ipaddress module has no support for scoped IPv6 addresses which prevents the use of ipaddress.ip_address() and ipaddress.IPv6Address() with (always available by default on IPv6 systems) RFC conforming IPv6 link local addresses that specify interface scope.

https://tools.ietf.org/html/rfc4007

This is bad because interface scope is required for connect() and bind() operations on multihomed machines, and virtualized or software defined networking will make this case very common.

eg.

>>> ipaddress.IPv6Address('fe80::dead:dead:beef:ffff')
IPv6Address('fe80::dead:dead:beef:ffff')
>>> ipaddress.IPv6Address('fe80::dead:dead:beef:ffff%eth0')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ipaddress.py", line 1900, in __init__
    self._ip = self._ip_int_from_string(addr_str)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ipaddress.py", line 1716, in _ip_int_from_string
    raise AddressValueError("%s in %r" % (exc, ip_str)) from None
ipaddress.AddressValueError: Only hex digits permitted in 'ffff%eth0' in 'fe80::dead:dead:beef:ffff%eth0'
>>> ipaddress.IPv6Interface('fe80::dead:dead:beef:ffff%eth0')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ipaddress.py", line 2060, in __init__
    IPv6Address.__init__(self, addr[0])
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ipaddress.py", line 1900, in __init__
    self._ip = self._ip_int_from_string(addr_str)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/ipaddress.py", line 1716, in _ip_int_from_string
    raise AddressValueError("%s in %r" % (exc, ip_str)) from None
ipaddress.AddressValueError: Only hex digits permitted in 'ffff%eth0' in 'fe80::dead:dead:beef:ffff%eth0'