uTest: the Essential Test Framework for Scala

uTest is a testing framework for the Scala programming language. uTest aims to be both simple and convenient to use, to allow you to focus on what's most important: your tests and your code. This post will explore what makes uTest interesting, and why you should consider using it to build the test suite in your next Scala project.

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy the Author's book Hands-on Scala Programming

essential absolutely necessary; indispensable pertaining to or constituting the essence of a thing http://www.dictionary.com/browse/essential

uTest isn't "essential" in the sense that you have to use it: other testing frameworks like ScalaTest or Specs2 are always an option. Rather, uTest is essential in that it contains just the features that are absolutely necessary for testing your code, and trims them down to their core essence, without unnecessary fluff.

uTest supports the following tasks:

Writing snippets of code to run ("tests"), labeling them and organizing them neatly

Sharing initialization and other code between your various tests

assert -ing things to make sure they're true

Selecting which tests you want to run, and seeing if they blow up

Anything else: fluent matchers, conventions about how tests should be labelled using "should" or "must" or "given - when - then", mocking, whether you're writing unit vs integration tests or example vs bulk tests, all that is left up to the user. uTest doesn't need to know, or care: it will happily run your tests and report their results no matter how you choose to write them.

Traditional Scala testing libraries force you to make many arbitrary decisions: choosing what syntax you'd like to use to structure your tests, or what syntax you'd like to use when doing asserts. In many cases, the difference is superficial, but nonetheless adds to the mental effort needed to use the library:

You have to choose (and hopefully standardize!) on one of the half-dozen styles of defining tests,

Choose between a half-dozen styles each time you want to assert something,

And read code written by collaborators or colleagues who happened to make a different choice!

When I myself have used ScalaTest or Specs2 in the past, there has always been the feeling at the back of my mind that I don't fully understand how the test framework works, that I'm not properly using the tools the test framework is giving me, that I need to consciously limit myself to the subset of the framework that I think I understand. All this even as I'm already struggling to understand the complexities of my own code and test suite!

uTest chooses a different path: it strips these features down to the bare essentials. A single, simple way of defining tests and a single, simple syntax for smart asserts: in most things that uTest provides, there is one - and only one - way to do things. That one way is made powerful and flexible enough to accommodate a wide range of use cases, yet simple enough you can understand it in it's entirety. If that isn't enough, you can build your own tools and helpers on top of uTest specialized for your own specific need.

uTest is simple enough you will never bump into a dark corner of the library you aren't familiar with. This simplicity lets you focus on what's you should be focusing on - your own code and test suite - with full confidence that the test framework will yield no surprises.

Getting started with uTest

While you can run uTest standalone, most people will probably be using it as part of an SBT project. For that, you need to add uTest to your build.sbt file:

libraryDependencies += "com.lihaoyi" %% "utest" % "0.5.3" % "test" testFrameworks += new TestFramework("utest.runner.Framework")

Define a test file in your src/test/scala folder:

package test.utest.examples import utest._ object HelloTests extends TestSuite{ val tests = Tests{ 'test1 - { throw new Exception("test1") } 'test2 - { 1 } 'test3 - { val a = List[Byte](1, 2) a(10) } } }

Here, the 'test1 - { ... } syntax marks out a block of code as a test, which will then get run and reported separately from every other block.

You then run it via:

sbt myproject/test

And see the results:

-------------------------------- Running Tests -------------------------------- Setting up CustomFramework X test.utest.examples.HelloTests.test1 4ms java.lang.Exception: test1 test.utest.examples.HelloTests$.$anonfun$tests$2(HelloTests.scala:7) + test.utest.examples.HelloTests.test2.inner 0ms 1 X test.utest.examples.HelloTests.test3 0ms java.lang.IndexOutOfBoundsException: 10 scala.collection.LinearSeqOptimized.apply(LinearSeqOptimized.scala:63) scala.collection.LinearSeqOptimized.apply$(LinearSeqOptimized.scala:61) scala.collection.immutable.List.apply(List.scala:86) test.utest.examples.HelloTests$.$anonfun$tests$5(HelloTests.scala:16) Tearing down CustomFramework Tests: 3, Passed: 1, Failed: 2

And that's all there is to it! There's a lot more documentation in the uTest readme about how to use the project: nesting tests to group them, selecting which tests to run, how the assertions work, how to configure the framework. Feel free to spend more time digging through those docs if you want to learn more about how to use the library. The remainder of this blog post will focus less on how to use uTest, and more on what makes uTest interesting and different from traditional testing libraries like ScalaTest or Specs2.

Test Definitions

The first thing that uTest does differently is that it only provides a single syntax for defining tests and groups of tests. Whether it's the simple, flat test suite shown above:

'test1 - { throw new Exception("test1") } 'test2 - { 1 } 'test3 - { val a = List[Byte](1, 2) a(10) }

Or a more complex test suite with tests grouped together in blocks, with some values/fixtures shared between the various tests in a block ( x , y and z below):

val x = 1 'outer1 - { val y = x + 1 'inner1 - { assert(x == 1, y == 2) (x, y) } 'inner2 - { val z = y + 1 assert(z == 3) } } 'outer2 - { 'inner3 - { assert(x > 1) } }

In all cases, it's a simple 'foo - {...} (or "foo bar" - {...} if the label has spaces or other special characters in it) to define a test or block of tests, with the inner-most blocks automatically being treated as "tests" to be run and their results reported.

This is in contrast with libraries like ScalaTest, which defines no less that eight different styles of test definitions:

test("foo bar"){ ... } "foo" should "bar" in { ... } describe("foo bar"){ ... } "foo bar" - { ... } property("foo bar"){ ... } scenario("foo bar"){ ... } def `foo bar` { ... }

While ScalaTest provides lots of options of how to define your test suite, in essence it doesn't really matter whether you call something a property or a scenario , or whether you use the words should in describe or just - . All you want to do when defining your tests is to write labelled snippets of code and group them together to keep things neat. The label just needs to be something descriptive enough that it's clear (together with the context) what the snippet is intended to test.

Hence uTest just takes the - -based syntax from ScalaTest's FreeSpec:

// FooTests.scala "Should respond appropriately when action event happens" - { ... }

and extends it to work with 'symbol s:

// FooTests.scala 'actionEventHandler - { ... }

As well as anonymous bullets:

// FooTests.scala * - { ... }

While it may initially seem strange to be able to define anonymous test cases without any label at all, it comes in useful when sometimes what the test is testing is completely obvious from the code. For example, these few test cases from the Ammonite repo which exercise Ammonite's parser:

// ParserTests.scala def assertResult(x: String, pred: Option[fastparse.all.Parsed[_]] => Boolean) = { val res = ammonite.interp.Parsers.split(x) assert(pred(res)) } def assertComplete(x: String) = assertResult(x, _.isDefined) 'endOfCommand - { * - assertComplete("{}") * - assertComplete("foo.bar") * - assertComplete("foo.bar // line comment") * - assertComplete("foo.bar /* block comment */") * - assertComplete("va va") // postfix * - assertComplete("") // ... }

Here, it is relatively clear from the surrounding endOfCommand label (in this ParserTests suite) that this is testing to make sure that these inputs parse to a complete result. And what sorts of inputs each test is exercising is also relatively clear from the string being passed to assertComplete .

uTest leaves it up to the developer to decide how much verbosity is needed. I personally find this encourages you to write helper methods and bang out relatively large numbers of simple test cases, which makes it easier to hit edge cases than with a small number of verbosely-labeled, complex test cases. Apart from that, uTest only provides a single syntax for defining test cases and groups of test cases, since that's all you really need to label snippets of code and keep things neat.

Test Running

Both ScalaTest and Specs2 have syntaxes for running individual tests within a suite:

# ScalaTest sbt> testOnly mypackage.MySuite -- -z testCaseNumberTwo

# Specs2 sbt> testOnly mypackage.ExampleSpec -- -ex testCaseNumberTwo

uTest removes the distinction between "test suite" and "test case":

sbt> testOnly -- mypackage.ExampleTests.testCaseNumberTwo

in uTest, there are only tests and groups of tests. It doesn't matter whether you are running a package full of test suites:

sbt> testOnly -- mypackage

All the tests in a test suite:

sbt> testOnly -- mypackage.ExampleTests

A group of tests within the test suite:

sbt> testOnly -- mypackage.ExampleTests.fooRelatedTests

Or a specific test case:

sbt> testOnly -- mypackage.ExampleTests.fooRelatedTests.testCaseNumberTwo

To uTest, you can run any grouping of tests, no matter how small or large, with the same uniform syntax. That uniform syntax also applies when running multiple tests or groups:

sbt> testOnly -- {mypackage,yourpackage} sbt> testOnly -- mypackage.{ExampleTests,HelloTests} sbt> testOnly -- mypackage.HelloTests.{fooRelatedTests,barRelatedTests} sbt> testOnly -- mypackage.HelloTests.fooRelatedTests.{testCaseOne,testCaseNumberTwo}

While in Scala you are forced to wrap code in an object within a source file, that wrapper object is not core to the essence of testing. The real goal is just to organize things hierarchially: whether multiple tests within a single file, or multiple files within a folder.

With uTest, it doesn't matter whether you are running tests within a Scala package, a single TestSuite object, a specific group of tests within a TestSuite , or even an individual test. You have a single hierarchy of "tests" organized in a tree, and can choose which bits of the tree to run to see if it blows up. And that is the essential part of what running tests is about.

Smart Asserts

Both ScalaTest and Specs2 have a large library of "matchers" to help you perform asserts of various sorts on your data. This ranges from simple equality checks:

// ScalaTest foo should equal (bar) foo should === (bar) foo should be (bar) foo shouldEqual bar foo shouldBe bar

// Specs2 foo must beEqualTo(bar) foo must be_==(bar) foo must_== bar foo mustEqual bar foo should_== bar foo === bar foo must be equalTo(bar)

To more specific matchers, e.g. to assert properties on Strings:

// ScalaTest string should startWith ("Hello") string should endWith ("world") string should include ("seven") string should startWith regex "Hel*o" string should endWith regex "wo.ld" string should include regex "wo.ld"

// Specs2 beMatching or be matching // check if a string matches a regular expression =~(s) // shortcut for beMatching("(.|\\s)*"+s+"(.|\\s)*") find(exp).withGroups(a, b, c) // check if some groups are found in a string have length // check the length of a string have size // check the size of a string (seen as an Iterable[Char]) be empty // check if a string is empty beEqualTo(b).ignoreCase // check if 2 strings are equal regardless of casing beEqualTo(b).ignoreSpace // check if 2 strings are equal when you replaceAll("\\s", "") beEqualTo(b).trimmed // check if 2 strings are equal when trimmed beEqualTo(b).ignoreSpace.ignoreCase // you can compose them contain(b) // check if a string contains another one startWith(b) // check if a string starts with another one endWith(b) // check if a string ends with another one

With similar matchers for Seq s, Array s, numbers, case class es, and a hodge-podge of other supported types.

ScalaTest also provides useful "Smart" assert methods that automatically print out the values of variables involved in the assertion:

val a = 1 val b = 2 val c = 3 val d = 4 assert(a == b || c >= d) // Error message: 1 did not equal 2, and 3 was not greater than or equal to 4

To me, the implementation of "matchers" in these libraries is flawed for a couple of reasons:

They are extremely redundant! I don't need 7 different ways to check assert(a == b) , just one will do

They use a strange, english-like syntax: I can use a should equal (b) and a shouldEqual b but not a should equal b , when in my mind I'm just thinking assert(a == b)

They pollute your namespace! By flooding your namespace with names like include , endWith , empty , length , size , this ends up causing name clashes e.g. here where Specs2's matcher should syntax collides with it's test-definition should syntax

Overall, these matchers are not essential to what tests are about. While they do provide nicer error messages of plain-old assert(a == b) asserts, ScalaTest's smart asserts are already able to give you good error messages, without needing an entire matcher framework.

Hence, uTest does without matchers entirely, and offers a relatively small number of built-in assertions. The main ones are:

Smart Asserts, similar to ScalaTest's, which automatically print out the code that failed, together with the values and types of the values involved in the assertion:

val x = 1 val y = "2" assert( x > 0, x == y ) // utest.AssertionError: x == y // x: Int = 1 // y: String = 2

Intercept, which asserts that the given code throws an exception, capturing it and returning it so you can perform checks on its properties:

val e = intercept[MatchError]{ (0: Any) match { case _: String => } } println(e) // scala.MatchError: 0 (of class java.lang.Integer)

assertMatch, which lets you check if the given value matches a pattern. This is similar to assert(a == b) but a lot more flexible, since the pattern can have _ holes in it for parts which you do not care about asserting:

assertMatch(Seq(1, 2, 3)){case Seq(1, _) =>} // AssertionError: Matching failed Seq(1, 2, 3)

compileError, which lets you assert that a given expression doesn't compile, and assert further that it produces a particular error message. It's a good practice to test runtime error cases and make sure the error messages are what you expect, and there's no reason this shouldn't apply to compile-time error cases too:

compileError("true * false") // CompileError.Type("value * is not a member of Boolean") compileError("(}") // CompileError.Parse("')' expected but '}' found.")

I think that these few core asserts cut to the essence of what people want from their asserts when they write tests: they want to be able to write code, check some property, and if the check fails (or the code blows up) they want to see what values likely caused the code to fail or blow up.

On top of that, people inevitably want more customized asserts, more customized comparisons, with their own customized error messages and diagnostics when things go wrong. While uTest's assert doesn't let you provide a custom error message, Predef.assert is still available if you want to do an assert with a custom error message. After that, it is then up to the user of uTest to write their own "check" functions to perform whatever more custom checks they want.

For example, Fansi's test suite has a simple check function that round-trips a fansi.Str through rendering and parsing and makes sure that the output is the same as the input.

'parsing - { def check(frag: fansi.Str) = { val parsed = fansi.Str(frag.render) assert(parsed == frag) parsed } * - check(fansi.Color.True(255, 0, 0)("lol")) * - check(fansi.Color.True(1, 234, 56)("lol")) * - check(fansi.Color.True(255, 255, 255)("lol")) * - check(fansi.Color.True(10000)("lol")) * - check(square(for(i <- 0 to 255) yield fansi.Color.True(i,i,i))) }

PPrint uses a custom check function to make sure escaped ASCII characters match the expected output string:

'escapeChar{ def check(c: Char, expected: String) = { val escaped = pprint.Util.escapeChar(c, new StringBuilder).toString assert(escaped == expected) } check('a', "a") check('-', "-") check('

', "\

") check('\\', "\\\\") check('\t', "\\t") }

uTest just provides the basic tools that "everyone" wants: assert s that print out the values involved when they go wrong. Past that, test suites are just Scala code: it's easy for the developer using uTest to define their own helpers to perform whatever checks they want, and so uTest leaves it up to them.

Shared Fixtures

When writing code in Scala, you are always aware of lexical scope: that things defined in broader scopes are available to code running in narrower, enclosed scopes.

When you have a test suite with nested blocks of tests, it is then natural to want to put shared "fixture" definitions in the outer blocks, so the inner blocks can access them. It doesn't matter whether those fixtures are val s, def s or something else, although test fixtures have a tendency to fail just like any other code in your test suite.

It turns out that this causes problems in many traditional frameworks like ScalaTest. Consider the following ScalaTest suite:

import org.scalatest.FreeSpec class SetSpec extends FreeSpec { "outer1" - { throw new Exception("Outer One") "inner1" in { throw new Exception("Inner One") } "inner2" in { throw new Exception("Inner Two") } } "outer2" - { throw new Exception("Outer Two") "inner3" in { throw new Exception("Inner Three") } } }

Here, we're using throw new Exception as a stand-in for code that might fail. While this test suite is of course nonsensical, the way it can throw exceptions in various places is not unlike how many perfectly sensible test suites work.

Running this suite gives the following error:

SetSpec: SetSpec *** ABORTED *** java.lang.Exception was thrown inside "outer1" -, construction cannot continue: "Outer One" (SetSpec.scala:5)

Here, we can see that in ScalaTest, any failure in the "outer" blocks causes the entire test suite to error out without running properly.

On the other hand, if you run a set of nested uTest tests like this:

import utest._ object MyTests extends TestSuite{ val tests = Tests{ 'outer1 - { throw new Exception("Outer One") 'inner1 - { throw new Exception("Inner One") } 'inner2 - { throw new Exception("Inner One") } } 'outer2 - { throw new Exception("Outer Two") 'inner3 - { throw new Exception("Inner Two") } } } }

You will get the following output:

----------------------------------- Results ----------------------------------- - MyTests 0ms - outer1 0ms X inner1 0ms java.lang.Exception: Outer One mypackage.MyTests.$anonfun$tests$2(MyTests.sc:3) X inner2 0ms java.lang.Exception: Outer One mypackage.MyTests.$anonfun$tests$2(MyTests.sc:3) - outer2 0ms X inner3 0ms java.lang.Exception: Outer Two mypackage.MyTests.$anonfun$tests$5(MyTests.sc:12)

Note how despite the fact that outer1 and outer2 failed with exceptions Outer One and Outer Two , uTest was still aware of all the tests in the suite and able to report their failure.

Another case worth looking at is what happens to mutable variables defined in the enclosing scopes. In ScalaTest, these mutable variables are shared between the various individual tests:

import org.scalatest.FreeSpec class SetSpec extends FreeSpec { var x = 0 "outer1" - { x += 1 "inner1" in { x += 2 println(x) // 7, from 0 + 1 + 4 + 2 } "inner2" in { x += 3 println(x) // 10, from 7 + 3 } } "outer2" - { x += 4 "inner3" in { x += 5 println(x) // 15, from 10 + 5 } } }

This can cause confusing issues, e.g. your test suite may pass or fail differently depending on how you re-order the tests within it.

However, in the equivalent uTest test suite, each test gets its own copy of any fixtures initialized in the Tests{ ... } block. This means that each test is only affected by code in their surrounding blocks, and won't see interference from other tests next to it:

import utest._ object SeparateSetupTests extends TestSuite{ val tests = Tests{ var x = 0 'outer1 - { x += 1 'inner1 - { x += 2 assert(x == 3) // 0 + 1 + 2 x } 'inner2 - { x += 3 assert(x == 4) // 0 + 1 + 3 x } } 'outer2 - { x += 4 'inner3 - { x += 5 assert(x == 9) // 0 + 4 + 5 x } } } }

Lexical scope is a convenient, well-understood way of sharing code: whatever you put in outer blocks is available to code in inner blocks. However, with a framework like ScalaTest, you are unable to use this fact to define shared fixtures that can be shared by all your tests: if defining the fixture throws an exception, that causes the suite to error out without running properly, and any mutable fixtures are shared which can lead to tests interfering with each other in confusing ways.

In uTest, on the other hand, these problems are solved: you can define shared initialization code in outer blocks, whether val s of def s, just like in normal Scala code. Furthermore, these definitions are by-default not shared between different tests in the suite, to avoid accidental interference. Thus, defining shared fixtures is as simple as defining them in the block enclosing the tests that need to access it: an intuitive approach that really gets to the core essence of what shared test fixtures are all about.

At some point, your test initialization code will grow too large and messy to include in the outer blocks of a test suite. The solution is to extract it into a separate file, the same way you deal with any too-long block in normal code.

Missing Features from uTest

uTest is no where near as feature-rich as traditional frameworks like ScalaTest or Specs2. Apart from a lot of missing features I consider "unessential", there are some things which I think are pretty useful and should eventually end up in uTest in some shape or form:

Tagging tests : uTest only allows you to organize your tests hierarchically. If you wanted to conveniently "only run integration tests", you would need to put all those tests in a separate integration package. This works for many cases, but there are cases where there are multiple different ways you may want to select parts of your test suite, and a convenient way to tag tests would help achieve that.

More complex test queries : using globs ( foo.*.bar ), negation ("all test in foo.bar except those in foo.bar.baz ), partial matches ( foo.*BarTests ). While these tend to be sloppy and should not be the default way of picking individual tests, it would be great to have them available.

Better SBT integration : things like taking test configuration from SBT, or test arguments from SBT.

Fine-grained teardown hooks : while uTest allows you to put "setup" code in a variety of places -the object bodies, or in various scopes within the Test{} block - it only lets you put teardown code in one global "teardown" handler that runs at the end of each test run.

Customizable reporting: currently, uTest only outputs to the console and to a junit.xml results file via it's SBT integration. It would be nice to expose hooks such that uTest's results could be easily post-processed after a test run is complete, to generate custom reports e.g. showing the slowing tests in the test suite.

All these are are useful things that eventually should find their way into uTest. None of them are in-principle difficult to implement, just that I have never found the need for them yet in my own projects, and so they simply haven't been implemented. While uTest is likely to grow in breadth and functionality over time, it is unlikely to ever become as large and complex as the other test frameworks out there.

Conclusion

While there are endless possibilities for what a test framework could do, I believe that only a few of those things are really essential:

Writing snippets of code to run ("tests"), labeling them and organizing them neatly

Sharing initialization and other code between your various tests

assert -ing things to make sure they're true

Selecting which tests you want to run, and seeing if they blow up

uTest is a test framework optimized for these features - making them as simple, intuitive and powerful as possible.

As a project grows, it is natural to want more things: perhaps a custom organization of your test suite and test data, your own libraries of shared test setup fixtures, or asserts that check properties specific to your codebase. Rather than trying to be all things to all people, uTest provides the basics and then stays out of the way, to let you focus on building out your own test infrastructure specialized to your specific need.

uTest is not a new project. The initial code was written in 2014, as it was written first to be a test framework to support Scala.js, back when no other libraries supported it. However, apart from it's Scala.js support (which is no longer unique, with ScalaTest and other libraries now offering it) I believe there are still many good reasons why you may want to consider using uTest over the alternatives.

The recent 0.5.3 release has fixed a large number of long-standing issues that historically made uTest uncomfortable to use. What remains should is, I hope, a tight and polished package that provides the core tools needed to build your test suites. An essential test framework for the Scala programming language.

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy the Author's book Hands-on Scala Programming