I don't have Python 3.7 available to me, but in 3.5 the behaviour of u.startswith(v) with an empty v seems consistent to me:


py> "alpha".startswith("", 20, 30)
True
py> "alpha"[20:30].startswith("")
True

py> "".startswith("", 20, 30)
True
py> ""[20:30].startswith("")
True

So I can't see any inconsistency that might be fixed by always returning True in the case v="", as that appears to already be the case.