aiopg: Using "localhost" as host rather than "127.0.0.1 blocks connections

Using the example code from the documentation:

import asyncio
import aiopg

dsn = 'dbname=aiopg user=aiopg password=passwd host=localhost'

async def go():
    async with aiopg.create_pool(dsn) as pool:
        async with pool.acquire() as conn:
            async with conn.cursor() as cur:
                await cur.execute("SELECT 1")
                ret = []
                async for row in cur:
                    ret.append(row)
                assert ret == [(1,)]

loop = asyncio.get_event_loop()
loop.run_until_complete(go())

Results in this:

$ time python pgtest.py 
Traceback (most recent call last):
  File "pgtest.py", line 17, in <module>
    loop.run_until_complete(go())
  File "/usr/lib/python3.5/asyncio/base_events.py", line 466, in run_until_complete
    return future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 293, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 241, in _step
    result = coro.throw(exc)
  File "pgtest.py", line 7, in go
    async with aiopg.create_pool(dsn) as pool:
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/utils.py", line 77, in __aenter__
    self._obj = yield from self._coro
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/pool.py", line 46, in _create_pool
    yield from pool._fill_free_pool(False)
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/pool.py", line 203, in _fill_free_pool
    **self._conn_kwargs)
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/utils.py", line 67, in __iter__
    resp = yield from self._coro
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/connection.py", line 74, in _connect
    yield from conn._poll(waiter, timeout)
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/connection.py", line 239, in _poll
    yield from asyncio.shield(cancel(), loop=self._loop)
  File "/usr/lib/python3.5/asyncio/futures.py", line 380, in __iter__
    yield self  # This tells Task to wait for completion.
  File "/usr/lib/python3.5/asyncio/tasks.py", line 304, in _wakeup
    future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 293, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/home/mikaelk/venvs/csc-backend/lib/python3.5/site-packages/aiopg/connection.py", line 225, in cancel
    self._conn.cancel()
psycopg2.OperationalError: asynchronous connection attempt underway

real	1m0.133s
user	0m0.104s
sys	0m0.012s

If I change it to “127.0.0.1” it works perfectly, however.