Update os.path for PEP 519/__fspath__()

[brettcannon]

BPO	27524
Nosy	@brettcannon, @ned-deily, @ethanfurman
Dependencies	bpo-26027: Support Path objects in the posix module bpo-27184: Support path objects in the ntpath module
Files	fspath_tests.diff os_path.diff: Tests and fixes not involving stat or lstat

^{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/brettcannon'
closed_at = <Date 2016-08-26.21:46:15.496>
created_at = <Date 2016-07-15.21:06:32.888>
labels = ['type-bug', 'library', 'release-blocker']
title = 'Update os.path for PEP 519/__fspath__()'
updated_at = <Date 2016-09-06.22:59:12.300>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2016-09-06.22:59:12.300>
actor = 'python-dev'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2016-08-26.21:46:15.496>
closer = 'brett.cannon'
components = ['Library (Lib)']
creation = <Date 2016-07-15.21:06:32.888>
creator = 'brett.cannon'
dependencies = ['26027', '27184']
files = ['43740', '43941']
hgrepos = []
issue_num = 27524
keywords = ['patch']
message_count = 6.0
messages = ['270513', '270519', '271645', '271654', '273733', '274655']
nosy_count = 4.0
nosy_names = ['brett.cannon', 'ned.deily', 'ethan.furman', 'python-dev']
pr_nums = []
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27524'
versions = ['Python 3.6']

[brettcannon]

As per PEP-519, os.path needs to be updated to support __fspath__().

[brettcannon]

Here are tests for genericpath, posixpath, and ntpath (which should in the end cover all of os.path).

[brettcannon]

Ran into the first nasty snag: people are abusing os.path.commonprefix() and it's extremely generic approach. Instead of passing in a list of file paths they are passing a list of lists of file path *parts*, e.g. instead of ["a/b", "a/c"] they are passing in [["a", "b"], ["a", "c"]]. I have special-cased when the list contains another list or tuple, but I still feel dirty doing it.

[brettcannon]

Here is a patch that includes the previous tests as long with fixes which don't involve stat or lstat (once those functions are updated themselves then os.path will be ported).

[python-dev]

New changeset b64f83d6ff24 by Brett Cannon in branch 'default':
Issue bpo-26027, bpo-27524: Add PEP 519/fspath() support to os and
https://hg.python.org/cpython/rev/b64f83d6ff24

[python-dev]

New changeset 9be0286772bf by Brett Cannon in branch 'default':
Issue bpo-26027, bpo-27524: Document the support for path-like objects in os and os.path.
https://hg.python.org/cpython/rev/9be0286772bf