39

In a Ruby unit test, how do I assert that a string contains a substring? Something like:

assert_contains string_to_test, substring_to_verify
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6 Answers 6

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52

You could go with assert_match pattern, string, [message] which is true if string =~ pattern:

assert_match substring_to_verify, string_to_test

e.g.

assert_match /foo/, "foobar"

If you use this very often, why not write your own assertion?

require 'test/unit'

module Test::Unit::Assertions
  def assert_contains(expected_substring, string, *args)
    assert_match expected_substring, string, *args
  end
end

Alternatively, using the method described by @IvayloStrandjev (way easier to understand), you could define

require 'test/unit'

module Test::Unit::Assertions
  def assert_contains(expected_substring, string, *args)
    assert string.include?(expected_substring), *args
  end
end

The usage is exactly as you requested in your question, e.g.

class TestSimpleNumber < Test::Unit::TestCase
  def test_something
    assert_contains 'foo', 'foobar'
  end

  def test_something_fails
    assert_contains 'x', 'foobar', 'Does not contain x'
  end
end

Which will produce

Run options:

# Running tests:

.F

Finished tests in 0.000815s, 2453.9877 tests/s, 2453.9877 assertions/s.

  1) Failure:
test_something_fails(TestSimpleNumber) [assertion.rb:15]:
Does not contain x

2 tests, 2 assertions, 1 failures, 0 errors, 0 skips

Edit

As requested, with automated message:

module Test::Unit::Assertions
  def assert_contains(exp_substr, obj, msg=nil)
    msg = message(msg) { "Expected #{mu_pp obj} to contain #{mu_pp exp_substr}" }
    assert_respond_to obj, :include?
    assert obj.include?(exp_substr), msg
  end
end

adapted from the original assert_match source. This actually also works with Arrays!

assert_contains 3, [1,2,3]
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4 Comments

Hi. Cool answer! +1. I have several questions, maybe too long for comments. Would you mind checking chat.stackoverflow.com/rooms/28565 ?
Hi, sorry i couldn't make it. Do you still have questions?
yeah, can you take a look at the link in my comment?
Doesn't work on TestUnit 3.3 w/ Ruby 2.7: "NoMethodError: undefined method `message' "
12

There is assert_includes:

assert_includes 'foobar', 'foo'

will assert that foobar contains foo.

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3 Comments

The format should be assert_includes result, 'foo' in order to test for the substring 'foo'.
Surely this should be the accepted answer?
@SamCosta1 It works, but it invalidates the paradigm where the left side of the expression is the "expected" result, whereas in this case is the thing we are testing instead.
8

You can write assert string_to_test.include?(string_to_verify) for instance. You can not expect to have asserts for all the checks you would like to perform, so just go the the classic check of a boolean condition.

Also have a look here to see a list of all available assertions.

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4 Comments

is it possible to use assert_match? What pattern should I use?
@LouisRhys I believe so, though I never used it(and it is only now that I notice it). Maybe try: assert_match(".*#{string_to_verify}.*", string_to_test). Should work if I am not wrong.
Sure this works but the error message on failures Expected false to be truthy. is not very helpful compared to using assert_match.
@Andrew assert takes an optional message that will be printed in case the assertion fails. You can use that to get more informative message
2

I'd use one of these:

assert(string_to_test[substring_to_verify])
assert_equal(substring_to_verify, string_to_test[substring_to_verify])

They accomplish the same thing so the first is my usual choice.

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Comments

0

Like this:

assert string_to_test.index(substring_to_verify)

The .index method returns nil if the substring is not found, which will cause the assert to fail.

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0

I would use assert_match:

require 'test/unit'

class MyTest < Test::Unit::TestCase
  def test_match
    assert_match( /aa/, 'xxaaxx')
  end
  def test_match_fail
    #~ assert_match( /aa/, 'xxbbxx')  #fails
  end
end

If you need it often, you could extend TestCase:

require 'test/unit'

module Test
  module Unit
    class TestCase
      #Define new assertion
      def assert_contains(string_to_test, substring_to_verify)
        assert_match( string_to_test, substring_to_verify)
      end
      def assert_not_contains(string_to_test, substring_to_verify)
        assert_not_match( string_to_test, substring_to_verify)
      end
    end
  end
end
class MyTest < Test::Unit::TestCase
  def test_contains()
    assert_contains( /aa/, 'xxaaxx')
    assert_contains( 'aa', 'xxaaxx')
  end
  #~ def test_contains_fail()
    #~ assert_contains( 'aa', 'xxxxxx')
    #~ assert_contains( /aa/, 'xxxxxx')
  #~ end
  #~ def test_contains_not_fail()
    #~ assert_not_contains( /aa/, 'xxaaxx')
    #~ assert_not_contains( 'aa', 'xxaaxx')
  #~ end
  def test_contains_not()
    assert_not_contains( 'aa', 'xxxxxx')
    assert_not_contains( /aa/, 'xxxxxx')
  end
  def test_contains_special_characters()
    assert_contains( '[aa', 'xx[aaxx')
    #~ assert_contains( /[aa/, 'xx[aaxx')
  end
end

Remarks:

  • When you use special regexp-characters (like []....) then you can use it in strings (at least my experiment in test_contains_special_characters worked.
  • You can define your own regexps.
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