Asserting that a particular exception is thrown in Cucumber

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北海茫月
北海茫月 2021-02-02 13:20

Scenario

I\'m writing a library (no Ruby on Rails) for which I\'d like to have very detailed Cucumber features. This especially includes describing erro

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  • 2021-02-02 13:46

    It is possible to raise an exception in a When block and then make assertions about it in the following Then blocks.

    Using your example:

    When /^I do something unwanted$/ do
      @result = -> { throw_an_exception! }
    end
    
    Then /^an "(.*)" should be thrown$/ do |error|
      expect{ @result.call }.to raise_error(error)
    end
    

    That example uses RSpec's matchers but the important part is the -> (Lambda); which allows the reference to the throw_an_exception! method to be passed around.

    I hope that helps!

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  • 2021-02-02 13:50

    I thought about it once more, and maybe the answer is:

    There is no elegant solution, because the Given-When-Then-Scheme is violated in your case. You expect that "Then an exception should be thrown" is the outcome of "When I do something unwanted".

    But when you think about it, this is not true! The exception is not the outcome of this action, in fact the exception just shows that the "When"-Statement failed.

    My solution to this would be to test at a higher level:

    When I do something unwanted
    Then an error should be logged
    

    or

    When I do something unwanted
    Then the user should get an error message
    

    or

    When I do something unwanted
    Then the program should be locked in state "error"
    

    or a combination of these.

    Then you would "cache the exception" in your program - which makes perfect sense, as you most likely need to do that anyway.

    The two problems you've stated would be solved, too.

    In case you really must test for exceptions

    Well, i guess then cucumber isn't the right test suite, hmm? ;-)

    As the Given-When-Then-Scheme is violated anyway, I would simply write

    When I do something unwanted it should fail with "ArgumentError"
    

    and in the step definitions something like (untested, please correct me if you try it)

    When /^I do something unwanted it should fail with "(.*)"$/ do |errorstring|
      expect {
        throw_an_exception!
      }.to raise_error(errorstring)
    end
    

    As said above, that is horribly wrong as the scheme is broken, but it would serve the purpose, wouldn't it? ;-)

    You'll find further documentation at testing errors at rspec expectations.

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  • 2021-02-02 13:53

    I'm answering from the perspective of someone who uses Cucumber features in a Behavior-Driven Development situation, so take it or leave it...

    Scenarios should be written to test a 'feature' or functionality of the application, as opposed to being used to test the code itself. An example being:

    When the service is invoked
    Then a success code should be returned
    

    It sounds like your test case (i.e. If I do this, then this exception should be thrown) is a candidate for unit or integration testing - in my case, we would use some Mocking or unit testing framework.

    My suggestion would be to re-evaluate your feature scenarios to see if they are really testing what you intend them to test. From personal experience, I've found that if my test classes are becoming abnormally complex, then my features are 'wrong.'

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  • 2021-02-02 13:55

    One option is to mark the scenario with @allow-rescue and check the page's output and status code. For example

    In my_steps.rb

    Then(/^the page (?:should have|has) content (.+)$/) do |content|
      expect(page).to have_content(content)
    end
    
    Then(/^the page should have status code (\d+)$/) do |status_code|
      expect(page.status_code.to_s).to eq(status_code)
    end
    
    Then /^I should see an error$/ do
      expect(400..599).to include(page.status_code)
    end
    

    In my_feature.feature

    @allow-rescue
    Scenario: Make sure user can't do XYZ
      Given some prerequisite
      When I do something unwanted
      Then the page should have content Routing Error
      And the page should have status code 404
    

    or alternatively:

    @allow-rescue
    Scenario: Make sure user can't do XYZ
      Given some prerequisite
      When I do something unwanted
      Then I should see an error
    

    This may not be exactly what you were hoping for, but it might be an acceptable workaround for some people who come across this page. I think it will depend on the type of exception, since if the exception is not rescued at any level then the scenario will still fail. I have used this approach mostly for routing errors so far, which has worked fine.

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