Inconsistency in startswith/endswith

[serhiy-storchaka]

BPO	24284
Nosy	@rhettinger, @bitdancer, @vadmium, @serhiy-storchaka
Files	str_tailmatch.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2015-05-31.06:17:31.577>
created_at = <Date 2015-05-25.11:41:27.565>
labels = ['interpreter-core', 'type-bug']
title = 'Inconsistency in startswith/endswith'
updated_at = <Date 2015-05-31.06:17:31.577>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-05-31.06:17:31.577>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-05-31.06:17:31.577>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2015-05-25.11:41:27.565>
creator = 'serhiy.storchaka'
dependencies = []
files = ['39492']
hgrepos = []
issue_num = 24284
keywords = ['patch']
message_count = 8.0
messages = ['244027', '244036', '244068', '244106', '244124', '244139', '244520', '244521']
nosy_count = 5.0
nosy_names = ['rhettinger', 'r.david.murray', 'python-dev', 'martin.panter', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24284'
versions = ['Python 3.5', 'Python 3.6']

[serhiy-storchaka]

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.

[bitdancer]

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.

[vadmium]

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?

[bitdancer]

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.

[serhiy-storchaka]

Could you please help me with wording?

[vadmium]

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*.

[serhiy-storchaka]

Thank you Martin.

[python-dev]

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