Ok, I fixed the bug in Python 2.7, 3.4, 3.5 and 3.6. (Python 2.7 was also impacted for custom *unicode* strings.)

Thanks for your bug report paul!

> ISTM this is a case where Python's core shouldn't be using assert. It's possible for userland code to trigger an assertion failure, which means it should be a regular if(..) raise.

Right, this check is implemented in PyUnicode_AsEncodedString(). Moreover, PyUnicode_AsEncodedString() calls directly the codec, it doesn't call the encode() method of the input string.

(Sorry, I wrote PyUnicode_AsEncodedObject() which has a different purpose.)

> @haypo, what do you mean by "fuzzing"?

This: https://en.wikipedia.org/wiki/Fuzz_testing