Python 3.4.2 (default, Oct  8 2014, 10:45:20)
>>> u'..'.encode('idna')
Traceback (most recent call last):
  File "/usr/lib/python3.4/encodings/idna.py", line 165, in encode
    raise UnicodeError("label empty or too long")
UnicodeError: label empty or too long

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeError: encoding with 'idna' codec failed (UnicodeError: label empty or too long)

→ I was expecting that this raises either not at all or UnicodeEncodeError.

>>> b'..'.decode('idna')
'..'
→ Why doesn't this raise then, too?

The error message is also messed up which wasn't the case in python 2.7. It could be cleaned up.