I remember when FePy was first coming out, and Jim Hugunin was encountering considerable slowness in regard to exceptions, and he conferred with Guido about it

I remember when FePy was first coming out, and Jim Hugunin was encountering considerable slowness in regard to exceptions, and he conferred with Guido about it because the fail-first idiom is a preferred python strategy. Anyways, Guido was like, you are functionally equivalent so it doesn't matter. (I'm just regurgitating my rough memory of Jim's blog about it.) But that was also the first time I learned how disruptive an exception was to .Net program flow. I've also seen it in general with other .Net languages -- exceptions kill performance, and defensive programming is very worth the extra effort.