NullSort instead of None

A more descriptive placeholder for "don't sort".

beets/dbcore/db.py

@@ -752,7 +752,6 @@ class Database(object):
        `sort_order` is either a SQLite ORDER BY clause for sorting or a
         Sort object.
         """
-
         sql, subvals, query, sort = build_sql(model_cls, query, sort_order)
 
         with self.transaction() as tx:

beets/dbcore/query.py

@@ -531,7 +531,7 @@ class Sort(object):
     def sort(self, items):
         """Sort the list of objects and return a list.
         """
-        return sorted(items, key=lambda x: x)
+        return sorted(items)
 
     def is_slow(self):
         """Indicate whether this query is *slow*, meaning that it cannot
@@ -544,8 +544,8 @@ class MultipleSort(Sort):
     """Sort that encapsulates multiple sub-sorts.
     """
 
-    def __init__(self):
-        self.sorts = []
+    def __init__(self, sorts=None):
+        self.sorts = sorts or []
 
     def add_sort(self, sort):
         self.sorts.append(sort)
@@ -612,6 +612,9 @@ class MultipleSort(Sort):
             items = sort.sort(items)
         return items
 
+    def __repr__(self):
+        return u'MultipleSort({0})'.format(repr(self.sorts))
+
 
 class FieldSort(Sort):
     """An abstract sort criterion that orders by a specific field (of
@@ -628,6 +631,13 @@ class FieldSort(Sort):
         return sorted(objs, key=attrgetter(self.field),
                       reverse=not self.ascending)
 
+    def __repr__(self):
+        return u'<{0}: {1}{2}>'.format(
+            type(self).__name__,
+            self.field,
+            '+' if self.ascending else '-',
+        )
+
 
 class FixedFieldSort(FieldSort):
     """Sort object to sort on a fixed field.
@@ -653,7 +663,7 @@ class SmartArtistSort(Sort):
             field = 'artist'
         return ('(CASE {0}_sort WHEN NULL THEN {0} '
                 'WHEN "" THEN {0} '
-                'ELSE {0}_sort END) {2} ').format(field, order)
+                'ELSE {0}_sort END) {1} ').format(field, order)
 
 
 class SlowFieldSort(FieldSort):
@@ -664,6 +674,15 @@ class SlowFieldSort(FieldSort):
         return True
 
 
+class NullSort(Sort):
+    """No sorting. Leave results unsorted."""
+    def sort(items):
+        return items
+
+    def __nonzero__(self):
+        return False
+
+
 def build_sql(model_cls, query, sort):
     """ Generate a sql statement (and the values that must be injected into it)
     from a query, sort and a model class. Query and sort objects are returned

beets/dbcore/queryparse.py

@@ -124,11 +124,18 @@ def query_from_strings(query_cls, model_cls, prefixes, query_parts):
 
 
 def construct_sort_part(model_cls, part):
-    """ Creates a Sort object from a single criteria. Returns a `Sort` instance.
+    """Create a `Sort` from a single string criterion.
+
+    `model_cls` is the `Model` being queried. `part` is a single string
+    ending in ``+`` or ``-`` indicating the sort.
     """
-    sort = None
+    assert part, "part must be a field name and + or -"
     field = part[:-1]
-    is_ascending = (part[-1] == '+')
+    assert field, "field is missing"
+    direction = part[-1]
+    assert direction in ('+', '-'), "part must end with + or -"
+    is_ascending = direction == '+'
+
     if field in model_cls._fields:
         sort = query.FixedFieldSort(field, is_ascending)
     elif field == 'smartartist':
@@ -141,11 +148,12 @@ def construct_sort_part(model_cls, part):
 
 
 def sort_from_strings(model_cls, sort_parts):
-    """Creates a Sort object from a list of sort criteria strings.
+    """Create a `Sort` from a list of sort criteria (strings).
     """
     if not sort_parts:
-        return None
-    sort = query.MultipleSort()
-    for part in sort_parts:
-        sort.add_sort(construct_sort_part(model_cls, part))
-    return sort
+        return query.NullSort()
+    else:
+        sort = query.MultipleSort()
+        for part in sort_parts:
+            sort.add_sort(construct_sort_part(model_cls, part))
+        return sort

test/test_dbcore.py

@@ -434,7 +434,7 @@ class SortFromStringsTest(unittest.TestCase):
 
     def test_zero_parts(self):
         s = self.sfs([])
-        self.assertIsNone(s)
+        self.assertIsInstance(s, dbcore.query.NullSort)
 
     def test_one_parts(self):
         s = self.sfs(['field+'])