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Bind method may easily fail on Unix if there is no permission to bind to a privileged port:
>>> try: TCPServer(("", 80), ...)
... except Exception as err: err
... 
PermissionError(13, 'Permission denied')
>>> gc.collect()
__main__:1: ResourceWarning: unclosed <socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketType.SOCK_STREAM, proto=0, laddr=('0.0.0.0', 0)>
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This problem is inherited by HTTPServer and WSGIServer. My current workaround includes this code in a BaseServer fixup mixin, invoking server_close() if __init__() fails:
class Server(BaseServer, Context):
 def __init__(self, ...):
 try:
 super().__init__((host, port), RequestHandlerClass)
 except: # Workaround for socketserver.TCPServer leaking socket
 self.close()
 raise
 
 def close(self):
 return self.server_close()