Add documentation for NotQuery, cleanup

* Add changelog and query.rst documentation entries for the usage of negated
queries.
* Cleanup NotQuery class as suggested during code review (PEP conforming
docstring, clarification on empty clause match behaviour).

beets/dbcore/query.py

@@ -449,7 +449,8 @@ class OrQuery(MutableCollectionQuery):
 
 class NotQuery(Query):
     """A query that matches the negation of its `subquery`, as a shorcut for
-    performing `not(subquery)` without using regular expressions."""
+    performing `not(subquery)` without using regular expressions.
+    """
     def __init__(self, subquery):
         self.subquery = subquery
 
@@ -458,7 +459,8 @@ class NotQuery(Query):
         if clause:
             return 'not ({0})'.format(clause), subvals
         else:
-            # special case for RegexpQuery, (None, ())
+            # If there is no clause, there is nothing to negate. All the logic
+            # is handled by match() for slow queries.
             return clause, subvals
 
     def match(self, item):

docs/changelog.rst

@@ -20,6 +20,10 @@ New:
   :doc:`/plugins/discogs` also adopts the same setting.
 * :doc:`/plugins/embyupdate`: A plugin to trigger a library refresh on a
   `Emby Server`_ if database changed.
+* Queries can now use "not" logic: if you prepend a query term with "-" or
+  "^", items or albums matching that term will be excluded from the results.
+  For example, ``beet ls foo ^artist:bar`` will get all the items matching
+  `foo` but whose artist do not match `bar`. See :ref:`not_query`. :bug:`819`
 
 For developers:
 

docs/reference/query.rst

@@ -181,6 +181,35 @@ Find all items with a file modification time between 2008-12-01 and
 
     $ beet ls 'mtime:2008-12-01..2008-12-02'
 
+.. _not_query:
+
+Query Term Negation
+-------------------
+
+Query terms can also be negated, acting like a Boolean "not", by prepending
+them with ``-`` or ``^``. This has the effect of returning all the items that
+do **not** match the query term. For example, this command::
+
+    $ beet list ^love
+
+matches all the songs in the library that do not have "love" in any of their
+fields.
+
+Negation can be combined with the rest of the query mechanisms, allowing to
+negate specific fields, regular expressions, etc. For example, this command::
+
+    $ beet list -a artist:dylan ^year:1980..1990 "^album::the(y)?"
+
+matches all the albums with an artist containing "dylan", but excluding those
+released on the eighties and those that have "the" or "they" on the title.
+
+Note that the ``-`` character is treated by most shells as a reserved character
+for passing arguments, and as such needs to be escaped if using it for query
+negation. In most UNIX derivatives shells, using a double dash ``--``
+(indicating that everything after that point should not be treated as
+arguments) before the query terms should prevent conflicts, such as::
+
+    $ beet list -a -- artist:dylan -year:1980..1990 "-album::the(y)?"
 
 .. _pathquery:
 

test/test_query.py

@@ -782,8 +782,6 @@ class NotQueryTest(DummyDataTestCase):
     """Test `query.NotQuery` against the dummy data:
     - `test_type_xxx`: tests for the negation of a particular XxxQuery class.
     - `test_get_yyy`: tests on query strings (similar to `GetTest`)
-
-    TODO: add test_type_bytes, for ByteQuery?
     """
     def assertNegationProperties(self, q):
         """Given a Query `q`, assert that: