Allow TXXX:ALBUMARTISTSORT to be a fallback for albumartistsort

I noticed that my previous fix for duplicate key registration of
albumartistsort would break reading files already having
TXXX:ALBUMARTISTSORT so this commit fixes it by making
TXXX:ALBUMARTISTSORT a fallback for albumartistsort if the first
getter can't get a value and also by making EasyID3 delete
TXXX:ALBUMARTISTSORT frames when deleting albumartistsort.

Still, when setting an albumartistsort value only TSO2 frames
are used.

This also adds tests for all those cases.

mutagen/easyid3.py

@@ -12,6 +12,7 @@
 """
 
 from typing import Callable, Dict
+from functools import partial
 
 import mutagen.id3
 
@@ -542,6 +543,48 @@ def peakgain_list(id3, key):
     EasyID3.RegisterTXXXKey(key, desc)
 
 
+def fallback_getter_helper(id3, key, original, fallback):
+    try:
+        return original(id3, key)
+    except KeyError as e:
+        try:
+            return fallback(id3, key)
+        except KeyError:
+            raise e
+
+def fallback_deleter_helper(id3, key, original, fallback):
+    try:
+        original(id3, key)
+    except KeyError as e:
+        exception1 = e
+    else:
+        exception1 = None
+
+    try:
+        return fallback(id3, key)
+    except KeyError:
+        if exception1:
+            raise exception1
+
+def txxxkey_getter(id3, key, frameid):
+    return list(id3[frameid])
+
+def txxxkey_deleter(id3, key, frameid):
+    del id3[frameid]
+
+for key, extra_source in {
+    "albumartistsort": "TXXX:ALBUMARTISTSORT",
+}.items():
+    EasyID3.Get[key] = partial(fallback_getter_helper,
+                               original=EasyID3.Get[key],
+                               fallback=partial(txxxkey_getter,
+                                                frameid=extra_source))
+    EasyID3.Delete[key] = partial(fallback_deleter_helper,
+                                  original=EasyID3.Delete[key],
+                                  fallback=partial(txxxkey_deleter,
+                                                   frameid=extra_source))
+
+
 class EasyID3FileType(ID3FileType):
     """EasyID3FileType(filething=None)
 

tests/test_easyid3.py

@@ -3,7 +3,7 @@
 import pickle
 
 from mutagen import MutagenError
-from mutagen.id3 import ID3FileType, ID3, RVA2, CHAP, TDRC, CTOC, TSO2
+from mutagen.id3 import ID3FileType, ID3, RVA2, CHAP, TDRC, CTOC, TSO2, TXXX
 from mutagen.easyid3 import EasyID3, error as ID3Error
 
 from tests import TestCase, DATA_DIR, get_temp_copy
@@ -434,3 +434,39 @@ def test_albumartistsort(self):
         self.id3.save(self.filename)
         id3 = EasyID3(self.filename)
         self.failUnlessEqual(id3["albumartistsort"], [u"someartist"])
+
+        self.id3["albumartistsort"] = [u"otherartist"]
+        self.assertEqual(self.realid3["TSO2"], [u"otherartist"])
+
+        del self.id3["albumartistsort"]
+        self.assertEqual(len(self.id3), 0)
+
+    def test_albumartistsort_from_TXXX(self):
+        self.realid3.add(TXXX(desc="ALBUMARTISTSORT", text=u"someartist"))
+        self.id3.save(self.filename)
+        id3 = EasyID3(self.filename)
+        self.failUnlessEqual(id3["albumartistsort"], [u"someartist"])
+
+        self.id3["albumartistsort"] = [u"otherartist"]
+        self.assertEqual(self.realid3["TSO2"], [u"otherartist"])
+
+        del self.id3["albumartistsort"]
+        self.assertEqual(len(self.id3), 0)
+
+    def test_albumartistsort_with_both_frames(self):
+        self.realid3.add(TSO2(text=u"someartist"))
+        self.realid3.add(TXXX(desc="ALBUMARTISTSORT", text=u"someotherartist"))
+        self.id3.save(self.filename)
+        id3 = EasyID3(self.filename)
+        self.failUnlessEqual(id3["albumartistsort"], [u"someartist"])
+        self.assertEqual(len(self.id3), 1)
+        self.assertEqual(len(self.realid3), 2)
+
+        self.id3["albumartistsort"] = [u"otherartist"]
+        self.assertEqual(self.realid3["TSO2"], [u"otherartist"])
+
+        del self.id3["albumartistsort"]
+        self.assertEqual(len(self.id3), 0)
+        self.assertEqual(len(self.realid3), 0)
+
+        self.failUnlessRaises(KeyError, self.id3.__delitem__, "albumartistsort")