Changes as requested

beets/util/__init__.py

@@ -1108,3 +1108,25 @@ def lazy_property(func):
         return value
 
     return wrapper
+
+
+def decode_commandline_path(path):
+    """Prepare a path for substitution into commandline template.
+
+    On Python 3, we need to construct the subprocess commands to invoke as a
+    Unicode string. On Unix, this is a little unfortunate---the OS is
+    expecting bytes---so we use surrogate escaping and decode with the
+    argument encoding, which is the same encoding that will then be
+    *reversed* to recover the same bytes before invoking the OS. On
+    Windows, we want to preserve the Unicode filename "as is."
+    """
+    if six.PY2:
+        # On Python 2, substitute the bytestring directly into the template.
+        return path
+    else:
+        # On Python 3, the template is a Unicode string, which only supports
+        # substitution of Unicode variables.
+        if platform.system() == 'Windows':
+            return path.decode(_fsencoding())
+        else:
+            return path.decode(arg_encoding(), 'surrogateescape')

beetsplug/convert.py

@@ -16,7 +16,7 @@
 """Converts tracks or albums to external directory
 """
 from __future__ import division, absolute_import, print_function
-from beets.util import par_map
+from beets.util import par_map, decode_commandline_path
 
 import os
 import threading
@@ -25,7 +25,6 @@ import tempfile
 import shlex
 import six
 from string import Template
-import platform
 
 from beets import ui, util, plugins, config
 from beets.plugins import BeetsPlugin
@@ -205,20 +204,8 @@ class ConvertPlugin(BeetsPlugin):
         if not quiet and not pretend:
             self._log.info(u'Encoding {0}', util.displayable_path(source))
 
-        # On Python 3, we need to construct the command to invoke as a
-        # Unicode string. On Unix, this is a little unfortunate---the OS is
-        # expecting bytes---so we use surrogate escaping and decode with the
-        # argument encoding, which is the same encoding that will then be
-        # *reversed* to recover the same bytes before invoking the OS. On
-        # Windows, we want to preserve the Unicode filename "as is."
-        if not six.PY2:
-            command = command.decode(util.arg_encoding(), 'surrogateescape')
-            if platform.system() == 'Windows':
-                source = source.decode(util._fsencoding())
-                dest = dest.decode(util._fsencoding())
-            else:
-                source = source.decode(util.arg_encoding(), 'surrogateescape')
-                dest = dest.decode(util.arg_encoding(), 'surrogateescape')
+        source = decode_commandline_path(source)
+        dest = decode_commandline_path(dest)
 
         # Substitute $source and $dest in the argument list.
         args = shlex.split(command)

beetsplug/duplicates.py

@@ -17,15 +17,13 @@
 """
 from __future__ import division, absolute_import, print_function
 
-import platform
 import shlex
 import six
 
-from beets import util
 from beets.plugins import BeetsPlugin
 from beets.ui import decargs, print_, Subcommand, UserError
 from beets.util import command_output, displayable_path, subprocess, \
-    bytestring_path, MoveOperation
+    bytestring_path, MoveOperation, decode_commandline_path
 from beets.library import Item, Album
 
 
@@ -200,20 +198,8 @@ class DuplicatesPlugin(BeetsPlugin):
         output as flexattr on a key that is the name of the program, and
         return the key, checksum tuple.
         """
-        # On Python 3, we need to construct the command to invoke as a
-        # Unicode string. On Unix, this is a little unfortunate---the OS is
-        # expecting bytes---so we use surrogate escaping and decode with the
-        # argument encoding, which is the same encoding that will then be
-        # *reversed* to recover the same bytes before invoking the OS. On
-        # Windows, we want to preserve the Unicode filename "as is."
-        if not six.PY2:
-            if platform.system() == 'Windows':
-                args = [p.format(file=item.path.decode(util._fsencoding()))
-                        for p in shlex.split(prog)]
-            else:
-                args = [p.format(file=item.path.decode(util.arg_encoding(),
-                        'surrogateescape')) for p in shlex.split(prog)]
-
+        args = [p.format(file=decode_commandline_path(item.path))
+                for p in shlex.split(prog)]
         key = args[0]
         checksum = getattr(item, key, False)
         if not checksum: