using proper debugging techniques and understanding the inner workings of Kotest's error reporting, you can diagnose and fix unexpected test results more effectively.

FAQ

Q1: What is the purpose of withClue in Kotest?

withClue is used to add custom clues to assertions. If an assertion fails, Kotest will include the clue in the error message, helping to provide more context for debugging.

Q2: What should I do if my shouldThrowWithMessage assertion is not working?

Ensure that:

The correct exception type is being thrown.

The exception message contains the expected value.

The test setup correctly handles propagation of exceptions.

Q3: How can I ensure my Kotest test fails with a custom error message?

Use withClue to attach a custom message to the failing assertion. Make sure the message is in the right format and that you are checking for the correct exception type.

Q4: Why does my shouldThrowWithMessage not match the expected message?

Kotest matches both the exception type and the message. If the message differs, the assertion will fail. Ensure that your code throws the correct message and that Kotest captures it properly.