Why does Python pathlib relative_to allow multiple input paths?

user_534298 0
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Why is *other used here. What does it mean if multiple paths are passed in?

PurePath.relative_to(*other)

https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.relative_to

It seems only the last one matters

In [1]: p = Path('/etc/pass')
[PYFLYBY] from pathlib import Path

In [2]: p.relative_to('arsta', '/etc', '/etc')
Out[2]: PosixPath('pass')

In [3]: p.relative_to('arsta', '/etc', '/etc/')
Out[3]: PosixPath('pass')

In [4]: p.relative_to('arsta', '/etc', '/etc/arstar')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-1669e7092659> in <module>
----> 1 p.relative_to('arsta', '/etc', '/etc/arstar')

/opt/schrodinger/suites2022-1/internal/lib/python3.8/pathlib.py in relative_to(self, *other)
    906         if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
    907             formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
--> 908             raise ValueError("{!r} does not start with {!r}"
    909                              .format(str(self), str(formatted)))
    910         return self._from_parsed_parts('', root if n == 1 else '',

ValueError: '/etc/pass' does not start with '/etc/arstar'

Additional test cases

In [8]: p.relative_to('/etc', 'pass')
Out[8]: PosixPath('.')

In [9]: p.relative_to('axx', '/etc', 'pass')
Out[9]: PosixPath('.')

In [10]: p.relative_to('/axx', '/etc', 'pass')
Out[10]: PosixPath('.')

Maybe the intended usage is ['/etc', 'pass']? How would one transform back and forth between this valid list form and /etc/pass?

pathlib python-3.x python
2 Answers

The arguments to relative_to are concatenate in order to form a complete path (if they are relative). That is, this:

>>> path = Path('/usr/bin/mkdir')
>>> path.relative_to('/usr/bin')
PosixPath('mkdir')

Is the same as:

>>> path = Path('/usr/bin/mkdir')
>>> p.relative_to('/usr', 'bin')
PosixPath('mkdir')

If one the arguments is an absolute path, that masks any preceding path:

>>> path = Path('/usr/bin/mkdir')
>>> path.relative_to('/usr', 'bin', '/etc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.10/pathlib.py", line 816, in relative_to
    raise ValueError("{!r} is not in the subpath of {!r}"
ValueError: '/usr/bin/mkdir' is not in the subpath of '/etc' OR one path is relative and the other is absolute.
user_1473560

Yes the intention is to allow specifying paths from separate parts, without needing to use a (platform-dependent) path separator.

This is clear from the test suite (src):

def test_relative_to_common(self):
    P = self.cls
    p = P('a/b')
    ...
    self.assertEqual(p.relative_to('a/b'), P())
    # With several args.
    self.assertEqual(p.relative_to('a', 'b'), P())

And it is also analogous to other interfaces in pathlib which accept *args, for example:

>>> Path("/usr").joinpath("local", "bin")
PosixPath('/usr/local/bin')
>>> Path(".").joinpath("a", "b", "c")
PosixPath('a/b/c')
>>> Path(Path.home(), "music", "mp3s")
PosixPath('/home/wim/music/mp3s')
user_6740390
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