os.path.relpath() needlessly calls abspath() when given two paths with matching anchors #99199

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1. barneygale

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Nov 7, 2022
2. eryksun commented on Nov 7, 2022

   

   eryksun

   on Nov 7, 2022

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   On Windows, abspath() also normalizes a path, such as stripping any trailing spaces and dots from the final component. For example:

   >>> ntpath.relpath('spam/eggs. . .', 'spam/eggs/foo')
   '..'

   If we want to retain the ".." result here, then trailing dots and spaces will have to be stripped from the final component of each path. Otherwise the result will be "..\..\eggs. . .". There are also cases with DOS device names that will change if abspath() isn't called. For example, "foo/bar/nul" resolves to "\\.\nul".
3. barneygale commented on Nov 9, 2022

   

   barneygale

   on Nov 9, 2022

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   Funnily enough normpath() doesn't apply that normalisation. The support for Windows normalisation rules in Python seems pretty patchy IMO. There's no provision for it in pathlib, for example.
4. eryksun commented on Nov 9, 2022

   

   eryksun

   on Nov 9, 2022

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   Funnily enough normpath() doesn't apply that normalisation. The support for Windows normalisation rules in Python seems pretty patchy IMO. There's no provision for it in pathlib, for example.

   I'd prefer for ntpath.normpath() to strip dots and spaces from the final component, except for a ".." name. This would also improve ntpath.abspath() on POSIX¹.

   Normalizing DOS device names is problematic because it depends on the Windows version. There's still consistent behavior for bare device names, such as "con" -> "\\.\con", but not "./con"² or "con.txt".

   Footnotes

   1. The fallback on POSIX also needs better support for drives, and to use "C:" as the working drive. It should get drive, path = splitdrive(path), and then resolve the absolute path as join(drive or 'C:', cwd, path). ↩

   2. ntpath.abspath() is implemented as _getfullpathname(normpath(path)). normpath() aggressively normalizes "./con" as "con", so abspath('./con') returns "\\.\con" even if the current Windows version doesn't reserve "./con". In general, removing a leading "." component is wrong on Windows. For example, "./C:spam" (a file named "C" with a data stream named "spam") differs from the drive-relative path "C:spam". ↩
5. barneygale commented on Nov 12, 2022

   

   barneygale

   on Nov 12, 2022

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   Ack, this doesn't work due to differences in collapsing leading ../ segments:

   ======================================================================
   ERROR: test_relpath (test.test_ntpath.TestNtpath.test_relpath)
   ----------------------------------------------------------------------
   Traceback (most recent call last):
     File "/Users/barney.gale/code/cpython/Lib/test/test_ntpath.py", line 689, in test_relpath
       tester('ntpath.relpath("a", "../b")', '..\\'+currentdir+'\\a')
     File "/Users/barney.gale/code/cpython/Lib/test/test_ntpath.py", line 57, in tester
       raise TestFailed("%s should return: %s but returned: %s" \
   test.support.TestFailed: ntpath.relpath("a", "../b") should return: ..\@test_90826_tmpæ\a but returned: ..\..\a
   
   ======================================================================
   FAIL: test_relpath (test.test_posixpath.PosixPathTest.test_relpath)
   ----------------------------------------------------------------------
   Traceback (most recent call last):
     File "/Users/barney.gale/code/cpython/Lib/test/test_posixpath.py", line 610, in test_relpath
       self.assertEqual(posixpath.relpath("a", "../b"), "../"+curdir+"/a")
   AssertionError: '../../a' != '../bar/a'
   - ../../a
   + ../bar/a
   
6. barneygale commented on Nov 12, 2022

   

   barneygale

   on Nov 12, 2022

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   Closing this issue as the handling of ../ segments in os.path and pathlib can't be reconciled in this case.
7. barneygale

   closed this as completedon Nov 12, 2022