"getaddrinfo() argument 1 must be string or None" error starting dev_appserver.py on Linux

I'm getting re-started with App Engine after not having used it in a while. I'm using the 64-bit Linux Go runtime, version 1.8.1.

I believe I'm following the steps from the documentation correctly, and I believe I'm doing what's worked correctly in the past, but I'm getting this error when attempting to launch dev_appserver.py:

<pre>
$ dev_appserver.py .
INFO 2013-07-11 07:24:45,919 sdk_update_checker.py:244] Checking for updates to the SDK.
INFO 2013-07-11 07:24:46,230 sdk_update_checker.py:288] This SDK release is newer than the advertised release.
WARNING 2013-07-11 07:24:46,443 simple_search_stub.py:955] Could not read search indexes from /tmp/appengine.batterybotinfo.darshan/search_indexes
Traceback (most recent call last):
File "/home/darshan/bin/dev_appserver.py", line 182, in <module>
_run_file(file, globals())
File "/home/darshan/bin/dev_appserver.py", line 178, in run_file
execfile(script_path, globals)
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 695, in <module>
main()
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 688, in main
dev_server.start(options)
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/devappserver2.py", line 659, in start
apis.start()
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/api_server.py", line 137, in start
super(APIServer, self).start()
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/wsgi_server.py", line 295, in start
if self._start_all_dynamic_port(host_ports):
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/wsgi_server.py", line 348, in _start_all_dynamic_port
server.start()
File "/home/darshan/software/google_appengine/google/appengine/tools/devappserver2/wsgi_server.py", line 194, in start
socket.SOCK_STREAM, 0, socket.AI_PASSIVE)
TypeError: getaddrinfo() argument 1 must be string or None
</pre>

My first thought was that I might be using an incorrect version of Python. Sure enough, I'm using 2.7.5, and the documentation clearly states that 2.5 is necessary. However, the documentation seems to be outdated, because after installing 2.5 and setting my system to use it, I got this error:

<pre>
Error: Python 2.5 is not supported. Please use version 2.7.
</pre>

Okay, so back to 2.7.5 and my initial error.

I'm not sure if this is a bug in the dev_appserver.py Python code (I'm guessing not, as it's been out for a month), an issue with my Python installation, or something else about my system that isn't configured according to Google's expectations.

I'd rather not mess with the dev_appserver.py code unless necessary, but I'm happy to poke at it to help figure out what's going wrong. The error is on line 194; here are lines 190-195:

<!-- language: lang-python -->

# AF_INET or AF_INET6 socket
# Get the correct address family for our host (allows IPv6 addresses)
host, port = self.bind_addr
try:
  info = socket.getaddrinfo(host, port, socket.AF_UNSPEC,
                            socket.SOCK_STREAM, 0, socket.AI_PASSIVE)

I've determined that the containing method is called twice. The first time host is always "127.0.0.1" and port is 0. The second time is the one that crashes; host is always 10 (an int, not a string), and port is a seemingly-random five-digit int.

I've tried hard-coding host to "127.0.0.1" and port to either 8080 or 0, but then I get another error. I feel in over my head, and I suspect I'm not going to resolve the real issue by changing things I don't really understand. Googling for the error message hasn't helped.

Persistent Googling eventually paid off. Despite this question having a very different (and much more informative) error message, the solution turned out to be the same: ensure that /etc/hosts contains only a single entry for localhost.

Notably, my system contained both of these lines:

<!-- language: lang-conf -->

127.0.0.1  localhost
::1        localhost

Commenting out the second (and adding a comment to document why) solved my issue:

<!-- language: lang-conf -->

127.0.0.1  localhost

# Having multiple localhost entries causes App Enginge dev_appserver.py to fail.
#  IPv6 not currently needed, and the dev server IS needed, so commenting out.
#::1        localhost

dev_appserver.py now starts and works properly.

If you edit lines 189-194 to this, it should work until Google releases an update. This is basically just checking the type of host and returning early if it isn't a string.

189    # AF_INET or AF_INET6 socket
190    # Get the correct address family for our host (allows IPv6 addresses)
191     host, port = self.bind_addr
192     try:
193       if type(host) is not str:
194         return
195       info = socket.getaddrinfo(host, port, socket.AF_UNSPEC,
196                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE)