Thank you for the bug report Ronan, but I'm afraid that I have no idea what you think the problematic behaviour is. I'm not going to spend the time installing the third-party hypothesis module, and learning how to use it, just to decipher your "actual spec". Where did this spec come from? The documentation is fairly sparse:

https://docs.python.org/3/library/stdtypes.html#str.startswith

so I'm not sure where your spec comes from. The title of this ticket is uninformative: what implementation details are being leaked? 

Saying "The problem is the complexity of the actual behaviour of these methods." explains nothing. Which actual behaviour? Please provide simple examples that contrast expected behaviour from actual behaviour, and justification for the expected behaviour.