Can I stub STDERR in RSpec?

Question

I have a simple function that I'd like to test (perhaps mostly to appease simplecov). The function is:

module Utils
  extend self

  def blather(msg)
    msg = "=== " + msg
    STDERR.puts(msg)
    Rails.logger.debug(msg)
  end

end

The RSpec documentation for stubbing says that:

Messages can be stubbed on any class, including those in Ruby's core library.

But the following:

# file: spec/lib/utils_spec.rb
require 'spec_helper'
describe Utils do
  context "blather" do
    it "should print to STDERR" do
      STDERR.any_instance.should_receive(:puts).with("=== zoo")    
      Utils.blather("zoo")
    end
  end
end

... I get an error

undefined method `any_instance' for #<IO:<STDERR>>

Putting aside questions as to whether this test makes any sense, is it possible to stub STDERR (the IO class)? Is this failing because it's a class method? Or is there a more sensible strategy for this kind of test?

Answer 1

Firstly, you should typically use $stderr rather than STDERR.

module Utils
  extend self

  def blather(msg)
    msg = "=== " + msg
    $stderr.puts(msg)
    Rails.logger.debug(msg)
  end

end

To answer your question, you can do the following in RSpec:

describe Utils do
  context "blather" do
    it "should print to stderr" do
      $stderr.should_receive(:puts).with("=== zoo")
      Utils.blather("zoo")
    end
  end
end

You just stub the method via $stderr.should_receive. Because $stderr is a normal object, you can stub methods on it like normal as well as set up expectations on it.