Avoid string copy when split char doesn't match

[smontanaro]

BPO	1538
Nosy	@gvanrossum, @smontanaro
Files	string-split.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 = None
closed_at = <Date 2007-12-08.15:34:09.129>
created_at = <Date 2007-12-02.07:51:02.919>
labels = ['interpreter-core', 'type-feature']
title = "Avoid string copy when split char doesn't match"
updated_at = <Date 2007-12-08.15:34:09.127>
user = 'https://github.com/smontanaro'

bugs.python.org fields:

activity = <Date 2007-12-08.15:34:09.127>
actor = 'skip.montanaro'
assignee = 'none'
closed = True
closed_date = <Date 2007-12-08.15:34:09.129>
closer = 'skip.montanaro'
components = ['Interpreter Core']
creation = <Date 2007-12-02.07:51:02.919>
creator = 'skip.montanaro'
dependencies = []
files = ['8864']
hgrepos = []
issue_num = 1538
keywords = ['patch']
message_count = 4.0
messages = ['58081', '58145', '58169', '58295']
nosy_count = 2.0
nosy_names = ['gvanrossum', 'skip.montanaro']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue1538'
versions = ['Python 2.6']

[smontanaro]

The topic of avoiding string copies in certain string methods came up in
the
ChiPy list:

http://mail.python.org/pipermail/chicago/2007-December/002975.html.

The attached patch modifies the split and rsplit implementations to
avoid
making a copy of self when the split fails to find anything to split on:

    >>> s = "abc def"
    >>> x = s.split(';')
    >>> x[0] is s
    True
    >>> y = s.rsplit('-')
    >>> y[0] is s
    True
    >>> t = "abcdef"
    >>> x = t.split()
    >>> x[0] is t
    True
    >>> y = t.rsplit()
    >>> y[0] is t
    True

All tests pass. Given that this is just a small optimization I
don't believe any changes to the docs or the existing tests are
necessary.

[gvanrossum]

I think this is not quite right; you shouldn't return self unless it is
an exact string instance. If it is a subclass of PyString should make a
copy.

[smontanaro]

I'm not sure why a string subclass shouldn't work, but I've attached a new
version of the patch that calls PyString_CheckExact() to prevent using a
string subclass.

[smontanaro]

In the absence of any more feedback, I checked this in as r59420.