Documentation claims that PySequence_Fast returns a tuple, when it actually returns a list.

[sfllaw]

BPO	16395
Nosy	@rhettinger, @larryhastings, @merwok, @serhiy-storchaka, @MojoVampire
Files	16395.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 2014-04-08.14:49:17.729>
created_at = <Date 2012-11-03.15:18:21.787>
labels = ['type-bug', 'docs']
title = 'Documentation claims that PySequence_Fast returns a tuple, when it actually returns a list.'
updated_at = <Date 2014-04-08.14:49:17.728>
user = 'https://bugs.python.org/sfllaw'

bugs.python.org fields:

activity = <Date 2014-04-08.14:49:17.728>
actor = 'python-dev'
assignee = 'docs@python'
closed = True
closed_date = <Date 2014-04-08.14:49:17.729>
closer = 'python-dev'
components = ['Documentation']
creation = <Date 2012-11-03.15:18:21.787>
creator = 'sfllaw'
dependencies = []
files = ['27857']
hgrepos = []
issue_num = 16395
keywords = ['patch']
message_count = 8.0
messages = ['174633', '174636', '174637', '174654', '215728', '215729', '215761', '215763']
nosy_count = 8.0
nosy_names = ['rhettinger', 'larry', 'sfllaw', 'eric.araujo', 'docs@python', 'python-dev', 'serhiy.storchaka', 'josh.r']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue16395'
versions = ['Python 2.7', 'Python 3.2']

[sfllaw]

The documentation in Python 2.7, 3.2, and 3.3 claim that:

PyObject* PySequence_Fast(PyObject *o, const char *m)
Return value: New reference.
Returns the sequence o as a tuple, unless it is already a tuple or list, in which case o is returned...

Unfortunately, the code does this in Objects/abstract.c:

    v = PySequence_List(it);

And the header file in Include/abstract.h matches the documentation:

     PyAPI_FUNC(PyObject *) PySequence_Fast(PyObject *o, const char* m);
       /*
     Returns the sequence, o, as a tuple, unless it's already a
     tuple or list.
       */

[sfllaw]

It looks like this was caught in the 3.3 branch, but only fixed it in the comment:

changeset: 75448:d8f68195210e
user: Larry Hastings <larry@hastings.org>
date: Mon Mar 05 22:59:13 2012 -0800
summary: Fix a comment: PySequence_Fast() creates a list, not a tuple.

The included patch applies cleanly to Python 2.7 and 3.2. When applying to 3.3, include the failure in Objects/abstract.c because the same change has already been made.

[merwok]

Larry, any objection to backporting this?

[serhiy-storchaka]

PySequence_Tuple() was changed to PySequence_List() in the changeset 4714aff4bf75 in 2004. Are there any measurements which show PySequence_List() faster than PySequence_Tuple() on modern Python? I don't see any references in 4714aff4bf75 description.

[MojoVampire]

Any news on this? I was about to open a bug of my own for this, since the docs and code are still out of sync.

[MojoVampire]

As far as performance goes, presumably the length hinting API reduces the number of cases in which we're working with completely unsized iterables, right?

[larryhastings]

I have no objections to someone backporting this.

[python-dev]

New changeset b2187b82a658 by Benjamin Peterson in branch '3.4':
PySequence_Fast generally returns a list not a tuple (closes bpo-16395)
http://hg.python.org/cpython/rev/b2187b82a658

New changeset b235db467cd5 by Benjamin Peterson in branch '2.7':
PySequence_Fast generally returns a list not a tuple (closes bpo-16395)
http://hg.python.org/cpython/rev/b235db467cd5

New changeset c833c35aa13a by Benjamin Peterson in branch 'default':
merge 3.4 (bpo-16395)
http://hg.python.org/cpython/rev/c833c35aa13a