add python2.7 commonpath backport

Author: tfeldmann

fs/_pathcompat.py

@@ -0,0 +1,40 @@
+try:
+    from os.path import commonpath
+except ImportError:
+    # Return the longest common sub-path of the sequence of paths given as input.
+    # The paths are not normalized before comparing them (this is the
+    # responsibility of the caller). Any trailing separator is stripped from the
+    # returned path.
+
+    def commonpath(paths):
+        """Given a sequence of path names, returns the longest common sub-path."""
+
+        if not paths:
+            raise ValueError("commonpath() arg is an empty sequence")
+
+        paths = tuple(paths)
+        if isinstance(paths[0], bytes):
+            sep = b"/"
+            curdir = b"."
+        else:
+            sep = "/"
+            curdir = "."
+
+        split_paths = [path.split(sep) for path in paths]
+
+        try:
+            (isabs,) = set(p[:1] == sep for p in paths)
+        except ValueError:
+            raise ValueError("Can't mix absolute and relative paths")
+
+        split_paths = [[c for c in s if c and c != curdir] for s in split_paths]
+        s1 = min(split_paths)
+        s2 = max(split_paths)
+        common = s1
+        for i, c in enumerate(s1):
+            if c != s2[i]:
+                common = s1[:i]
+                break
+
+        prefix = sep if isabs else sep[:0]
+        return prefix + sep.join(common)

fs/move.py

@@ -1,23 +1,22 @@
 """Functions for moving files between filesystems.
 """
 
-from __future__ import print_function
-from __future__ import unicode_literals
+from __future__ import print_function, unicode_literals
 
-from os.path import commonpath
 import typing
 
 from . import open_fs
-from .copy import copy_dir
-from .copy import copy_file
+from ._pathcompat import commonpath
+from .copy import copy_dir, copy_file
 from .errors import FSError
 from .opener import manage_fs
 from .path import frombase
 
 if typing.TYPE_CHECKING:
-    from .base import FS
     from typing import Text, Union
 
+    from .base import FS
+
 
 def move_fs(
     src_fs,  # type: Union[Text, FS]
@@ -80,7 +79,7 @@ def move_file(
                     with _src_fs.lock(), _dst_fs.lock():
                         with open_fs(common, writeable=True) as base:
                             base.move(rel_src, rel_dst, preserve_time=preserve_time)
-                    return  # optimization worked, exit early
+                            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.