Letting AsyncIO choose a port for you

Say you are writing a test case for an AsyncIO-based network function. You want to write a test server and have the code being tested connect to it. You can choose a port number and hope it’s not taken when the test is run, or you can have a free port chosen for you each time the test is run. Simply pass 0 as the local port number:

{% highlight python %}
transport, protocol = await loop.create_datagram_endpoint(
    ServerProto, local_addr=('127.0.0.1', 0))
{% endhighlight %}

Afterwards, you can query the transport object for the port number chosen for you:

{% highlight python %}
server_addr, server_port = transport._sock.getsockname()
{% endhighlight %}

And that’s it; now you know the port number. I’m not sure if using a member variable with an underscore at the start of its name is the best or the official way to do this but it’s the best method I’ve found so far.