[IshKebab]

Most Python users don't do any form of static analysis though, so both changes would only be found at runtime.

But I think changing the behaviour and not the name is still a very bad idea.

[kevincox]

Yes, but "Unknown function datetime.utcnow" is much easier to diagnose than somewhere in your application, possibly inside a third-party library or maybe even in a different service altogether (because the change traversed serialization and deserilialization) throwing an exception because you can't compare two datetimes.