Asserting state with #anticipate in Swift Testing – Donny Wals

I do not suppose I’ve ever heard of a testing library that does not have some mechanism to check assertions. An assertion within the context of testing is actually an assumption that you’ve about your code that you just need to guarantee is appropriate.

For instance, if I have been to write down a operate that is supposed so as to add one to any given quantity, then I might need to assert that if I put 10 into that operate I get 11 out of it. A testing library that might not be capable to do that’s not price a lot. And so it must be no shock in any respect that Swift testing has a manner for us to carry out assertions.

Swift testing makes use of the #anticipate macro for that.

On this submit, we’re going to check out the #anticipate macro. We’ll get began by utilizing it for a easy Boolean assertion after which work our manner as much as extra advanced assertions that contain errors.

Testing easy boolean circumstances with #anticipate

The commonest manner that you just’re most likely going to be utilizing #anticipate is to guarantee that sure circumstances are evaluated to betrue. For instance, I’d need to take a look at that the operate beneath really returns 5 every time I name it.

func returnFive() -> Int {
  return 0
}

In fact this code is a little bit bit foolish, it does not actually do this a lot, however you might think about {that a} extra sophisticated piece of code would must be examined extra totally.

Since I have never really carried out my returnFive operate but, it simply returns 0. What I can do now’s write a take a look at as proven beneath.

@Check func returnFiveWorks() async throws {
  let functionOutput = Incrementer().returnFive()
  #anticipate(5 == functionOutput)
}

This take a look at goes to check that after I name my operate, we get quantity 5 again. Discover the road the place it says #anticipate(5 == functionOutput).

That’s an assertion.

I’m attempting to claim that 5 equals the output of my operate by utilizing the #anticipate macro.

When our operate returns 5, my expression (5 == functionOutput) evaluated to true and the take a look at will move. When the expression is false, the take a look at will fail with an error that appears a bit like this:

Expectation failed: 5 == (functionOutput → 0)

This error will present up as an error on the road of code the place the expectation failed. That implies that we are able to simply see what went improper.

We are able to present extra context to our take a look at failures by including a remark. For instance:

@Check func returnFiveWorks() async throws {
  let functionOutput = Incrementer().returnFive()
  #anticipate(5 == functionOutput, "returnFive() ought to all the time return 5")
}

If we replace our exams to look a little bit bit extra like this, if the take a look at fails we are going to see an output that is a little more elaborate (as you may see beneath).

Expectation failed: 5 == (functionOutput → 0)
returnFive() ought to all the time return 5

I all the time like to write down a remark in my expectations as a result of this may present a little bit bit extra context about what I anticipated to occur, making debugging my code simpler in the long term.

Usually talking, you are both going to be passing one or two arguments to the anticipate macro:

1. The primary argument is all the time going to be a Boolean worth
2. A remark that can be proven upon take a look at failure

So within the take a look at you noticed earlier, I had my comparability between 5 and the operate output within my expectation macro as follows:

5 == functionOutput

If I have been to vary my code to appear like this the place I put the comparability exterior of the macro, the output of my failing take a look at goes to look a little bit bit totally different. This is what it is going to appear like:

@Check func returnFiveWorks() async throws {
  let functionOutput = Incrementer().returnFive()
  let didReturnFive = 5 == functionOutput
  #anticipate(didReturnFive, "returnFive() ought to all the time return 5")
}

// produces the next failure message:
// Expectation failed: didReturnFive
// returnFive() ought to all the time return 5

Discover how I am not getting any suggestions proper now about what might need gone improper. I merely get a message that claims “Expectation failed: didReturnFive” and no context as to what precisely might need gone improper.

I all the time suggest attempting to place your expressions contained in the anticipate macro as a result of that’s merely going to make your take a look at output much more helpful as a result of it is going to examine variables that you just inserted into your anticipate macro and it’ll say “you anticipated 5 however you have acquired 0”.

On this case I solely know that I didn’t get 5, which goes to be so much more durable to debug.

We are able to even have a number of variables that we’re utilizing within anticipate and have the testing framework inform us about these as properly.

So think about I’ve a operate the place I enter a quantity and the quantity that I need to increment the quantity by. And I anticipate the operate to carry out the maths increment the enter by the quantity given. I might write a take a look at that appears like this.

@Check func incrementWorks() async throws {
  let enter = 1
  let incrementBy = 2
  let functionOutput = Incrementer().increment(enter: enter, by: incrementBy)
  #anticipate(functionOutput == enter + incrementBy, "increment(enter:by:) ought to add the 2 numbers collectively")
}

This take a look at defines an enter variable and the quantity that I need to increment the primary variable by.

It passes them each to an increment operate after which does an assertion that checks whether or not the operate output equals the enter plus the increment quantity. If this take a look at fails, I get an output that appears as follows:

Expectation failed: (functionOutput → 4) == (enter + incrementBy → 3)
increment(enter:by:) ought to add the 2 numbers collectively

Discover how I fairly conveniently see that my operate returned 4, and that’s not equal to enter + increment (which is 3). It is actually like this stage of element in my failure messages.

It’s particularly helpful while you pair this with the take a look at arguments that I coated in my submit on parameterized testing. You’ll be able to simply see a transparent report on what your inputs have been, what the output was, and what could have gone improper for every totally different enter worth.

Along with boolean circumstances like we’ve seen up to now, you may need to write exams that verify whether or not or not your operate threw an error. So let’s check out testing for errors utilizing anticipate subsequent.

Testing for errors with #anticipate

Typically, the purpose of a unit take a look at is not essentially to verify that the operate produces the anticipated output, however that the operate produces the anticipated error or that the operate merely does not throw an error. We are able to use the anticipate macro to claim this.

For instance, I might need a operate that throws an error if my enter is both smaller than zero or bigger than 50. This is what that take a look at might appear like with the anticipate macro:

@Check func errorIsThrownForIncorrectInput() async throws {
  let enter = -1
  #anticipate(throws: ValidationError.valueTooSmall, "Values lower than 0 ought to throw an error") {
    strive checkInput(enter)
  }
}

The syntax for the anticipate macro while you’re utilizing it for errors is barely totally different than you may anticipate based mostly on what the Boolean model seemed like. This macro is available in numerous flavors, and I choose the one you simply noticed for my normal objective error exams.

The primary argument that we move is the error that we anticipate to be thrown. The second argument that we move is the remark that we need to print every time one thing goes improper. The third argument is a closure. On this closure we run the code that we need to verify thrown errors for.

So for instance on this case I am calling strive checkInput which implies that I anticipate that code to throw the error that I specified as the primary argument in my #anticipate.

If every thing works as anticipated and checkInput throws an error, my take a look at will move so long as that error matches ValidationError.valueTooSmall.

Now as an instance that I by accident throw a distinct error for this operate the output will look a little bit bit like this

Expectation failed: anticipated error "valueTooSmall" of sort ValidationError, however "valueTooLarge" of sort ValidationError was thrown as an alternative
Values lower than 0 ought to throw an error

Discover how the message explains precisely which error we obtained (valueTooLarge) and the error that we anticipated (valueTooSmall). It is fairly handy that the #anticipate macro will really inform us what we obtained and what we anticipated, making it straightforward to determine what might have gone improper.

Including a little bit remark identical to we did with the Boolean model makes it simpler to cause about what we anticipated to occur or what may very well be taking place.

If the take a look at doesn’t throw an error in any respect, the output would look as proven beneath

ExpectMacro.swift:42:3: Expectation failed: an error was anticipated however none was thrown
Values lower than 0 ought to throw an error

This error fairly clearly tells us that no error was thrown whereas we did anticipate an error to be thrown.

There is also conditions the place you do not actually care concerning the precise error being thrown, however simply that an error of a particular sort was thrown. For instance, I won’t care that my “worth too small” or “worth too giant” error was thrown, however I do care that the kind of error that acquired thrown was a validation error. I can write my take a look at like this to verify for that.

@Check func errorIsThrownForIncorrectInput() async throws {
  let enter = -1
  #anticipate(throws: ValidationError.self, "Values lower than 0 ought to throw an error") {
    strive checkInput(enter)
  }
}

As a substitute of specifying the precise case on validation error that I anticipate to be thrown, I merely move ValidationError.self. This can permit my take a look at to move when any validation error is thrown. If for no matter cause I throw a distinct type of error, the take a look at would fail.

There is a third model of anticipate in relation to errors that we might use. This one would first permit us to specify a remark like we are able to in any anticipate. We are able to then move a closure that we need to execute (e.g. calling strive checkInput) and a second closure that receives no matter error we obtained. We are able to carry out some checks on that after which we are able to return whether or not or not that was what we anticipated.

For instance, in case you have a bit extra sophisticated setup the place you are throwing an error with an related worth you may need to examine the related worth as properly. This is what that would appear like.

@Check func errorIsThrownForIncorrectInput() async throws {
  let enter = -1
  #anticipate {
    strive checkInput(enter)
  } throws: { error in 
    guard let validationError = error as? ValidationError else {
      return false
    }

    change validationError {
    case .valueTooSmall(let margin) the place margin == 1:
      return true
    default:
      return false
    }
  }
}

On this case, our validation logic for the error is fairly fundamental, however we might broaden this in the true world. That is actually helpful when you may have a sophisticated error or sophisticated logic to find out whether or not or not the error was precisely what you anticipated.

Personally, I discover that most often I’ve fairly easy error checking, so I’m usually utilizing the very first model of anticipate that you just noticed on this part. However I’ve positively dropped right down to this one after I needed to examine extra sophisticated circumstances to find out whether or not or not I acquired what I anticipated from my error.

What you want is, after all, going to rely by yourself particular scenario, however know that there are three variations of anticipate that you need to use when checking for errors, and that all of them have form of their very own downsides that you just may need to consider.

In Abstract

Often, I consider testing libraries by how highly effective or expressive their assertion APIs are. Swift Testing has carried out a extremely good job of offering us with a reasonably fundamental however highly effective sufficient API within the #anticipate macro. There’s additionally the #require macro that we’ll discuss extra in a separate submit, however the #anticipate macro by itself is already an effective way to start out writing unit exams. It supplies a number of context about what you are doing as a result of it is a macro and it’ll broaden into much more data behind the scenes. The API that we write is fairly clear, fairly concise, and it is highly effective to your testing wants.

Be certain that to take a look at this class of Swift testing on my web site as a result of I had a number of totally different posts with Swift testing, and I plan to broaden this class over time. If there’s something you need me to speak about when it comes to Swift testing, be sure you discover me on social media, I’d love to listen to from you.