datadog agent not reachable from inside docker container
You will need to set non_local_traffic: yes in your /etc/dd-agent/datadog.conf file. Otherwise the agent will reject metrics from containers.
After setting, you will need to restart the agent for the change to take effect: sudo /etc/init.d/datadog-agent restart or sudo service datadog-agent restart
The docker-dd-agent image enables non_local_traffic: yes by default.
If dd-agent listens on localhost it can receive data only from localhost (127.0.0.1). Try to change the dd-agent host to 0.0.0.0 instead of localhost.
We are using docker-dd-agent and it works OOTB.
You don't actually want to use the IP of the host in this case. If you're running the docker dd-agent, there are two environment variables you can tap into:
statsd.connect(DOGSTATSD_PORT_8125_UDP_ADDR, DOGSTATSD_PORT_8125_UDP_PORT)
That should do the trick. If not, you should be able to find the relevant info to your problem in this section of the Datadog docs.
Also, I should point out that the only Python library that Datadog shows in their docs is datadogpy.