Redirecting STDOUT

In writing spec in Ruby, you would often meet a situation that you have to reduce outputting messages to STDOUT.

def f
  p 123
end

describe 'f' do
  it 'outputs "123\n" to stdout' do
    ...
  end
end

How do you write it? Usually I assign a stringio object to $stdout. I guess many people use this approach.

def f
  p 123
end

# spec
require 'stringio'
results = $stdout = StringIO.new
f()
results.close_write
results.rewind
$stdout = STDOUT
p results.read #=> "123\n"

I started having an idea that IO#reopen may be a better solution of it. The code previously given looks obviously dirty.

Finally I figured out the new approach was also dirty.

def f
  p 123
end

require 'tempfile'
results = Tempfile.new('a').path
a = STDOUT.dup
STDOUT.reopen(results, 'w')
f()
STDOUT.reopen(a)
p File.read(results)

STDOUT.reopen doesn't accept an IO object, but only accepts a Filename.

capture_io of minitest

Eric Hodel and Ryan Davis mentioned about capture_io in minitest. It uses the former approache.

def capture_io
  require 'stringio'

  orig_stdout, orig_stderr         = $stdout, $stderr
  captured_stdout, captured_stderr = StringIO.new, StringIO.new
  $stdout, $stderr                 = captured_stdout, captured_stderr

  yield

  return captured_stdout.string, captured_stderr.string
ensure
  $stdout = orig_stdout
  $stderr = orig_stderr
end

Reference

http://www.ruby-forum.com/topic/187367

Eric Hodel++