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This is what major version numbers are for.

Or issue an actual runtime warning: http://docs.python.org/2/library/warnings.html

That's why you should avoid using default arguments in library functions.

Great point! I have never heard that, but it makes perfect sense. Hopefully I will remember in the future.

I don't agree, you should just know what they are. Defaults are helpful.

Until they change, and your code breaks, and you have to go figure out why. The point of not (silently) using defaults is that as long as you're explicit with your arguments, you won't get surprised by changing defaults. Of course, you might get surprised by any number of other changes, but you can at least reduce your failure surface area a little bit :-)