In traditional unit testing, you write tests that describe precisely what the test will do: create these objects, wire them together, call these functions, assert on the results, and so on. It is clear and deterministic, but also limited, because it only covers the exact situations you think to test. In most cases, it is not feasible to test all of the possible combinations of data that might arise in real-world use.

Property-based testing works the other way around. You describe properties -- rules that you expect your classes to live by -- and describe how to test those properties. The test system then generates relatively large amounts of synthetic data (with an emphasis on edge cases that tend to make things break), so that you can see if the properties hold true in these situations.

As a result, property-based testing is scientific in the purest sense: you are stating a hypothesis about how things should work (the property), and the system is trying to falsify that hypothesis. If the tests pass, that doesn't prove the property holds, but it at least gives you some confidence that you are probably correct.

Property-based testing is deliberately a bit random: while the edge cases get tried upfront, the system also usually generates a number of random values to try out. This makes things a bit non-deterministic -- each run will be tried with somewhat different data. To make it easier to debug, and to build regression tests, the system provides tools to re-run a failed test with precisely the same data.

Background

TODO: Bill should insert a brief section on QuickCheck, ScalaCheck, etc, and how this system is similar and different.

Using Property Checks

In order to use the tools described here, you should import this package:

import org.scalatest._
import org.scalatest.prop._

This library is designed to work well with the types defined in Scalactic, and some functions take types such as PosZInt as parameters. So it can also be helpful to import those with:

import org.scalactic.anyvals._

In order to call forAll, the function that actually performs property checks, you will need to either extend or import GeneratorDrivenPropertyChecks, like this:

class DocExamples extends FlatSpec with Matchers with GeneratorDrivenPropertyChecks {

There's nothing special about FlatSpec, though -- you may use any of ScalaTest's styles with property checks. GeneratorDrivenPropertyChecks extends CommonGenerators, so it also provides access to the many utilities found there.

What Does a Property Look Like?

Let's check a simple property of Strings -- that if you concatenate a String to itself, its length will be doubled:

"Strings" should "have the correct length when doubled" in {
  forAll { (s: String) =>
    val s2 = s * 2
    s2.length should equal (s.length * 2)
  }
}

(Note that the examples here are all using the FlatSpec style, but will work the same way with any of ScalaTest's styles.)

As the name of the tests suggests, the property we are testing is the length of a String that has been doubled.

The test begins with forAll. This is usually the way you'll want to begin property checks, and that line can be read as, "For all Strings, the following should be true".

The test harness will generate a number of Strings, with various contents and lengths. For each one, we compute s * 2. (* is a function on String, which appends the String to itself as many times as you specify.) And then we check that the length of the doubled String is twice the length of the original one.

Using Specific Generators

Let's try a more general version of this test, multiplying arbitrary Strings by arbitrary multipliers:

"Strings" should "have the correct length when multiplied" in {
  forAll { (s: String, n: PosZInt) =>
    val s2 = s * n.value
    s2.length should equal (s.length * n.value)
  }
}

Again, you can read the first line of the test as "For all Strings, and all non-negative Integers, the following should be true". (PosZInt is a type defined in Scalactic, which can be any positive integer, including zero. It is appropriate to use here, since multiplying a String by a negative number doesn't make sense.)

This intuitively makes sense, but when we try to run it, we get a JVM Out of Memory error! Why? Because the test system tries to test with the "edge cases" first, and one of the more important edge cases is Int.MaxValue. It is trying to multiply a String by that, which is far larger than the memory of even a big computer, and crashing.

So we want to constrain our test to sane values of n, so that it doesn't crash. We can do this by using more specific Generators.

When we write a forAll test like the above, ScalaTest has to generate the values to be tested -- the semi-random Strings, Ints and other types that you are testing. It does this by calling on an implicit Generator for the desired type. The Generator generates values to test, starting with the edge cases and then moving on to randomly-selected values.

ScalaTest has built-in Generators for many major types, including String and PosZInt, but these Generators are generic: they will try any value, including values that can break your test, as shown above. But it also provides tools to let you be more specific.

Here is the fixed version of the above test:

"Strings" should "have the correct length when multiplied" in {
  forAll(strings, posZIntsBetween(0, 1000))
  { (s: String, n: PosZInt) =>
    val s2 = s * n.value
    s2.length should equal (s.length * n.value)
  }
}

This is using a variant of forAll, which lets you specify the Generators to use instead of just picking the implicit one. CommonGenerators.strings is the built-in Generator for Strings, the same one you were getting implicitly. (The other built-ins can be found in CommonGenerators. They are mixed into GeneratorDrivenPropertyChecks, so they are readily available.)

But CommonGenerators.posZIntsBetween is a function that creates a Generator that selects from the given values. In this case, it will create a Generator that only creates numbers from 0 to 1000 -- small enough to not blow up our computer's memory. If you try this test, this runs correctly.

The moral of the story is that, while using the built-in Generators is very convenient, and works most of the time, you should think about the data you are trying to test, and pick or create a more-specific Generator when the test calls for it.

CommonGenerators contains many functions that are helpful in common cases. In particular:

xxsBetween (where xxs might be Int, Long, Float or most other significant numeric types) gives you a value of the desired type in the given range, as in the posZIntsBetween() example above.
CommonGenerators.specificValue and CommonGenerators.specificValues create Generators that produce either one specific value every time, or one of several values randomly. This is useful for enumerations and types that behave like enumerations.
CommonGenerators.evenly and CommonGenerators.frequency create higher-level Generators that call other Generators, either more or less equally or with a distribution you define.

Testing Your Own Types

Testing the built-in types isn't very interesting, though. Usually, you have your own types that you want to check the properties of. So let's build up an example piece by piece.

Say you have this simple type:

sealed trait Shape {
  def area: Double
}
case class Rectangle(width: Int, height: Int) extends Shape {
  require(width > 0)
  require(height > 0)
  def area: Double = width * height
}

Let's confirm a nice straightforward property that is surely true: that the area is greater than zero:

"Rectangles" should "have a positive area" in {
   forAll { (w: PosInt, h: PosInt) =>
     val rect = Rectangle(w, h)
     rect.area should be > 0.0
   }
 }

Note that, even though our class takes ordinary Ints as parameters (and checks the values at runtime), it is actually easier to generate the legal values using Scalactic's PosInt type.

This should work, right? Actually, it doesn't -- if we run it a few times, we quickly hit an error!

[info] Rectangles
[info] - should have a positive area *** FAILED ***
[info]   GeneratorDrivenPropertyCheckFailedException was thrown during property evaluation.
[info]    (DocExamples.scala:42)
[info]     Falsified after 2 successful property evaluations.
[info]     Location: (DocExamples.scala:42)
[info]     Occurred when passed generated values (
[info]       None = PosInt(399455539),
[info]       None = PosInt(703518968)
[info]     )
[info]     Init Seed: 1568878346200

TODO: fix the above error to reflect the better errors we should get when we merge in the code being forward-ported from 3.0.5.

Looking at it, we can see that the numbers being used are pretty large. What happens when we multiply them together?

scala> 399455539 * 703518968
res0: Int = -2046258840

We're hitting an Int overflow problem here: the numbers are too big to multiply together and still get an Int. So we have to fix our area function:

case class Rectangle(width: Int, height: Int) extends Shape {
  require(width > 0)
  require(height > 0)
  def area: Double = width.toLong * height.toLong
}

Now, when we run our property check, it consistently passes. Excellent -- we've caught a bug, because ScalaTest tried sufficiently large numbers.

Composing Your Own Generators

Doing things as shown above works, but having to generate the parameters and construct a Rectangle every time is a nuisance. What we really want is to create our own Generator that just hands us Rectangles, the same way we can do for PosInt. Fortunately, this is easy.

Generators can be composed in for comprehensions. So we can create our own Generator for Rectangle like this:

implicit val rectGenerator = for {
  w <- posInts
  h <- posInts
}
  yield Rectangle(w, h)

Taking that line by line:

w <- posInts

CommonGenerators.posInts is the built-in Generator for positive Ints. So this line puts a randomly-generated positive Int in w, and

h <- posInts

this line puts another one in h. Finally, this line:

yield Rectangle(w, h)

combines w and h to make a Rectangle.

That's pretty much all you need in order to build any normal case class -- just build it out of the Generators for the type of each field. (And if the fields are complex data structures themselves, build Generators for them the same way, until you are just using primitives.)

Now, our property check becomes simpler:

"Generated Rectangles" should "have a positive area" in {
   forAll { (rect: Rectangle) =>
     rect.area should be > 0.0
   }
 }

That's about as close to plain English as we can reasonably hope for!

Filtering Values with whenever()

Sometimes, not all of your generated values make sense for the property you want to check -- you know (via external information) that some of these values will never come up. In cases like this, you can create a custom Generator that only creates the values you do want, but it's often easier to just use Whenever.whenever. (Whenever is mixed into GeneratorDrivenPropertyChecks, so this is available when you need it.)

The Whenever.whenever function can be used inside of GeneratorDrivenPropertyChecks.forAll. It says that only the filtered values should be used, and anything else should be discarded. For example, look at this property:

"Fractions" should "get smaller when squared" in {
  forAll { (n: Float) =>
    whenever(n > 0 && n < 1) {
      (n * n) should be < n
    }
  }
}

We are testing a property of numbers less than 1, so we filter away everything that is not the numbers we want. This property check succeeds, because we've screened out the values that would make it fail.

Discard Limits

You shouldn't push Whenever.whenever too far, though. This system is all about trying random data, but if too much of the random data simply isn't usable, you can't get valid answers, and the system tracks that.

For example, consider this apparently-reasonable test:

"Space Chars" should "not also be letters" in {
  forAll { (c: Char) =>
    whenever (c.isSpaceChar) {
      assert(!c.isLetter)
    }
  }
}

Although the property is true, this test will fail with an error like this:

[info] Lowercase Chars
[info] - should upper-case correctly *** FAILED ***
[info]   Gave up after 0 successful property evaluations. 49 evaluations were discarded.
[info]   Init Seed: 1568855247784

Because the vast majority of Chars are not spaces, nearly all of the generated values are being discarded. As a result, the system gives up after a while. In cases like this, you usually should write a custom Generator instead.

The proportion of how many discards to permit, relative to the number of successful checks, is configuration-controllable. See GeneratorDrivenPropertyChecks for more details.

Randomization

The point of Generator is to create pseudo-random values for checking properties. But it turns out to be very inconvenient if those values are actually random -- that would mean that, when a property check fails occasionally, you have no good way to invoke that specific set of circumstances again for debugging. We want "randomness", but we also want it to be deterministic, and reproducible when you need it.

To support this, all "randomness" in ScalaTest's property checking system uses the Randomizer class. You start by creating a Randomizer using an initial seed value, and call that to get your "random" value. Each call to a Randomizer function returns a new Randomizer, which you should use to fetch the next value.

GeneratorDrivenPropertyChecks.forAll uses Randomizer under the hood: each time you run a forAll-based test, it will automatically create a new Randomizer, which by default is seeded based on the current system time. You can override this, as discussed below.

Since Randomizer is actually deterministic (the "random" values are unobvious, but will always be the same given the same initial seed), this means that re-running a test with the same seed will produce the same values.

If you need random data for your own Generators and property checks, you should use Randomizer in the same way; that way, your tests will also be re-runnable, when needed for debugging.

Debugging, and Re-running a Failed Property Check

In Testing Your Own Types above, we found to our surprise that the property check failed with this error:

[info] Rectangles
[info] - should have a positive area *** FAILED ***
[info]   GeneratorDrivenPropertyCheckFailedException was thrown during property evaluation.
[info]    (DocExamples.scala:42)
[info]     Falsified after 2 successful property evaluations.
[info]     Location: (DocExamples.scala:42)
[info]     Occurred when passed generated values (
[info]       None = PosInt(399455539),
[info]       None = PosInt(703518968)
[info]     )
[info]     Init Seed: 1568878346200

There must be a bug here -- but once we've fixed it, how can we make sure that we are re-testing exactly the same case that failed?

This is where the pseudo-random nature of Randomizer comes in, and why it is so important to use it consistently. So long as all of our "random" data comes from that, then all we need to do is re-run with the same seed.

That's why the Init Seed shown in the message above is crucial. We can re-use that seed -- and therefore get exactly the same "random" data -- by using the -S flag to ScalaTest.

So you can run this command in sbt to re-run exactly the same property check:

testOnly *DocExamples -- -z "have a positive area" -S 1568878346200

Taking that apart:

testOnly *DocExamples says that we only want to run suites whose paths end with DocExamples
-z "have a positive area" says to only run tests whose names include that string.
-S 1568878346200 says to run all tests with a "random" seed of 1568878346200

By combining these flags, you can re-run exactly the property check you need, with the right random seed to make sure you are re-creating the failed test. You should get exactly the same failure over and over until you fix the bug, and then you can confirm your fix with confidence.

Configuration

In general, forAll() works well out of the box. But you can tune several configuration parameters when needed. See GeneratorDrivenPropertyChecks for info on how to set configuration parameters for your test.

Table-Driven Properties

Sometimes, you want something in between traditional hard-coded unit tests and Generator-driven, randomized tests. Instead, you sometimes want to check your properties against a specific set of inputs.

(This is particularly useful for regression tests, when you have found certain inputs that have caused problems in the past, and want to make sure that they get consistently re-tested.)

ScalaTest supports these, by mixing in TableDrivenPropertyChecks. See the documentation for that class for the full details.

Definition Classes: scalatest

package propspec

Definition Classes: scalatest

package tagobjects

Definition Classes: scalatest

package time

Definition Classes: scalatest

package tools

Definition Classes: scalatest

package words

Definition Classes: scalatest

package wordspec

Definition Classes: scalatest

org.scalatest

exceptions

package exceptions

Type Members

class DiscardedEvaluationException extends RuntimeException
Exception that indicates an evaluation of a property should be discarded, because a condition required by a property was not met by the passed values.
Exception that indicates an evaluation of a property should be discarded, because a condition required by a property was not met by the passed values.
This exception is thrown by the whenever method defined in trait Whenever when the given condition is false. The forAll methods defined in trait PropertyChecks catch the DiscardedEvaluationException and ignore it, moving on to try the next set of parameter values it is checking th property against.
class DuplicateTestNameException extends StackDepthException
Exception that indicates an attempt was made to register a test that had the same name as a test already registered in the same suite.
Exception that indicates an attempt was made to register a test that had the same name as a test already registered in the same suite. The purpose of this exception is to encapsulate information about the stack depth at which the line of code that made this attempt resides, so that information can be presented to the user that makes it quick to find the problem line of code. (In other words, the user need not scan through the stack trace to find the correct filename and line number of the offending code.)
Exceptions thrown
NullArgumentException if testName is null
class GeneratorDrivenPropertyCheckFailedException extends PropertyCheckFailedException
Exception that indicates a ScalaCheck property check failed.
Exception that indicates a ScalaCheck property check failed.
Exceptions thrown
NullArgumentException if any parameter is null or Some(null).
trait ModifiableMessage[T <: Throwable] extends AnyRef
Trait implemented by exception types that can modify their detail message.
Trait implemented by exception types that can modify their detail message.
This trait facilitates the withClue construct provided by trait Assertions. This construct enables extra information (or "clues") to be included in the detail message of a thrown exception. Although both assert and expect provide a way for a clue to be included directly, assertThrows, intercept, and ScalaTest matcher expressions do not. Here's an example of clues provided directly in assert:
```
assert(1 + 1 === 3, "this is a clue")
```
and in expect:
```
expect(3, "this is a clue") { 1 + 1 }
```
The exceptions thrown by the previous two statements will include the clue string, "this is a clue", in the exceptions detail message. To get the same clue in the detail message of an exception thrown by a failed assertThrows call requires using withClue:
```
withClue("this is a clue") {
  assertThrows[IndexOutOfBoundsException] {
    "hi".charAt(-1)
  }
}
```
Similarly, to get a clue in the exception resulting from an exception arising out of a ScalaTest matcher expression, you need to use withClue. Here's an example:
```
withClue("this is a clue") {
  1 + 1 should === (3)
}
```
Exception types that mix in this trait have a modifyMessage method, which returns an exception identical to itself, except with the detail message option replaced with the result of invoking the passed function, supplying the current detail message option as the lone String parameter.
trait ModifiablePayload[T <: Throwable] extends AnyRef
Trait implemented by PayloadField exception types that can modify their payload.
Trait implemented by PayloadField exception types that can modify their payload.
This trait facilitates the withPayload construct provided by trait Payloads. This construct enables a payload object (or modified payload object) to be included as the payload of a thrown exception. The payload can then be included in the ScalaTest event that results from that exception. For example, the payload included in a TestFailedException will be included as the payload of the resulting TestFailed event. Here's an example in which a GUI snapshot is included as a payload when a test fails:
```
withPayload(generateGUISnapshot()) {
  1 + 1 should === (3)
}
```
Exception types that mix in this trait have a modifyPayload method, which returns an exception identical to itself, except with the payload option replaced with the result of invoking the passed function, supplying the current payload option as the lone Option[Any] parameter.
class NotAllowedException extends StackDepthException
Exception that indicates something was attempted in test code that is not allowed.
Exception that indicates something was attempted in test code that is not allowed. For example, in a FeatureSpec, it is not allowed to nest a feature clause inside another feature clause. If this is attempted, the construction of that suite will fail with a NotAllowedException.
Exceptions thrown
NullArgumentException if either message or failedCodeStackDepthFun is null
trait PayloadField extends AnyRef
Trait implemented by exception types that carry an optional payload.
Trait implemented by exception types that carry an optional payload.
Many ScalaTest events include an optional "payload" field that can be used to pass information to a custom reporter. This trait facilitates such customization, by allowing test code to include a payload in an exception (such as TestFailedException). ScalaTest looks for this trait and fires any payloads it finds in the relevant ScalaTest event stimulated by the exception, such as a TestFailed event stimulated by a TestFailedException. (Although in its initial release there is only two subclasses of PayloadField in ScalaTest, TestFailedException and TestCanceledException, in future version of ScalaTest, there could be more)
For an example of how payloads could be used, see the documentation for trait Payloads.
abstract class PropertyCheckFailedException extends TestFailedException
Exception that indicates a property check failed.
Exception that indicates a property check failed.
Exceptions thrown
NullArgumentException if any parameter is null or Some(null).
trait StackDepth extends AnyRef
Trait that encapsulates the information required of an exception thrown by ScalaTest's assertions and matchers, which includes a stack depth at which the failing line of test code resides.
Trait that encapsulates the information required of an exception thrown by ScalaTest's assertions and matchers, which includes a stack depth at which the failing line of test code resides.
This trait exists so that it can be mixed into two exception superclasses, StackDepthException, from which extend several exceptions that do not depend on JUnit, and JUnitTestFailedError, which does depend on JUnit. The latter, which requires JUnit be in the classpath, ensures failed ScalaTest assertions are reported as "failures," not "errors," by JUnit.
abstract class StackDepthException extends RuntimeException with StackDepth
Exception class that encapsulates information about the stack depth at which the line of code that failed resides, so that information can be presented to the user that makes it quick to find the failing line of code.
Exception class that encapsulates information about the stack depth at which the line of code that failed resides, so that information can be presented to the user that makes it quick to find the failing line of code. (In other words, the user need not scan through the stack trace to find the correct filename and line number of the problem code.) Having a stack depth is more useful in a testing environment in which test failures are implemented as thrown exceptions, as is the case in ScalaTest's built-in suite traits.
Exceptions thrown
NullArgumentException if either messageFun, cause or failedCodeStackDepthFun is null, or Some(null).
class TableDrivenPropertyCheckFailedException extends PropertyCheckFailedException
Exception that indicates a table-driven property check failed.
Exception that indicates a table-driven property check failed.
For an introduction to using tables, see the documentation for trait TableDrivenPropertyChecks.
Exceptions thrown
NullArgumentException if any parameter is null or Some(null).
class TestCanceledException extends StackDepthException with ModifiableMessage[TestCanceledException] with PayloadField with ModifiablePayload[TestCanceledException]
Exception thrown to indicate a test has been canceled.
Exception thrown to indicate a test has been canceled.
A canceled test is one that is unable to run because a needed dependency, such as an external database, is missing.
Canceled tests are ones that complete abruptly with a TestCanceledException after starting.
class TestFailedDueToTimeoutException extends TestFailedException with TimeoutField
Subclass of TestFailedException representing tests that failed because of a timeout.
Subclass of TestFailedException representing tests that failed because of a timeout.
This exception is thrown by the failAfter method of trait Timeouts, the eventually method of trait Eventually, and the await methods of trait AsyncAssertions.
Exceptions thrown
NullArgumentException if either messageFun, cause or failedCodeStackDepthFun is null, or Some(null).
class TestFailedException extends StackDepthException with ModifiableMessage[TestFailedException] with PayloadField with ModifiablePayload[TestFailedException]
Exception that indicates a test failed.
Exception that indicates a test failed.
One purpose of this exception is to encapsulate information about the stack depth at which the line of test code that failed resides, so that information can be presented to the user that makes it quick to find the failing line of test code. (In other words, the user need not scan through the stack trace to find the correct filename and line number of the failing test.)
Another purpose of this exception is to encapsulate a payload, an object to be included in a TestFailed event as its payload, so it can be consumed by a custom reporter that understands the payload. For example, tests could take a screen shot image of a GUI when a test fails, and include that as a payload. A custom reporter could listen for such payloads and display the screen shots to the user.
Exceptions thrown
NullArgumentException if either messageFun, cause or failedCodeStackDepthFun is null, or Some(null).
class TestPendingException extends RuntimeException
Exception thrown to indicate a test is pending.
Exception thrown to indicate a test is pending.
A pending test is one that has been given a name but is not yet implemented. The purpose of pending tests is to facilitate a style of testing in which documentation of behavior is sketched out before tests are written to verify that behavior (and often, the before the behavior of the system being tested is itself implemented). Such sketches form a kind of specification of what tests and functionality to implement later.
To support this style of testing, a test can be given a name that specifies one bit of behavior required by the system being tested. The test can also include some code that sends more information about the behavior to the reporter when the tests run. At the end of the test, it can call method pending, which will cause it to complete abruptly with TestPendingException. Because tests in ScalaTest can be designated as pending with TestPendingException, both the test name and any information sent to the reporter when running the test can appear in the report of a test run. (In other words, the code of a pending test is executed just like any other test.) However, because the test completes abruptly with TestPendingException, the test will be reported as pending, to indicate the actual test, and possibly the functionality, has not yet been implemented.
class TestRegistrationClosedException extends StackDepthException
Exception that indicates an action that is only allowed during a suite's test registration phase, such as registering a test to run or ignore, was attempted after registration had already closed.
Exception that indicates an action that is only allowed during a suite's test registration phase, such as registering a test to run or ignore, was attempted after registration had already closed.
In suites that register tests as functions, such as FunSuite and FunSpec, tests are normally registered during construction. Although it is not the usual approach, tests can also be registered after construction by invoking methods that register tests on the already constructed suite so long as run has not been invoked on that suite. As soon as run is invoked for the first time, registration of tests is "closed," meaning that any further attempts to register a test will fail (and result in an instance of this exception class being thrown). This can happen, for example, if an attempt is made to nest tests, such as in a FunSuite:
```
test("this test is fine") {
  test("but this nested test is not allowed") {
  }
}
```
This exception encapsulates information about the stack depth at which the line of code that made this attempt resides, so that information can be presented to the user that makes it quick to find the problem line of code. (In other words, the user need not scan through the stack trace to find the correct filename and line number of the offending code.)
Exceptions thrown
NullArgumentException if either message or failedCodeStackDepthFun is null
trait TimeoutField extends AnyRef
Trait mixed into exceptions thrown due to a timeout, which offers a timeout field providing a Span representing the timeout that expired.
Trait mixed into exceptions thrown due to a timeout, which offers a timeout field providing a Span representing the timeout that expired.
This trait is used by trait TimeLimitedTests to detect exceptions thrown because of timeouts, and for such exceptions, to modify the message to more clearly indicate a test timed out. (Although in its initial release there is only one subclass of TimeoutField in ScalaTest, TestFailedDueToTimeoutException, in future version of ScalaTest, there could be more)

Packages

Introduction to Property-based Testing