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

[Miss Islington (bot)]

https://github.com/python/cpython/commit/8d0f36940e728989822c3789025b0813a8fe249a
commit: 8d0f36940e728989822c3789025b0813a8fe249a
branch: 3.8
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2019-12-16T04:42:20-08:00
summary:

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


Commit 6b5b013bcc22 ("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 at redhat.com>
(cherry picked from commit 092435e932dee1802784ec28f39454f50fdd879a)

Co-authored-by: Toke Høiland-Jørgensen <toke at redhat.com>

files:
A Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst
M Lib/pathlib.py
M Lib/test/test_pathlib.py

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index c42bde459b368..8ed3c883c6006 100644
--- a/Lib/pathlib.py
+++ b/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
 
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index 4543e3773dbf8..b127343982ca9 100644
--- a/Lib/test/test_pathlib.py
+++ b/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'
diff --git a/Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst b/Misc/NEWS.d/next/Library/2019-11-15-18-06-04.bpo-38811.AmdQ6M.rst
new file mode 100644
index 0000000000000..0e4a7f5bdf538
--- /dev/null
+++ b/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.