Strange Error when Unit Testing a particular function in Ruby

Question

I'm working on this Exercise 49 in Learn Ruby the Hard Way

The exercise asks to write a unit test for each function provided. One of the unit tests I am writing is giving me an error.

Here is the code I am testing (word_list is an array of Pair structs)

Pair = Struct.new(:token, :word)

def peek(word_list)
    begin
        word_list.first.token
    rescue
        nil
    end
end

def match(word_list, expecting)
    begin
        word = word_list.shift
        if word.token == expecting
            word
        else
            nil
        end
    rescue
        nil
    end
end

def skip(word_list, token)
    while peek(word_list) == token
        match(word_list, token)
    end
end

Here is the test:

def test_skip
    small_list = [Pair.new(:verb, 'go'), Pair.new(:verb, 'do')]
    skip(small_list, :verb)
    assert_equal([],small_list) 
end

Here is the error I get when I run the unit test:

  1) Error:
test_skip(SentenceTests):
TypeError: backtrace must be Array of String
    test_sentence.rb:23:in `test_skip'

In this case, line 23 refers to "skip(small_list, :verb)". I am not sure why this error is happening, the above two functions were unit tested as well and those tests came out fine.

@Zabba, I did put them exactly as specified in the exercise above:

class ParserError < Exception

end

If anyone needs to see the exact files I am using here is the link to a gist: https://gist.github.com/1190148

Remove the `rescue` clause from `match` and `peek`, one at a time, and see what exception you get if any. — Zabba, Sep 02 '11 at 21:53
I removed rescue from match and peek, one at a time, and still got the same error in all cases. — Andrew Lauer Barinov, Sep 02 '11 at 22:00
What if you remove all the stuff after `raise` wherever it is being used .. do you still get that error? — Zabba, Sep 02 '11 at 22:23
What else is involved here? How exactly are you running this code? There is a `backtrace` method on Exception and that's supposed to return an Array. — mu is too short, Sep 02 '11 at 22:27
You can view the full code on the link provided above. My file is exactly the same. For the sake of clarity, I only provided functions/code directly relevant to this particular test. — Andrew Lauer Barinov, Sep 02 '11 at 22:34
You can see the complete files in a gist link I added above. — Andrew Lauer Barinov, Sep 02 '11 at 22:57
If you now reintroduce the `rescue` part in `peek` it works. `word_list.first` is finally `nil` and then `nil.token` raises an exception. Still I don't get why you see such a strange error message. Which Ruby version are you using? — undur_gongor, Sep 02 '11 at 23:17
I'm using ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin10.8.0] — Andrew Lauer Barinov, Sep 02 '11 at 23:48
@Andrew Barinov: See the update to my answer. Please note that the other point is still valid. — undur_gongor, Sep 03 '11 at 00:22
Should we introduce a tag for "Learn Ruby The Hard Way", like we have for Ruby Koans? — Andrew Grimm, Sep 04 '11 at 23:47

undur_gongor · Accepted Answer · 2011-09-03T00:19:23.993

2

The line

skip(small_list, :verb)

makes your test skip through the whole list (2 verbs). Finally, it is empty. So small_list.firstis nil which has no method/field word.

I get the error

undefined method `word' for nil:NilClass (NoMethodError)

though.

Probably in your code this exception is caught by some other code which itself raises the exception you are seeing.

UPDATE

It turns out in Ruby 1.9's test/unit (more specifically in the minitest/unit it is based on), there is a method skip which collides with your skip. See minitest/unit.rb, line 610. That method raised the exception because it expected its 2nd parameter to be an array.

Just rename your skip.

edited Sep 03 '11 at 00:19

answered Sep 02 '11 at 22:21

undur_gongor

15,657
5
63
75

`def test_skip` `small_list = [Pair.new(:verb, 'go'), Pair.new(:verb, 'do')]` `skip(small_list, :verb)` `answer = small_list.first.word` `assert_equal(answer,nil)` `end` Raise the same error. – Andrew Lauer Barinov Sep 02 '11 at 22:25
If you want to adjust your test to your code, the correct assertion would be `assert_equal([], small_list)` ;-) ... and not calculate `nil.word`. – undur_gongor Sep 02 '11 at 22:27
Gotcha, I changed the test as you suggested but the same error is still coming up. – Andrew Lauer Barinov Sep 02 '11 at 22:32

Strange Error when Unit Testing a particular function in Ruby

1 Answers1