Passthrough for things that appear as hostnames (like IP addresses)

[wumpus]

Thanks for this code, it's nice to see support for modern urls in Python! The built-in codec raises for any hostname containing ß, complaining

UnicodeError: ('IDNA does not round-trip', b'xn--einla-pqa', b'einlass')

idna 2008 fixes this stupidity.

However, when you're dealing with general-purpose urls you end up with hostnames like '::1', which is localhost for ipv6. This means that your module isn't so easy to drop in as a replacement. I either need to wrap it or stick try / except idna.core.InvalidCodepoint in my code, which looks a bit odd. I think it might be prettier if idna encode/decode grew options to pass-thru in this case?

>>> u='::1'
>>> u.encode('idna')
b'::1'
>>> import idna
>>> idna.encode(u)
Traceback (most recent call last):
  File "/home/lindahl/.pyenv/versions/3.6.4/lib/python3.6/site-packages/idna/core.py", line 263, in alabel
    ulabel(label)
  File "/home/lindahl/.pyenv/versions/3.6.4/lib/python3.6/site-packages/idna/core.py", line 299, in ulabel
    check_label(label)
  File "/home/lindahl/.pyenv/versions/3.6.4/lib/python3.6/site-packages/idna/core.py", line 253, in check_label
    raise InvalidCodepoint('Codepoint {0} at position {1} of {2} not allowed'.format(_unot(cp_value), pos+1, repr(label)))
idna.core.InvalidCodepoint: Codepoint U+003A at position 1 of '::1' not allowed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/lindahl/.pyenv/versions/3.6.4/lib/python3.6/site-packages/idna/core.py", line 355, in encode
    result.append(alabel(label))
  File "/home/lindahl/.pyenv/versions/3.6.4/lib/python3.6/site-packages/idna/core.py", line 265, in alabel
    raise IDNAError('The label {0} is not a valid A-label'.format(label))
idna.core.IDNAError: The label b'::1' is not a valid A-label

[wumpus]

Python 3.7 will fix the built-in codec to work.

[JanZerebecki]

I stumbled upon this because of urllib3/urllib3#1269