Monkey Patching

Monkey patching means reopening a class that already exists and changing or extending it at runtime, without editing its original source. Ruby makes this natural because its classes are “open”: you can define the same class as many times as you like, and each new definition adds to what is already there, even for built-ins like String. The power is real, since you can teach an existing class a new trick in a few lines:

class String
  def shout
    upcase + "!"
  end
end
"hi".shout  # => "HI!"

So is the danger: the last patch to load wins, there is no clean way to reach the original method, and a surprise patch can break code somewhere far away. Some Rubyists call it “duck punching,” a name from a RailsConf 2007 talk where Patrick Ewing joked that if a duck is not making the noise you want, “you’ve got to just punch that duck until it returns what you expect”.

Related: Everything is an Object · Duck Typing