Let’s imagine we’re writing a simple class to greet a user. Following a TDD approach, we’ll write the test first:

# spec/hello_spec.rb RSpec . describe Hello do it "greets" do result = Hello . run ( "Andy" ) expect ( result ). to eq ( "Hello, Andy" ) end it "raises an error if no name is supplied" do expect do Hello . new . run ( "" ) end . to raise_error end end

The implementation is straightforward::

# lib/hello.rb class Hello def self . run ( name ) raise ArgumentError , "No name given" if name . blank? "Hello, #{ name } " end end

We push the branch, see it passes in Travis (or whatever CI is in use), then merge it into master.

We just introduced a bug

Do you spot it? In the second test, we’re calling run as a instance method, but it’s actually a class method.

An error is being raised, so the test passes, but it’s not the error we were expecting.

If we were using RSpec 3 or newer, you might have spotted a warning displayed as part of the test output:

WARNING: Using the `raise_error` matcher without providing a specific error or message risks false positives, since `raise_error` will match when Ruby raises a `NoMethodError`, `NameError` or `ArgumentError`, potentially allowing the expectation to pass without even executing the method you are intending to call. Actual error raised was #<NoMethodError: undefined method `run' for #<Hello:0x007fdc39b50118>>. Instead consider providing a specific error class or message. This message can be suppressed by setting: `RSpec::Expectations.configuration.on_potential_false_positives = :nothing`. Called from spec/hello_spec.rb:11:in `block (2 levels) in <top (required)>'.

It’s saying the actual error raised was NoMethodError , rather than ArgumentError , and it’s warning us there’s risk of a false positive (in other words, the test passes when even though the implementation is wrong). The warning is useful, but on a large test suite which already outputs a lot of warnings, it might not be noticed.

We can improve the test so it checks for the specific error type:

it "raises an error if no name is supplied" do expect do Hello.new.run("") end.to raise_error ArgumentError, "No name given" end

Re-running the test now results in an actual failure:

expected ArgumentError with "No name given", got #<NoMethodError: undefined method `run' for #<Hello:0x007f8868b8d0d8>> with backtrace: # ./spec/hello_spec.rb:12:in `block (3 levels) in <top (required)>' # ./spec/hello_spec.rb:11:in `block (2 levels) in <top (required)>'

Configuration

You might have noticed that the RSpec output also describes how to suppress these warning messages. This is dangerous and you shouldn’t do that. You could miss genuine bugs in the code.

The default behaviour of on_potential_false_positives is :warn , which displays a message like we saw above.

However, there’s a better way to configure this. By setting it to :raise , your test will fail unless you specify the error type when using raise_error .

This will help to prevent future problems. If you have a large test suite, it might take some effort to update your test suite, but it’s worth it.

Final Thoughts

Pay attention to the warnings emitted from your code or test suite, and try to eliminate them. They exist to help you, not to annoy you.