Make FormattedMapping behave more like a dict

The collections.Mapping abstract base class provides all the nice dict-like
functionality we need.

beets/dbcore/db.py

@@ -20,6 +20,7 @@ from collections import defaultdict
 import threading
 import sqlite3
 import contextlib
+import collections
 
 import beets
 from beets.util.functemplate import Template
@@ -425,7 +426,7 @@ class Model(object):
             return string
 
 
-class FormattedMapping(object):
+class FormattedMapping(collections.Mapping):
     """A `dict`-like formatted view of a model.
 
     The accessor ``mapping[key]`` returns the formated version of
@@ -445,8 +446,11 @@ class FormattedMapping(object):
         else:
             raise KeyError(key)
 
-    def __contains__(self, key):
-        return key in self.model_keys
+    def __iter__(self):
+        return iter(self.model_keys)
+
+    def __len__(self):
+        return len(self.model_keys)
 
 
 # Database controller and supporting interfaces.

test/test_dbcore.py

@@ -269,6 +269,19 @@ class FormatTest(_common.TestCase):
         self.assertEqual(value, u'')
 
 
+class FormattedMappingTest(_common.TestCase):
+    def test_keys_equal_model_keys(self):
+        model = TestModel1()
+        formatted = model._formatted_mapping()
+        self.assertEqual(set(model.keys(True)), set(formatted.keys()))
+
+    def test_get_unset_field(self):
+        model = TestModel1()
+        formatted = model._formatted_mapping()
+        with self.assertRaises(KeyError):
+            formatted['other_field']
+
+
 class ParseTest(_common.TestCase):
     def test_parse_fixed_field(self):
         value = TestModel1._parse('field_one', u'2')