Without speaking to your specific situation, because I know neither you nor your coworker nor the full details of what happened...
I think every Python dev I've met (myself included) has a funny/embarrassing story about default initialization in Python. It's a behavior which exists the way it does for a good reason, but nonetheless goes against most people's intuitions. So, we tend to get bitten by it at least once, and then we remember our embarrassment and avoid it going forward.