My view is that exceptions are for exceptional code, not for simple logical
errors. As such anything that can be prevented by simple checks in your code
should not be caught but rather handled with checks(in the case the checks are
cheaper and not complex). KeyError is different than IndexError for the
complexity reason mentioned, to check if an index is valid it's a simple `a in
typeof(arr).low .. typeof(arr).high` whereas a checking if a key is in a table
is a hash of the type, which can be relatively expensive. Then there are also
procedures that let you avoid a `KeyError`. Defects inform the programmer they
have a logic error that can be prevented easily and cheaply, exceptions inform
the programmer there is something out of their control causing an error.
