Test for warnings using RSpec

Question

Is it somehow possible to test warnings i Ruby using RSpec?

Like this:

class MyClass
  def initialize
    warn "Something is wrong"
  end
end

it "should warn" do
  MyClass.new.should warn("Something is wrong")
end

Answer 1

There is a good article with custom expectation which solves exactly your problem: http://greyblake.com/blog/2012/12/14/custom-expectations-with-rspec/

So it would like:

expect { MyClass.new }.to write("Something is wrong").to(:error)

Base on that article you can create you own expectation to use it like this:

expect { MyClass.new }.to warn("Something is wrong")

Answer 2

If warning raises during initialization you can use output expectation:

expect do
  described_class.new
end.to output('test message').to_stderr

Answer 3

This is my solution, I define a custom matcher has_warn

require 'rspec'
require 'stringio'

module CustomMatchers
  class HasWarn
    def initialize(expected)
      @expected = expected
    end

    def matches?(given_proc)
      original_stderr = $stderr
      $stderr = StringIO.new
      given_proc.call
      @buffer = $stderr.string.strip
      @expected.include? @buffer.strip
    ensure
      $stderr = original_stderr
    end

    def supports_block_expectations?
      true
    end

    def failure_message_generator(to)
      %Q[expected #{to} get message:\n#{@expected.inspect}\nbut got:\n#{@buffer.inspect}]
    end

    def failure_message
      failure_message_generator 'to'
    end

    def failure_message_when_negated
      failure_message_generator 'not to'
    end

  end

  def has_warn(msg)
    HasWarn.new(msg)
  end
end

now you can use this function as follow after include the CustomMatchers:

expect{ MyClass.new }.to has_warn("warning messages")