This habit of monkeypatching core functions was what drove me away from Ruby. Even adorable little trapdoors like the map() example can lead to issues with performance, stability, documentation, or (ahem) unit tests that now have to reflect this new behavior but only in certain circumstances.

I understand the coolness of it. All I object to is this "__builtin__.old_map = map" fuckery. It's what people mean when they say "too clever, by half".