clarify fallthrough and add writeable=True

Author: tfeldmann

fs/move.py

@@ -59,8 +59,8 @@ def move_file(
             resources (defaults to `False`).
 
     """
-    with manage_fs(src_fs) as _src_fs:
-        with manage_fs(dst_fs, create=True) as _dst_fs:
+    with manage_fs(src_fs, writeable=True) as _src_fs:
+        with manage_fs(dst_fs, writeable=True, create=True) as _dst_fs:
             if _src_fs is _dst_fs:
                 # Same filesystem, may be optimized
                 _src_fs.move(
@@ -90,6 +90,8 @@ def move_file(
                         base.move(rel_src, rel_dst, preserve_time=preserve_time)
                     return  # optimization worked, exit early
                 except ValueError:
+                    # This is raised if we cannot find a common base folder.
+                    # In this case just fall through to the standard method.
                     pass
 
             # Standard copy and delete

tests/test_move.py

@@ -89,7 +89,9 @@ def test_move_file_read_only_source(self):
             dst = tmp.makedir("sub")
             with self.assertRaises(ResourceReadOnly):
                 fs.move.move_file(src, "file.txt", dst, "file.txt")
-            self.assertFalse(dst.exists("file.txt"))
+            self.assertFalse(
+                dst.exists("file.txt"), "file should not have been copied over"
+            )
             self.assertTrue(src.exists("file.txt"))
 
     def test_move_file_read_only_mem_source(self):
@@ -99,7 +101,9 @@ def test_move_file_read_only_mem_source(self):
             src_ro = read_only(src)
             with self.assertRaises(ResourceReadOnly):
                 fs.move.move_file(src_ro, "file.txt", sub, "file.txt")
-            self.assertFalse(sub.exists("file.txt"))
+            self.assertFalse(
+                sub.exists("file.txt"), "file should not have been copied over"
+            )
             self.assertTrue(src.exists("file.txt"))
 
     def test_move_dir(self):