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Here is a script which success with os.path.realpath(), but fails with pathlib.Path.resolve(). The `readlink -f` also success with these examples.

This is https://bitbucket.org/pitrou/pathlib/issue/9/pathresolve-fails-on-complex-symlinks
As noted there:
"""It's a ENAMETOOLONG error when calling readlink(), because there are many "." components. This is really an edge case caused by the specific test setup, I don't think you'd have that many "." components in real-world use."""

Path.resolve() also fails when last link is absolute.
mkdir testdir
ln -s 0/0 testdir/1
ln -s 1/1 testdir/2
ln -s "$(readlink -f testdir)" testdir/0
Path('testdir/2').resolve() fails:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/pathlib.py", line 1017, in resolve
 s = self._flavour.resolve(self)
 File "/home/serhiy/py/cpython/Lib/pathlib.py", line 273, in resolve
 raise RuntimeError("Symlink loop from %r" % cur)
RuntimeError: Symlink loop from '/home/serhiy/py/cpython/testdir/0'
Here is a patch which implements an algorithm similar to the algorithm used in posixpath.realpath().

Whoa, Serhiy, your patch won't fly on Windows Vista.
For starter, the patch does not use target_is_directory=True option in os.symlink. It needs that option when creating symlink to the directory on Windows Vista.
The maximum depth that pathlib can do on Windows Vista is 4 (Remember, this line: "for depth in 0, 1, 2, 3, 10, 100"?).
More than that:
OSError: [WinError 1921] The name of the file cannot be resolved by the system:
'C:\\Users\\vajrasky\\Code\\cpython\\@test_4220_tmp\\testdir\\link5'
Not sure whether the bug is in pathlib.py or _getfinalpathname (from nt module).

Thank you Vajrasky. Here is a patch with fixed tests.

It works. Just a coding nitpick, instead of hardcoding os.symlink(src, dst, target_is_directory=True) in the test, you can use helper function in the test itself, self.dirlink.

I would prefer if you made the test simpler. The current setup makes it difficult to understand and reproduce with the command line.

How the test can be simpler? It is already simple.
You ca use pathlib_resolve_test.py but replace `os.symlink('.', 'testdir/0')` by `os.symlink(os.path.abspath('testdir'), 'testdir/0')`. Or use following shell commands:
mkdir testdir
for i in $(seq 100); do ln -s $((i-1))/$((i-1)) testdir/$i; done
ln -s "$(pwd)/testdir" testdir/0
./python -c "import pathlib; print(pathlib.Path('testdir/100').resolve())"

> How the test can be simpler? It is already simple.
I mean, don't use loops but a simple test setup as in test_resolve_dot.
I am not interested in the pathologic "100 symlinks" case, just the case where the symlink is absolute.

And don't forget to use self.dirlink instead of os.symlink(src, dst, target_is_directory=True).

Here is a patch with unrolled loops.

> Here is a patch with unrolled loops.
I suppose that was some kind of joke, but what I meant was that we don't need to test with 100 levels of symlinks. 2 or 3 are enough...

Here is a patch with refactored tests. Vajrasky, do you want to test under Windows?

> I suppose that was some kind of joke, but what I meant was that we don't need to test with 100 levels of symlinks. 2 or 3 are enough...
Yes, sorry for this joke. Your tests LGTM, but why you repeat similar code 3-4 times instead using loops?

The patch passed on Windows Vista.

> why you repeat similar code 3-4 times instead using loops?
For no real reason :) I'll try with a loop.

Ah, I remember. Using subtests would make it more annoying to backport to 2.7.

New changeset 12a52186b4fd by Antoine Pitrou in branch 'default':
Issue #19887: Improve the Path.resolve() algorithm to support certain symlink chains.
http://hg.python.org/cpython/rev/12a52186b4fd 

Thanks a lot for the patch!

Hi,
This precise set of tests fails on Windows 7 on a NTFS partition (on revision c0b0e7aef360+ tip ), see below.
The problem is probably minor (drive letter case). I won't be able to develop a fix myself, but I'll be happy to test one.
cheers,
Philippe
======================================================================
FAIL: test_complex_symlinks_absolute (test.test_pathlib.PathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1724, in test_complex_symlinks_absolute
 self._check_complex_symlinks(BASE)
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
======================================================================
FAIL: test_complex_symlinks_relative (test.test_pathlib.PathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1728, in test_complex_symlinks_relative
 self._check_complex_symlinks('.')
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
======================================================================
FAIL: test_complex_symlinks_relative_dot_dot (test.test_pathlib.PathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1732, in test_complex_symlinks_relative_dot_dot
 self._check_complex_symlinks(os.path.join('dirA', '..'))
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
======================================================================
FAIL: test_complex_symlinks_absolute (test.test_pathlib.WindowsPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1724, in test_complex_symlinks_absolute
 self._check_complex_symlinks(BASE)
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
======================================================================
FAIL: test_complex_symlinks_relative (test.test_pathlib.WindowsPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1728, in test_complex_symlinks_relative
 self._check_complex_symlinks('.')
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
======================================================================
FAIL: test_complex_symlinks_relative_dot_dot (test.test_pathlib.WindowsPathTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1732, in test_complex_symlinks_relative_dot_dot
 self._check_complex_symlinks(os.path.join('dirA', '..'))
 File "c:\Users\Morgane\Documents000円\Dev\CPython\cpython\lib\test\test_pathlib.py", line 1692, in _check_complex_symlinks
 self.assertEqual(str(p), BASE)
AssertionError: 'C:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp' != 'c:\\Users\\Morgane\\Documents\000円\\Dev\\[53 chars]_tmp'
- C:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
+ c:\Users\Morgane\Documents000円\Dev\CPython\cpython\build\test_python_6060\@test_6060_tmp
? ^
----------------------------------------------------------------------
Ran 335 tests in 1.455s
FAILED (failures=6, skipped=93)
test test_pathlib failed
1 test failed:
 test_pathlib