How to unit test throwing functions in Swift?

How to test wether a function in Swift 2.0 throws or not? How to assert that the correct ErrorType is thrown?

EDIT: Updated the code for Swift 4.1 (still valid with Swift 5.2)

Here's the latest Swift version of Fyodor Volchyok's answer who used XCTAssertThrowsError:

    enum MyError: Error {
        case someExpectedError
        case someUnexpectedError
    }

    func functionThatThrows() throws {
        throw MyError.someExpectedError
    }

    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            XCTAssertEqual(error as! MyError, MyError.someExpectedError)
        }
    }

If your Error enum has associated values, you can either have your Error enum conform to Equatable, or use the if case statement:

    enum MyError: Error, Equatable {
        case someExpectedError
        case someUnexpectedError
        case associatedValueError(value: Int)
    }

    func functionThatThrows() throws {
        throw MyError.associatedValueError(value: 10)
    }

    // Equatable pattern: simplest solution if you have a simple associated value that can be tested inside 1 XCTAssertEqual
    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            XCTAssertEqual(error as! MyError, MyError.associatedValueError(value: 10))
        }
    }

    // if case pattern: useful if you have one or more associated values more or less complex (struct, classes...)
    func testFunctionThatThrows() {
        XCTAssertThrowsError(try functionThatThrows()) { error in
            guard case MyError.associatedValueError(let value) = error else {
		        return XCTFail()
		    }
        
            XCTAssertEqual(value, 10)
			// if you have several values or if they require more complex tests, you can do it here
        }
    }

At least of Xcode 7.3 (maybe earlier) you could use built-in XCTAssertThrowsError():

XCTAssertThrowsError(try methodThatThrows())

If nothing is thrown during test you'll see something like this:

If you want to check if thrown error is of some concrete type, you could use errorHandler parameter of XCTAssertThrowsError():

enum Error: ErrorType {
    case SomeExpectedError
    case SomeUnexpectedError
}

func functionThatThrows() throws {
    throw Error.SomeExpectedError
}

XCTAssertThrowsError(try functionThatThrows(), "some message") { (error) in
    XCTAssertEqual(error as? Error, Error.SomeExpectedError)
}

Given the following functions and declarations:

enum SomeError: ErrorType {
    case FifthError
    case FirstError
}

func throwingFunction(x: Int) throws {
    switch x {
    case 1:
        throw SomeError.FirstError
    case 5:
        throw SomeError.FifthError
    default:
        return
    }
}

This function will throw a FifthError if 5 is given to the function and FirstError if 1 is given.

To test, that a function successfully runs the unit test could look as follows:

func testNotError() {
    guard let _ = try? throwingFunction(2) else {
        XCTFail("Error thrown")
        return
    }
}

The let _ may also be replaced by any other name, so you can further test the output.

To assert that a function throws, no matter what ErrorType the unit test could look like this:

func testError() {
    if let _ = try? throwingFunction(5) {
        XCTFail("No error thrown")
        return
    }
}

If you want to test for a specific ErrorType it's done with a do-catch-statement. This is not the best way compared to other languages.

You have to make sure that you...

• return in the catch for the correct ErrorType
• XCTFail() and return for all other catch
• XCTFail() if no catch is executed

Given this requirements a test case could look like this:

func testFifthError() {
    do {
        try throwingFunction(5)
    } catch SomeError.FifthError {
        return
    } catch {
        XCTFail("Wrong error thrown")
        return
    }
    XCTFail("No error thrown")
}

Swift 4.1 Error throwing Test for associated values

enum ParseError: Error, Equatable {
    case unexpectedArgument(String)
}

func testWithNoSchemaButWithOneArgument() {
    XCTAssertThrowsError(try Args(withSchema: "", andArguments: ["-x"])) { error in
        XCTAssertEqual(error as? ParseError, ParseError.unexpectedArgument("Argument(s) -x unexpected."))
    }
}

You can use this function:

func XCTAssertThrowsError<T, E: Error & Equatable>(
  _ expression: @autoclosure () throws -> T,
  error: E,
  in file: StaticString = #file,
  line: UInt = #line
  ) {
  var thrownError: Error?
  XCTAssertThrowsError(
    try expression(),
    file: file,
    line: line) {
      thrownError = $0
  }
  
  XCTAssertTrue(
    thrownError is E,
    "Unexpected error type: \(type(of: thrownError))",
    file: file,
    line: line
  )
  
  XCTAssertEqual(
    thrownError as? E,
    error,
    file: file,
    line: line
  )
}

Example:

XCTAssertThrowsError(try funcThatThrowsSpecificError(), error: SpecificErrorEnum.someError)