Either use Atoms package manager or
apm install autocomplete-plus-jedi
APJ - let us just call it APJ, that's way shorter - uses the Python interpreter in your path. If you want to use it with a virtualenv just make sure the env is active and start atom from the command line.
Right now the completion daemon is started on port 7777 - please make sure no other service is using this port. In the next release the port will be configurable.
This addon is young and not feature complete. But it is under active development, so eventually things will improve.
2015-03-10 Timo Zimmermann firstname.lastname@example.org
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.