duplicates: Fix #999 (check "checksum" type)

beetsplug/duplicates.py

@@ -209,6 +209,10 @@ class DuplicatesPlugin(BeetsPlugin):
                 fmt += ': {0}'
 
             if checksum:
+                if not isinstance(checksum, basestring):
+                    raise UserError(
+                        'duplicates: "checksum" option must be a command'
+                    )
                 for i in items:
                     k, _ = _checksum(i, checksum)
                 keys = [k]

docs/changelog.rst

@@ -30,6 +30,8 @@ Fixes:
   beets and instead fail silently.
 * Slow queries, such as those over flexible attributes, should now be much
   faster when used with certain commands---notably, the :doc:`/plugins/play`.
+* :doc:`/plugins/duplicates`: Emit an error when the ``checksum`` config
+  option is set incorrectly.
 
 
 1.3.8 (September 17, 2014)