You're right, it does not reproduces on any machine. I'm using Arch Linux, still trying to figure out the repro steps.

My hosts file doesn't contain such entries for localhost, it's pretty standard. If I disable my external network interface, it retrieves my second IP from virtual interface. And only when I disable all interfaces, I can get 127.0.0.1 on source in IP packet.

The problem is, even if I enforce client to bind to "127.0.0.1", it substitutes as well (see https://gist.github.com/rrader/3e575cde56827b1f74a1).

strace is clean, Python calls all kernel functions with 127.0.0.1 IP. So, I suppose, the problem not in the Python code itself, but somewhere deeper (however it can be lack of some flags?)