[WJW]

Note how I said compiled languages, I expect Python to have some very similar capabilities. :) But as an example, take the cleanness of the following mock:

  allow_any_instance_of(Set).to receive(:member?).and_return(true)
  <rest of test code>

Contrast this for example with testing IO monad code in Haskell, where the best practice is to define a new monad 'underneath' your original monad stack, rewrite all your functions signatures to have a type constraint (of your new MonadIO type) and then switch out functions for their mocked equivalent as required by the test. Rspec can do the same switching out on any object or class, without any code required outside the test code. (Although as far as "handle with care" goes, monkeypatching the 'send' method has to score pretty high)