The behavior of startswith in corner case is inconsistent between str and bytes in Python 3, str and unicode in Python 2, and between str in Python 2 and Python 3.

Python 3:
>>> ''.startswith('', 1, 0)
True
>>> b''.startswith(b'', 1, 0)
False

Python 2:
>>> ''.startswith('', 1, 0)
False
>>> u''.startswith(u'', 1, 0)
True

If define s1.startswith(s2, start, end) for non-negative indices and non-tuple s2 as an equivalent to the expression start + len(s2) <= end and s2[start: start + len(s2)] == s2 or to s1.find(s2, start, end) == start, "".startswith("", 1, 0) should be False.

The same issue exists for endswith. See bpo-24243 for more detailed discussion.

Proposed patch fixes str.startswith and str.endswith.

I think this can only be applied in a feature release (and I think it should be, because of the backward-compatibility-with-python2 issue). However, since this is potentially controversial, we need some more opinions.

I can’t imagine much code would rely on either old or new behaviour. If you only put it into a feature release, would you have to document it as a change in behaviour?

Just in the what's new porting section, I think. The fact that there "should" be very little to no code that relies on this is why I'd like to see it fixed. The fact that the report was a theoretical one, and not one that broke code, is why I think we should fix it only in 3.5.

Could you please help me with wording?

How about this for What’s New:

• The :meth:`str.startswith` and :meth:`str.endswith` methods no longer return ``True`` when finding the empty string and the indexes are completely out of range. See :issue:`24284`.

Perhaps that is good enough, but if you wanted to be more precise, I think the actual condition is if the *start* index is beyond the end of the string, or *end* is before *start*.

Thank you Martin.

New changeset a82498f424fe by Serhiy Storchaka in branch '3.5':
Issue bpo-24284: The startswith and endswith methods of the str class no longer
https://hg.python.org/cpython/rev/a82498f424fe

New changeset 04162e06323f by Serhiy Storchaka in branch 'default':
Issue bpo-24284: The startswith and endswith methods of the str class no longer
https://hg.python.org/cpython/rev/04162e06323f