You should strive to not rely on type-checking, and trust and rely more on duck-typing. If at some point you need to ensure something is not None, test it and act accordingly in the alternate case. If something absolutely must not be None (or must have a specific method), ever, use assert() so that things blow up upfront where you can infer (or even read) the reason, and not deeper in the code where the actual error message, as you mention, has to be parsed according to the whole call stack to make sense of it.

Sometimes you may need to type-dispatch when you want to do smart functions. A typical example is a function that takes a sequence/iterable/generator of strings or a string. Since a string is iterable in python, you have no choice but to check for BaseString.