Testing STDOUT output in Rspec

Question

I am trying to build a spec for this statement. It is easy with \'puts\'

print \"\'#{@file}\' doesn\'t exist: Create Empty File (y/n)?\"

Answer 1

RSpec 3.0+

RSpec 3.0 added a new output matcher for this purpose:

expect { my_method }.to output("my message").to_stdout
expect { my_method }.to output("my error").to_stderr

Minitest

Minitest also has something called capture_io:

out, err = capture_io do
  my_method
end

assert_equals "my message", out
assert_equals "my error", err

RSpec < 3.0 (and others)

For RSpec < 3.0 and other frameworks, you can use the following helper. This will allow you to capture whatever is sent to stdout and stderr, respectively:

require 'stringio'

def capture_stdout(&blk)
  old = $stdout
  $stdout = fake = StringIO.new
  blk.call
  fake.string
ensure
  $stdout = old
end

def capture_stderr(&blk)
  old = $stderr
  $stderr = fake = StringIO.new
  blk.call
  fake.string
ensure
  $stderr = old
end

Now, when you have a method that should print something to the console

def my_method
  # ...
  print "my message"
end

you can write a spec like this:

it 'should print "my message"' do
  printed = capture_stdout do
    my_method # do your actual method call
  end

  printed.should eq("my message")
end

Answer 2

If your goal is only to be able to test this method, I would do it like this:

class Executable
  def initialize(outstream, instream, file)
    @outstream, @instream, @file = outstream, instream, file
  end

  def prompt_create_file
    @outstream.print "'#{@file}' doesn't exist: Create Empty File (y/n)?"
  end
end


# when executing for real, you would do something like
# Executable.new $stdout, $stdin, ARGV[0]

# when testing, you would do
describe 'Executable' do
  before { @input = '' }
  let(:instream)   { StringIO.new @input }
  let(:outstream)  { StringIO.new }
  let(:filename)   { File.expand_path '../testfile', __FILE__ }
  let(:executable) { Executable.new outstream, instream, filename }

  specify 'prompt_create_file prompts the user to create a new file' do
    executable.prompt_create_file
    outstream.string.should include "Create Empty File (y/n)"
  end
end

However, I want to point out that I would not test a method like this directly. Instead, I'd test the code that uses it. I was talking with a potential apprentice yesterday, and he was doing something very similar, so I sat down with him, and we reimplemented a portion of the class, you can see that here.

I also have a blog that talks about this kind of thing.