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If the hosts file contains invalid unicode, then the socket module will break when attempting to get the list of hosts. This renders modules such as Flask and Django unusable.
Background:
 I had a mapping to localghost (https://twitter.com/rfreebern/status/1214560971185778693), and Docker incorrectly rewrote the hosts file during installation, turning it from "127.0.0.1 xn--9q8h" to "127.0.0.1 xn-9q8h".
 The socket module was not able to handle that, and was failing with a UnicodeDecodeError in Python 3.6+ as it attempted to list the addresses for "127.0.0.1".
How to reproduce:
 # Add "127.0.0.1 xn-9q8h" to C:/Windows/System32/drivers/etc/hosts
 
 >>> socket.gethostbyaddr('127.0.0.1')
 UnicodeDecodeError: 'utf-8' codec can't decode byte 0x96 in position 2: invalid start byte
Expected:
 In Python 2.7 - 3.5, it returns 'xn\u20139q8h'.
 I feel returning "xn-9q8h" as a string would be best, but ignoring it could be an option.
 An alternative would be to raise socket.error.
Full traceback from Flask as an example:
 Traceback (most recent call last):
 File "__init__.py", line 285, in <module>
 app.run()
 File "C:\Users\Peter\AppData\Roaming\Python\Python37\site-packages\flask\app.py", line 990, in run
 run_simple(host, port, self, **options)
 File "C:\Users\Peter\AppData\Roaming\Python\Python37\site-packages\werkzeug\serving.py", line 1052, in run_simple
 inner()
 File "C:\Users\Peter\AppData\Roaming\Python\Python37\site-packages\werkzeug\serving.py", line 1005, in inner
 fd=fd,
 File "C:\Users\Peter\AppData\Roaming\Python\Python37\site-packages\werkzeug\serving.py", line 848, in make_server
 host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
 File "C:\Users\Peter\AppData\Roaming\Python\Python37\site-packages\werkzeug\serving.py", line 740, in __init__
 HTTPServer.__init__(self, server_address, handler)
 File "C:\Program Files\Python37\lib\socketserver.py", line 452, in __init__
 self.server_bind()
 File "C:\Program Files\Python37\lib\http\server.py", line 139, in server_bind
 self.server_name = socket.getfqdn(host)
 File "C:\Program Files\Python37\lib\socket.py", line 676, in getfqdn
 hostname, aliases, ipaddrs = gethostbyaddr(name)
 UnicodeDecodeError: 'utf-8' codec can't decode byte 0x96 in position 2: invalid start byte

I cannot reproduce the issue on Linux:
# echo "127.0.0.2 xn-9q8h" >> /etc/hosts
# python3.8
>>> import socket
>>> socket.gethostbyaddr("127.0.0.2")
('xn-9q8h', [], ['127.0.0.2'])

Ah, I just realised it may have been a different dash to the one that can be typed with the keyboard.
From the wiki article (https://en.wikipedia.org/wiki/Dash), using either the "en" or "em" dash will cause the issue for me on Windows.

This is covered by issue26227