bpo-38811: Check for presence of os.link method in pathlib (GH-17225)

Commit 6b5b013 ("bpo-26978: Implement pathlib.Path.link_to (Using os.link) (GH-12990)") introduced a new link_to method in pathlib. However, this makes pathlib crash when the 'os' module is missing a 'link' method. Fix this by checking for the presence of the 'link' method on pathlib module import, and if it's not present, turn it into a runtime error like those emitted when there is no lchmod() or symlink(). Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> (cherry picked from commit 092435e) Co-authored-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: miss-islington

Lib/pathlib.py

@@ -418,7 +418,12 @@ def lchmod(self, pathobj, mode):
 
  unlink = os.unlink
 
- link_to = os.link
+ if hasattr(os, "link"):
+ link_to = os.link
+ else:
+ @staticmethod
+ def link_to(self, target):
+ raise NotImplementedError("os.link() not available on this system")
 
  rmdir = os.rmdir
 

Lib/test/test_pathlib.py

@@ -1672,6 +1672,7 @@ def test_rmdir(self):
  self.assertFileNotFound(p.stat)
  self.assertFileNotFound(p.unlink)
 
+ @unittest.skipUnless(hasattr(os, "link"), "os.link() is not present")
  def test_link_to(self):
  P = self.cls(BASE)
  p = P / 'fileA'
@@ -1691,6 +1692,15 @@ def test_link_to(self):
  self.assertEqual(os.stat(r).st_size, size)
  self.assertTrue(q.stat)
 
+ @unittest.skipIf(hasattr(os, "link"), "os.link() is present")
+ def test_link_to_not_implemented(self):
+ P = self.cls(BASE)
+ p = P / 'fileA'
+ # linking to another path.
+ q = P / 'dirA' / 'fileAA'
+ with self.assertRaises(NotImplementedError):
+ p.link_to(q)
+
  def test_rename(self):
  P = self.cls(BASE)
  p = P / 'fileA'

Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst

@@ -0,0 +1 @@
+Fix an unhandled exception in :mod:`pathlib` when :meth:`os.link` is missing. Patch by Toke Høiland-Jørgensen.