prune is a no-op when file exists

i.e., when moving a file that's already at its destination.

beets/importer.py

@@ -461,13 +461,13 @@ class ImportTask(object):
     # Utilities.
 
     def prune(self, filename):
-        """Prune any empty directories above the given file, which must
-        not exist. If this task has no `toppath` or the file path
-        provided is not within the `toppath`, then this function has no
-        effect.
+        """Prune any empty directories above the given file. If this
+        task has no `toppath` or the file path provided is not within
+        the `toppath`, then this function has no effect. Similarly, if
+        the file still exists, no pruning is performed, so it's safe to
+        call when the file in question may not have been removed.
         """
-        assert not os.path.exists(filename)
-        if self.toppath:
+        if self.toppath and not os.path.exists(filename):
             util.prune_dirs(os.path.dirname(filename), self.toppath)
 
 
@@ -760,9 +760,7 @@ def manipulate_files(config):
                 # Just move the file.
                 old_path = item.path
                 lib.move(item, False)
-                # Clean up empty parent directory.
-                if task.toppath:
-                    task.prune(old_path)
+                task.prune(old_path)
             elif config.copy:
                 # If it's a reimport, move in-library files and copy
                 # out-of-library files. Otherwise, copy and keep track
@@ -810,7 +808,7 @@ def fetch_art(config):
                 album = lib.get_album(task.album_id)
                 album.set_art(artpath, not (config.delete or config.move))
 
-                if (config.delete or config.move) and task.toppath:
+                if config.delete or config.move:
                     task.prune(artpath)
 
 def finalize(config):
@@ -845,9 +843,7 @@ def finalize(config):
                 # Only delete files that were actually copied.
                 if old_path not in new_paths:
                     util.remove(syspath(old_path), False)
-                    # Clean up directory if it is emptied.
-                    if task.toppath:
-                        task.prune(old_path)
+                    task.prune(old_path)
 
         # Update progress.
         if config.resume is not False:

docs/changelog.rst

@@ -36,6 +36,8 @@ Changelog
   and give more context to "failed fingerprint generation" errors.
 * Interactive prompts are sent to stdout instead of stderr.
 * :doc:`/plugins/embedart`: Fix crash when audio files are unreadable.
+* Fix an assertion failure while importing with moving enabled when the file was
+  already at its destination.
 
 .. _artist credits: http://wiki.musicbrainz.org/Artist_Credit
 

test/test_importer.py

@@ -344,6 +344,16 @@ class ImportApplyTest(unittest.TestCase, _common.ExtraAsserts):
         _call_stages(config, [self.i], self.info, toppath=self.srcdir)
         self.assertNotExists(os.path.dirname(self.srcpath))
 
+    def test_manipulate_files_with_null_move(self):
+        """It should be possible to "move" a file even when the file is
+        already at the destination.
+        """
+        config = _common.iconfig(self.lib, move=True)
+        self.lib.move(self.i)  # Already at destination.
+        _call_stages(config, [self.i], self.info, toppath=self.srcdir,
+                     stages=[importer.manipulate_files])
+        self.assertExists(self.i.path)
+
 class AsIsApplyTest(unittest.TestCase):
     def setUp(self):
         self.dbpath = os.path.join(_common.RSRC, 'templib.blb')