guard clauses: good example

is_platypus.py

def is_platypus(self):
    # Not a platypus for everything below
    if not self.is_mammal():
        return False
    if not self.has_fur():
        return False
    if not self.has_beak():
        return False
    if not self.has_tail():
        return False
    if not self.can_swim():
        return False
    # Finally, it's a platypus!
    return True

I believe you've forgotten the not keyword in all but the first if statement

@AcideUK Thank you for the keen eye on this, fixed the code :)