Source

beets / beets / mediafile.py

The default branch has multiple heads

Full commit
   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
# This file is part of beets.
# Copyright 2013, Adrian Sampson.
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.

"""Handles low-level interfacing for files' tags. Wraps Mutagen to
automatically detect file types and provide a unified interface for a
useful subset of music files' tags.

Usage:

    >>> f = MediaFile('Lucy.mp3')
    >>> f.title
    u'Lucy in the Sky with Diamonds'
    >>> f.artist = 'The Beatles'
    >>> f.save()

A field will always return a reasonable value of the correct type, even
if no tag is present. If no value is available, the value will be false
(e.g., zero or the empty string).
"""
import mutagen
import mutagen.mp3
import mutagen.oggvorbis
import mutagen.mp4
import mutagen.flac
import mutagen.monkeysaudio
import mutagen.asf
import datetime
import re
import base64
import math
import struct
import imghdr
import os
import logging
import traceback
from beets.util.enumeration import enum

__all__ = ['UnreadableFileError', 'FileTypeError', 'MediaFile']


# Logger.
log = logging.getLogger('beets')


# Exceptions.

# Raised for any file MediaFile can't read.
class UnreadableFileError(Exception):
    pass

class FileIOError(UnreadableFileError, IOError):
    def __init__(self, exc):
        IOError.__init__(self, exc.errno, exc.strerror, exc.filename)

# Raised for files that don't seem to have a type MediaFile supports.
class FileTypeError(UnreadableFileError):
    pass


# Constants.

# Human-readable type names.
TYPES = {
    'mp3':  'MP3',
    'aac':  'AAC',
    'alac':  'ALAC',
    'ogg':  'OGG',
    'flac': 'FLAC',
    'ape':  'APE',
    'wv':   'WavPack',
    'mpc':  'Musepack',
    'asf':  'Windows Media',
}

MP4_TYPES = ('aac', 'alac')


# Utility.

def _safe_cast(out_type, val):
    """Tries to covert val to out_type but will never raise an
    exception. If the value can't be converted, then a sensible
    default value is returned. out_type should be bool, int, or
    unicode; otherwise, the value is just passed through.
    """
    if out_type == int:
        if val is None:
            return 0
        elif isinstance(val, int) or isinstance(val, float):
            # Just a number.
            return int(val)
        else:
            # Process any other type as a string.
            if not isinstance(val, basestring):
                val = unicode(val)
            # Get a number from the front of the string.
            val = re.match(r'[0-9]*', val.strip()).group(0)
            if not val:
                return 0
            else:
                return int(val)

    elif out_type == bool:
        if val is None:
            return False
        else:
            try:
                if isinstance(val, mutagen.asf.ASFBoolAttribute):
                    return val.value
                else:
                    # Should work for strings, bools, ints:
                    return bool(int(val))
            except ValueError:
                return False

    elif out_type == unicode:
        if val is None:
            return u''
        else:
            if isinstance(val, str):
                return val.decode('utf8', 'ignore')
            elif isinstance(val, unicode):
                return val
            else:
                return unicode(val)

    elif out_type == float:
        if val is None:
            return 0.0
        elif isinstance(val, int) or isinstance(val, float):
            return float(val)
        else:
            if not isinstance(val, basestring):
                val = unicode(val)
            val = re.match(r'[\+-]?[0-9\.]*', val.strip()).group(0)
            if not val:
                return 0.0
            else:
                return float(val)

    else:
        return val


# Image coding for ASF/WMA.

def _unpack_asf_image(data):
    """Unpack image data from a WM/Picture tag. Return a tuple
    containing the MIME type, the raw image data, a type indicator, and
    the image's description.

    This function is treated as "untrusted" and could throw all manner
    of exceptions (out-of-bounds, etc.). We should clean this up
    sometime so that the failure modes are well-defined.
    """
    type, size = struct.unpack_from("<bi", data)
    pos = 5
    mime = ""
    while data[pos:pos+2] != "\x00\x00":
        mime += data[pos:pos+2]
        pos += 2
    pos += 2
    description = ""
    while data[pos:pos+2] != "\x00\x00":
        description += data[pos:pos+2]
        pos += 2
    pos += 2
    image_data = data[pos:pos+size]
    return (mime.decode("utf-16-le"), image_data, type,
            description.decode("utf-16-le"))

def _pack_asf_image(mime, data, type=3, description=""):
    """Pack image data for a WM/Picture tag.
    """
    tag_data = struct.pack("<bi", type, len(data))
    tag_data += mime.encode("utf-16-le") + "\x00\x00"
    tag_data += description.encode("utf-16-le") + "\x00\x00"
    tag_data += data
    return tag_data


# iTunes Sound Check encoding.

def _sc_decode(soundcheck):
    """Convert a Sound Check string value to a (gain, peak) tuple as
    used by ReplayGain.
    """
    # SoundCheck tags consist of 10 numbers, each represented by 8
    # characters of ASCII hex preceded by a space.
    try:
        soundcheck = soundcheck.replace(' ', '').decode('hex')
        soundcheck = struct.unpack('!iiiiiiiiii', soundcheck)
    except (struct.error, TypeError):
        # SoundCheck isn't in the format we expect, so return default
        # values.
        return 0.0, 0.0

    # SoundCheck stores absolute calculated/measured RMS value in an
    # unknown unit. We need to find the ratio of this measurement
    # compared to a reference value of 1000 to get our gain in dB. We
    # play it safe by using the larger of the two values (i.e., the most
    # attenuation).
    maxgain = max(soundcheck[:2])
    if maxgain > 0:
        gain = math.log10(maxgain / 1000.0) * -10
    else:
        # Invalid gain value found.
        gain = 0.0

    # SoundCheck stores peak values as the actual value of the sample,
    # and again separately for the left and right channels. We need to
    # convert this to a percentage of full scale, which is 32768 for a
    # 16 bit sample. Once again, we play it safe by using the larger of
    # the two values.
    peak = max(soundcheck[6:8]) / 32768.0

    return round(gain, 2), round(peak, 6)

def _sc_encode(gain, peak):
    """Encode ReplayGain gain/peak values as a Sound Check string.
    """
    # SoundCheck stores the peak value as the actual value of the
    # sample, rather than the percentage of full scale that RG uses, so
    # we do a simple conversion assuming 16 bit samples.
    peak *= 32768.0

    # SoundCheck stores absolute RMS values in some unknown units rather
    # than the dB values RG uses. We can calculate these absolute values
    # from the gain ratio using a reference value of 1000 units. We also
    # enforce the maximum value here, which is equivalent to about
    # -18.2dB.
    g1 = min(round((10 ** (gain / -10)) * 1000), 65534)
    # Same as above, except our reference level is 2500 units.
    g2 = min(round((10 ** (gain / -10)) * 2500), 65534)

    # The purpose of these values are unknown, but they also seem to be
    # unused so we just use zero.
    uk = 0
    values = (g1, g1, g2, g2, uk, uk, peak, peak, uk, uk)
    return (u' %08X' * 10) % values


# Flags for encoding field behavior.

# Determine style of packing, if any.
packing = enum('SLASHED',   # pair delimited by /
               'TUPLE',     # a python tuple of 2 items
               'DATE',      # YYYY-MM-DD
               'SC',        # Sound Check gain/peak encoding
               name='packing')

class StorageStyle(object):
    """Parameterizes the storage behavior of a single field for a
    certain tag format.
     - key: The Mutagen key used to access the field's data.
     - list_elem: Store item as a single object or as first element
       of a list.
     - as_type: Which type the value is stored as (unicode, int,
       bool, or str).
     - packing: If this value is packed in a multiple-value storage
       unit, which type of packing (in the packing enum). Otherwise,
       None. (Makes as_type irrelevant).
     - pack_pos: If the value is packed, in which position it is
       stored.
     - suffix: When `as_type` is a string type, append this before
       storing the value.
     - float_places: When the value is a floating-point number and
       encoded as a string, the number of digits to store after the
       point.

    For MP3 only:
      - id3_desc: match against this 'desc' field as well
        as the key.
      - id3_frame_field: store the data in this field of the frame
        object.
      - id3_lang: set the language field of the frame object.
    """
    def __init__(self, key, list_elem=True, as_type=unicode,
                 packing=None, pack_pos=0, pack_type=int, 
                 id3_desc=None, id3_frame_field='text',
                 id3_lang=None, suffix=None, float_places=2):
        self.key = key
        self.list_elem = list_elem
        self.as_type = as_type
        self.packing = packing
        self.pack_pos = pack_pos
        self.pack_type = pack_type
        self.id3_desc = id3_desc
        self.id3_frame_field = id3_frame_field
        self.id3_lang = id3_lang
        self.suffix = suffix
        self.float_places = float_places

        # Convert suffix to correct string type.
        if self.suffix and self.as_type in (str, unicode):
            self.suffix = self.as_type(self.suffix)


# Dealing with packings.

class Packed(object):
    """Makes a packed list of values subscriptable. To access the packed
    output after making changes, use packed_thing.items.
    """
    def __init__(self, items, packstyle, out_type=int):
        """Create a Packed object for subscripting the packed values in
        items. The items are packed using packstyle, which is a value
        from the packing enum. Values are converted to out_type before
        they are returned.
        """
        self.items = items
        self.packstyle = packstyle
        self.out_type = out_type

        if out_type is int:
            self.none_val = 0
        elif out_type is float:
            self.none_val = 0.0
        else:
            self.none_val = None

    def __getitem__(self, index):
        if not isinstance(index, int):
            raise TypeError('index must be an integer')

        if self.items is None:
            return self.none_val

        items = self.items
        if self.packstyle == packing.DATE:
            # Remove time information from dates. Usually delimited by
            # a "T" or a space.
            items = re.sub(r'[Tt ].*$', '', unicode(items))

        # transform from a string packing into a list we can index into
        if self.packstyle == packing.SLASHED:
            seq = unicode(items).split('/')
        elif self.packstyle == packing.DATE:
            seq = unicode(items).split('-')
        elif self.packstyle == packing.TUPLE:
            seq = items # tuple: items is already indexable
        elif self.packstyle == packing.SC:
            seq = _sc_decode(items)

        try:
            out = seq[index]
        except:
            out = None

        if out is None or out == self.none_val or out == '':
            return _safe_cast(self.out_type, self.none_val)
        else:
            return _safe_cast(self.out_type, out)

    def __setitem__(self, index, value):
        # Interpret null values.
        if value is None:
            value = self.none_val

        if self.packstyle in (packing.SLASHED, packing.TUPLE, packing.SC):
            # SLASHED, TUPLE and SC are always two-item packings
            length = 2
        else:
            # DATE can have up to three fields
            length = 3

        # make a list of the items we'll pack
        new_items = []
        for i in range(length):
            if i == index:
                next_item = value
            else:
                next_item = self[i]
            new_items.append(next_item)

        if self.packstyle == packing.DATE:
            # Truncate the items wherever we reach an invalid (none)
            # entry. This prevents dates like 2008-00-05.
            for i, item in enumerate(new_items):
                if item == self.none_val or item is None:
                    del(new_items[i:]) # truncate
                    break

        if self.packstyle == packing.SLASHED:
            self.items = '/'.join(map(unicode, new_items))
        elif self.packstyle == packing.DATE:
            field_lengths = [4, 2, 2] # YYYY-MM-DD
            elems = []
            for i, item in enumerate(new_items):
                elems.append('{0:0{1}}'.format(int(item), field_lengths[i]))
            self.items = '-'.join(elems)
        elif self.packstyle == packing.TUPLE:
            self.items = new_items
        elif self.packstyle == packing.SC:
            self.items = _sc_encode(*new_items)


# The field itself.

class MediaField(object):
    """A descriptor providing access to a particular (abstract) metadata
    field. out_type is the type that users of MediaFile should see and
    can be unicode, int, or bool. id3, mp4, and flac are StorageStyle
    instances parameterizing the field's storage for each type.
    """
    def __init__(self, out_type=unicode, **kwargs):
        """Creates a new MediaField.
         - out_type: The field's semantic (exterior) type.
         - kwargs: A hash whose keys are 'mp3', 'mp4', 'asf', and 'etc'
           and whose values are StorageStyle instances
           parameterizing the field's storage for each type.
        """
        self.out_type = out_type
        if not set(['mp3', 'mp4', 'etc', 'asf']) == set(kwargs):
            raise TypeError('MediaField constructor must have keyword '
                            'arguments mp3, mp4, asf, and etc')
        self.styles = kwargs

    def _fetchdata(self, obj, style):
        """Get the value associated with this descriptor's field stored
        with the given StorageStyle. Unwraps from a list if necessary.
        """
        # fetch the value, which may be a scalar or a list
        if obj.type == 'mp3':
            if style.id3_desc is not None: # also match on 'desc' field
                frames = obj.mgfile.tags.getall(style.key)
                entry = None
                for frame in frames:
                    if frame.desc.lower() == style.id3_desc.lower():
                        entry = getattr(frame, style.id3_frame_field)
                        break
                if entry is None: # no desc match
                    return None
            else:
                # Get the metadata frame object.
                try:
                    frame = obj.mgfile[style.key]
                except KeyError:
                    return None

                entry = getattr(frame, style.id3_frame_field)

        else:  # Not MP3.
            try:
                entry = obj.mgfile[style.key]
            except KeyError:
                return None

        # Possibly index the list.
        if style.list_elem:
            if entry:  # List must have at least one value.
                # Handle Mutagen bugs when reading values (#356).
                try:
                    return entry[0]
                except:
                    log.error('Mutagen exception when reading field: %s' %
                              traceback.format_exc)
                    return None
            else:
                return None
        else:
            return entry

    def _storedata(self, obj, val, style):
        """Store val for this descriptor's field in the tag dictionary
        according to the provided StorageStyle. Store it as a
        single-item list if necessary.
        """
        # Wrap as a list if necessary.
        if style.list_elem:
            out = [val]
        else:
            out = val

        if obj.type == 'mp3':
            # Try to match on "desc" field.
            if style.id3_desc is not None:
                frames = obj.mgfile.tags.getall(style.key)

                # try modifying in place
                found = False
                for frame in frames:
                    if frame.desc.lower() == style.id3_desc.lower():
                        setattr(frame, style.id3_frame_field, out)
                        found = True
                        break

                # need to make a new frame?
                if not found:
                    assert isinstance(style.id3_frame_field, str)  # Keyword.
                    args = {
                        'encoding': 3,
                        'desc': style.id3_desc,
                        style.id3_frame_field: val,
                    }
                    if style.id3_lang:
                        args['lang'] = style.id3_lang
                    obj.mgfile.tags.add(mutagen.id3.Frames[style.key](**args))

            # Try to match on "owner" field.
            elif style.key.startswith('UFID:'):
                owner = style.key.split(':', 1)[1]
                frames = obj.mgfile.tags.getall(style.key)

                for frame in frames:
                    # Replace existing frame data.
                    if frame.owner == owner:
                        setattr(frame, style.id3_frame_field, val)
                else:
                    # New frame.
                    assert isinstance(style.id3_frame_field, str)  # Keyword.
                    frame = mutagen.id3.UFID(owner=owner,
                        **{style.id3_frame_field: val})
                    obj.mgfile.tags.setall('UFID', [frame])

            # Just replace based on key.
            else:
                assert isinstance(style.id3_frame_field, str)  # Keyword.
                frame = mutagen.id3.Frames[style.key](encoding = 3,
                    **{style.id3_frame_field: val})
                obj.mgfile.tags.setall(style.key, [frame])

        else:  # Not MP3.
            obj.mgfile[style.key] = out

    def _styles(self, obj):
        if obj.type in ('mp3', 'asf'):
            styles = self.styles[obj.type]
        elif obj.type in MP4_TYPES:
            styles = self.styles['mp4']
        else:
            styles = self.styles['etc']  # Sane styles.

        # Make sure we always return a list of styles, even when given
        # a single style for convenience.
        if isinstance(styles, StorageStyle):
            return [styles]
        else:
            return styles

    def __get__(self, obj, owner):
        """Retrieve the value of this metadata field.
        """
        # Fetch the data using the various StorageStyles.
        styles = self._styles(obj)
        if styles is None:
            out = None
        else:
            for style in styles:
                # Use the first style that returns a reasonable value.
                out = self._fetchdata(obj, style)
                if out:
                    break

            if style.packing:
                p = Packed(out, style.packing, out_type=style.pack_type)
                out = p[style.pack_pos]

            # Remove suffix.
            if style.suffix and isinstance(out, (str, unicode)):
                if out.endswith(style.suffix):
                    out = out[:-len(style.suffix)]

            # MPEG-4 freeform frames are (should be?) encoded as UTF-8.
            if obj.type in MP4_TYPES and style.key.startswith('----:') and \
                    isinstance(out, str):
                out = out.decode('utf8')

        return _safe_cast(self.out_type, out)

    def __set__(self, obj, val):
        """Set the value of this metadata field.
        """
        # Store using every StorageStyle available.
        styles = self._styles(obj)
        if styles is None:
            return

        for style in styles:

            if style.packing:
                p = Packed(self._fetchdata(obj, style), style.packing,
                           out_type=style.pack_type)
                p[style.pack_pos] = val
                out = p.items

            else:  # Unicode, integer, boolean, or float scalar.
                out = val

                # deal with Nones according to abstract type if present
                if out is None:
                    if self.out_type == int:
                        out = 0
                    elif self.out_type == float:
                        out = 0.0
                    elif self.out_type == bool:
                        out = False
                    elif self.out_type == unicode:
                        out = u''
                    # We trust that packed values are handled above.

                # Convert to correct storage type (irrelevant for
                # packed values).
                if self.out_type == float and style.as_type in (str, unicode):
                    # Special case for float-valued data.
                    out = u'{0:.{1}f}'.format(out, style.float_places)
                    out = style.as_type(out)
                elif style.as_type == unicode:
                    if out is None:
                        out = u''
                    else:
                        if self.out_type == bool:
                            # Store bools as 1/0 instead of True/False.
                            out = unicode(int(bool(out)))
                        elif isinstance(out, str):
                            out = out.decode('utf8', 'ignore')
                        else:
                            out = unicode(out)
                elif style.as_type == int:
                    if out is None:
                        out = 0
                    else:
                        out = int(out)
                elif style.as_type in (bool, str):
                    out = style.as_type(out)

                # Add a suffix to string storage.
                if style.as_type in (str, unicode) and style.suffix:
                    out += style.suffix

            # MPEG-4 "freeform" (----) frames must be encoded as UTF-8
            # byte strings.
            if obj.type in MP4_TYPES and style.key.startswith('----:') and \
                    isinstance(out, unicode):
                out = out.encode('utf8')

            # Store the data.
            self._storedata(obj, out, style)

class CompositeDateField(object):
    """A MediaFile field for conveniently accessing the year, month, and
    day fields as a datetime.date object. Allows both getting and
    setting of the component fields.
    """
    def __init__(self, year_field, month_field, day_field):
        """Create a new date field from the indicated MediaFields for
        the component values.
        """
        self.year_field = year_field
        self.month_field = month_field
        self.day_field = day_field

    def __get__(self, obj, owner):
        """Return a datetime.date object whose components indicating the
        smallest valid date whose components are at least as large as
        the three component fields (that is, if year == 1999, month == 0,
        and day == 0, then date == datetime.date(1999, 1, 1)). If the
        components indicate an invalid date (e.g., if month == 47),
        datetime.date.min is returned.
        """
        try:
            return datetime.date(
                max(self.year_field.__get__(obj, owner), datetime.MINYEAR),
                max(self.month_field.__get__(obj, owner), 1),
                max(self.day_field.__get__(obj, owner), 1)
            )
        except ValueError:  # Out of range values.
            return datetime.date.min

    def __set__(self, obj, val):
        """Set the year, month, and day fields to match the components of
        the provided datetime.date object.
        """
        self.year_field.__set__(obj, val.year)
        self.month_field.__set__(obj, val.month)
        self.day_field.__set__(obj, val.day)

class ImageField(object):
    """A descriptor providing access to a file's embedded album art.
    Holds a bytestring reflecting the image data. The image should
    either be a JPEG or a PNG for cross-format compatibility. It's
    probably a bad idea to use anything but these two formats.
    """
    @classmethod
    def _mime(cls, data):
        """Return the MIME type (either image/png or image/jpeg) of the
        image data (a bytestring).
        """
        kind = imghdr.what(None, h=data)
        if kind == 'png':
            return 'image/png'
        else:
            # Currently just fall back to JPEG.
            return 'image/jpeg'

    @classmethod
    def _mp4kind(cls, data):
        """Return the MPEG-4 image type code of the data. If the image
        is not a PNG or JPEG, JPEG is assumed.
        """
        kind = imghdr.what(None, h=data)
        if kind == 'png':
            return mutagen.mp4.MP4Cover.FORMAT_PNG
        else:
            return mutagen.mp4.MP4Cover.FORMAT_JPEG

    def __get__(self, obj, owner):
        if obj.type == 'mp3':
            # Look for APIC frames.
            for frame in obj.mgfile.tags.values():
                if frame.FrameID == 'APIC':
                    picframe = frame
                    break
            else:
                # No APIC frame.
                return None

            return picframe.data

        elif obj.type in MP4_TYPES:
            if 'covr' in obj.mgfile:
                covers = obj.mgfile['covr']
                if covers:
                    cover = covers[0]
                    # cover is an MP4Cover, which is a subclass of str.
                    return cover

            # No cover found.
            return None

        elif obj.type == 'flac':
            pictures = obj.mgfile.pictures
            if pictures:
                return pictures[0].data or None
            else:
                return None
        
        elif obj.type == 'asf':
            if 'WM/Picture' in obj.mgfile:
                pictures = obj.mgfile['WM/Picture']
                if pictures:
                    data = pictures[0].value
                    try:
                        return _unpack_asf_image(data)[1]
                    except:
                        return None
            return None

        else:
            # Here we're assuming everything but MP3, MPEG-4, and FLAC
            # use the Xiph/Vorbis Comments standard. This may not be
            # valid. http://wiki.xiph.org/VorbisComment#Cover_art

            if 'metadata_block_picture' not in obj.mgfile:
                # Try legacy COVERART tags.
                if 'coverart' in obj.mgfile and obj.mgfile['coverart']:
                    return base64.b64decode(obj.mgfile['coverart'][0])
                return None

            for data in obj.mgfile["metadata_block_picture"]:
                try:
                    pic = mutagen.flac.Picture(base64.b64decode(data))
                    break
                except TypeError:
                    pass
            else:
                return None

            return pic.data

    def __set__(self, obj, val):
        if val is not None:
            if not isinstance(val, str):
                raise ValueError('value must be a byte string or None')

        if obj.type == 'mp3':
            # Clear all APIC frames.
            obj.mgfile.tags.delall('APIC')
            if val is None:
                # If we're clearing the image, we're done.
                return

            picframe = mutagen.id3.APIC(
                encoding = 3,
                mime = self._mime(val),
                type = 3, # front cover
                desc = u'',
                data = val,
            )
            obj.mgfile['APIC'] = picframe

        elif obj.type in MP4_TYPES:
            if val is None:
                if 'covr' in obj.mgfile:
                    del obj.mgfile['covr']
            else:
                cover = mutagen.mp4.MP4Cover(val, self._mp4kind(val))
                obj.mgfile['covr'] = [cover]

        elif obj.type == 'flac':
            obj.mgfile.clear_pictures()

            if val is not None:
                pic = mutagen.flac.Picture()
                pic.data = val
                pic.mime = self._mime(val)
                obj.mgfile.add_picture(pic)

        elif obj.type == 'asf':
            if 'WM/Picture' in obj.mgfile:
                del obj.mgfile['WM/Picture']

            if val is not None:
                pic = mutagen.asf.ASFByteArrayAttribute()
                pic.value = _pack_asf_image(self._mime(val), val)
                obj.mgfile['WM/Picture'] = [pic]

        else:
            # Again, assuming Vorbis Comments standard.

            # Strip all art, including legacy COVERART.
            if 'metadata_block_picture' in obj.mgfile:
                if 'metadata_block_picture' in obj.mgfile:
                    del obj.mgfile['metadata_block_picture']
                if 'coverart' in obj.mgfile:
                    del obj.mgfile['coverart']
                if 'coverartmime' in obj.mgfile:
                    del obj.mgfile['coverartmime']

            # Add new art if provided.
            if val is not None:
                pic = mutagen.flac.Picture()
                pic.data = val
                pic.mime = self._mime(val)
                obj.mgfile['metadata_block_picture'] = [
                    base64.b64encode(pic.write())
                ]


# The file (a collection of fields).

class MediaFile(object):
    """Represents a multimedia file on disk and provides access to its
    metadata.
    """
    def __init__(self, path):
        """Constructs a new MediaFile reflecting the file at path. May
        throw UnreadableFileError.
        """
        self.path = path

        unreadable_exc = (
            mutagen.mp3.HeaderNotFoundError,
            mutagen.flac.FLACNoHeaderError,
            mutagen.monkeysaudio.MonkeysAudioHeaderError,
            mutagen.mp4.MP4StreamInfoError,
            mutagen.oggvorbis.OggVorbisHeaderError,
            mutagen.asf.ASFHeaderError,
        )
        try:
            self.mgfile = mutagen.File(path)
        except unreadable_exc as exc:
            log.debug(u'header parsing failed: {0}'.format(unicode(exc)))
            raise UnreadableFileError('Mutagen could not read file')
        except IOError as exc:
            raise FileIOError(exc)
        except Exception as exc:
            # Hide bugs in Mutagen.
            log.debug(traceback.format_exc())
            log.error('uncaught Mutagen exception: {0}'.format(exc))
            raise UnreadableFileError('Mutagen raised an exception')

        if self.mgfile is None: # Mutagen couldn't guess the type
            raise FileTypeError('file type unsupported by Mutagen')
        elif type(self.mgfile).__name__ == 'M4A' or \
             type(self.mgfile).__name__ == 'MP4':
            # This hack differentiates aac and alac until we find a more
            # deterministic approach.
            if hasattr(self.mgfile.info, 'sample_rate') and \
               self.mgfile.info.sample_rate > 0:
                self.type = 'aac'
            else:
                self.type = 'alac'
        elif type(self.mgfile).__name__ == 'ID3' or \
             type(self.mgfile).__name__ == 'MP3':
            self.type = 'mp3'
        elif type(self.mgfile).__name__ == 'FLAC':
            self.type = 'flac'
        elif type(self.mgfile).__name__ == 'OggVorbis':
            self.type = 'ogg'
        elif type(self.mgfile).__name__ == 'MonkeysAudio':
            self.type = 'ape'
        elif type(self.mgfile).__name__ == 'WavPack':
            self.type = 'wv'
        elif type(self.mgfile).__name__ == 'Musepack':
            self.type = 'mpc'
        elif type(self.mgfile).__name__ == 'ASF':
            self.type = 'asf'
        else:
            raise FileTypeError('file type %s unsupported by MediaFile' %
                                type(self.mgfile).__name__)

        # add a set of tags if it's missing
        if self.mgfile.tags is None:
            self.mgfile.add_tags()

    def save(self):
        self.mgfile.save()

    def delete(self):
        """Remove the current metadata tag from the file.
        """
        try:
            self.mgfile.delete()
        except NotImplementedError:
            # For Mutagen types that don't support deletion (notably,
            # ASF), just delete each tag individually.
            for tag in self.mgfile.keys():
                del self.mgfile[tag]


    # Field definitions.

    title = MediaField(
        mp3 = StorageStyle('TIT2'),
        mp4 = StorageStyle("\xa9nam"),
        etc = StorageStyle('TITLE'),
        asf = StorageStyle('Title'),
    )
    artist = MediaField(
        mp3 = StorageStyle('TPE1'),
        mp4 = StorageStyle("\xa9ART"),
        etc = StorageStyle('ARTIST'),
        asf = StorageStyle('Author'),
    )
    album = MediaField(
        mp3 = StorageStyle('TALB'),
        mp4 = StorageStyle("\xa9alb"),
        etc = StorageStyle('ALBUM'),
        asf = StorageStyle('WM/AlbumTitle'),
    )
    genre = MediaField(
        mp3 = StorageStyle('TCON'),
        mp4 = StorageStyle("\xa9gen"),
        etc = StorageStyle('GENRE'),
        asf = StorageStyle('WM/Genre'),
    )
    composer = MediaField(
        mp3 = StorageStyle('TCOM'),
        mp4 = StorageStyle("\xa9wrt"),
        etc = StorageStyle('COMPOSER'),
        asf = StorageStyle('WM/Composer'),
    )
    grouping = MediaField(
        mp3 = StorageStyle('TIT1'),
        mp4 = StorageStyle("\xa9grp"),
        etc = StorageStyle('GROUPING'),
        asf = StorageStyle('WM/ContentGroupDescription'),
    )
    track = MediaField(out_type=int,
        mp3 = StorageStyle('TRCK', packing=packing.SLASHED, pack_pos=0),
        mp4 = StorageStyle('trkn', packing=packing.TUPLE, pack_pos=0),
        etc = [StorageStyle('TRACK'),
               StorageStyle('TRACKNUMBER')],
        asf = StorageStyle('WM/TrackNumber'),
    )
    tracktotal = MediaField(out_type=int,
        mp3 = StorageStyle('TRCK', packing=packing.SLASHED, pack_pos=1),
        mp4 = StorageStyle('trkn', packing=packing.TUPLE, pack_pos=1),
        etc = [StorageStyle('TRACKTOTAL'),
               StorageStyle('TRACKC'),
               StorageStyle('TOTALTRACKS')],
        asf = StorageStyle('TotalTracks'),
    )
    disc = MediaField(out_type=int,
        mp3 = StorageStyle('TPOS', packing=packing.SLASHED, pack_pos=0),
        mp4 = StorageStyle('disk', packing=packing.TUPLE, pack_pos=0),
        etc = [StorageStyle('DISC'),
               StorageStyle('DISCNUMBER')],
        asf = StorageStyle('WM/PartOfSet'),
    )
    disctotal = MediaField(out_type=int,
        mp3 = StorageStyle('TPOS', packing=packing.SLASHED, pack_pos=1),
        mp4 = StorageStyle('disk', packing=packing.TUPLE, pack_pos=1),
        etc = [StorageStyle('DISCTOTAL'),
               StorageStyle('DISCC'),
               StorageStyle('TOTALDISCS')],
        asf = StorageStyle('TotalDiscs'),
    )
    lyrics = MediaField(
        mp3 = StorageStyle('USLT', list_elem=False, id3_desc=u''),
        mp4 = StorageStyle("\xa9lyr"),
        etc = StorageStyle('LYRICS'),
        asf = StorageStyle('WM/Lyrics'),
    )
    comments = MediaField(
        mp3 = StorageStyle('COMM', id3_desc=u''),
        mp4 = StorageStyle("\xa9cmt"),
        etc = [StorageStyle('DESCRIPTION'),
               StorageStyle('COMMENT')],
        asf = StorageStyle('WM/Comments'),
    )
    bpm = MediaField(out_type=int,
        mp3 = StorageStyle('TBPM'),
        mp4 = StorageStyle('tmpo', as_type=int),
        etc = StorageStyle('BPM'),
        asf = StorageStyle('WM/BeatsPerMinute'),
    )
    comp = MediaField(out_type=bool,
        mp3 = StorageStyle('TCMP'),
        mp4 = StorageStyle('cpil', list_elem=False, as_type=bool),
        etc = StorageStyle('COMPILATION'),
        asf = StorageStyle('WM/IsCompilation', as_type=bool),
    )
    albumartist = MediaField(
        mp3 = StorageStyle('TPE2'),
        mp4 = StorageStyle('aART'),
        etc = [StorageStyle('ALBUM ARTIST'),
               StorageStyle('ALBUMARTIST')],
        asf = StorageStyle('WM/AlbumArtist'),
    )
    albumtype = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'MusicBrainz Album Type'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Album Type'),
        etc = StorageStyle('MUSICBRAINZ_ALBUMTYPE'),
        asf = StorageStyle('MusicBrainz/Album Type'),
    )
    label = MediaField(
        mp3 = StorageStyle('TPUB'),
        mp4 = [StorageStyle('----:com.apple.iTunes:Label'),
               StorageStyle('----:com.apple.iTunes:publisher')],
        etc = [StorageStyle('LABEL'),
               StorageStyle('PUBLISHER')],  # Traktor
        asf = StorageStyle('WM/Publisher'),
    )
    artist_sort = MediaField(
        mp3 = StorageStyle('TSOP'),
        mp4 = StorageStyle("soar"),
        etc = StorageStyle('ARTISTSORT'),
        asf = StorageStyle('WM/ArtistSortOrder'),
    )
    albumartist_sort = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'ALBUMARTISTSORT'),
        mp4 = StorageStyle("soaa"),
        etc = StorageStyle('ALBUMARTISTSORT'),
        asf = StorageStyle('WM/AlbumArtistSortOrder'),
    )
    asin = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'ASIN'),
        mp4 = StorageStyle("----:com.apple.iTunes:ASIN"),
        etc = StorageStyle('ASIN'),
        asf = StorageStyle('MusicBrainz/ASIN'),
    )
    catalognum = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'CATALOGNUMBER'),
        mp4 = StorageStyle("----:com.apple.iTunes:CATALOGNUMBER"),
        etc = StorageStyle('CATALOGNUMBER'),
        asf = StorageStyle('WM/CatalogNo'),
    )
    disctitle = MediaField(
        mp3 = StorageStyle('TSST'),
        mp4 = StorageStyle("----:com.apple.iTunes:DISCSUBTITLE"),
        etc = StorageStyle('DISCSUBTITLE'),
        asf = StorageStyle('WM/SetSubTitle'),
    )
    encoder = MediaField(
        mp3 = StorageStyle('TENC'),
        mp4 = StorageStyle("\xa9too"),
        etc = [StorageStyle('ENCODEDBY'),
               StorageStyle('ENCODER')],
        asf = StorageStyle('WM/EncodedBy'),
    )
    script = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'Script'),
        mp4 = StorageStyle("----:com.apple.iTunes:SCRIPT"),
        etc = StorageStyle('SCRIPT'),
        asf = StorageStyle('WM/Script'),
    )
    language = MediaField(
        mp3 = StorageStyle('TLAN'),
        mp4 = StorageStyle("----:com.apple.iTunes:LANGUAGE"),
        etc = StorageStyle('LANGUAGE'),
        asf = StorageStyle('WM/Language'),
    )
    country = MediaField(
        mp3 = StorageStyle('TXXX',
                           id3_desc=u'MusicBrainz Album Release Country'),
        mp4 = StorageStyle("----:com.apple.iTunes:MusicBrainz Album "
                           "Release Country"),
        etc = StorageStyle('RELEASECOUNTRY'),
        asf = StorageStyle('MusicBrainz/Album Release Country'),
    )
    albumstatus = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'MusicBrainz Album Status'),
        mp4 = StorageStyle("----:com.apple.iTunes:MusicBrainz Album Status"),
        etc = StorageStyle('MUSICBRAINZ_ALBUMSTATUS'),
        asf = StorageStyle('MusicBrainz/Album Status'),
    )
    media = MediaField(
        mp3 = StorageStyle('TMED'),
        mp4 = StorageStyle("----:com.apple.iTunes:MEDIA"),
        etc = StorageStyle('MEDIA'),
        asf = StorageStyle('WM/Media'),
    )
    albumdisambig = MediaField(
        # This tag mapping was invented for beets (not used by Picard, etc).
        mp3 = StorageStyle('TXXX', id3_desc=u'MusicBrainz Album Comment'),
        mp4 = StorageStyle("----:com.apple.iTunes:MusicBrainz Album Comment"),
        etc = StorageStyle('MUSICBRAINZ_ALBUMCOMMENT'),
        asf = StorageStyle('MusicBrainz/Album Comment'),
    )

    # Release date.
    year = MediaField(out_type=int,
        mp3 = StorageStyle('TDRC', packing=packing.DATE, pack_pos=0),
        mp4 = StorageStyle("\xa9day", packing=packing.DATE, pack_pos=0),
        etc = [StorageStyle('DATE', packing=packing.DATE, pack_pos=0),
               StorageStyle('YEAR')],
        asf = StorageStyle('WM/Year', packing=packing.DATE, pack_pos=0),
    )
    month = MediaField(out_type=int,
        mp3 = StorageStyle('TDRC', packing=packing.DATE, pack_pos=1),
        mp4 = StorageStyle("\xa9day", packing=packing.DATE, pack_pos=1),
        etc = StorageStyle('DATE', packing=packing.DATE, pack_pos=1),
        asf = StorageStyle('WM/Year', packing=packing.DATE, pack_pos=1),
    )
    day = MediaField(out_type=int,
        mp3 = StorageStyle('TDRC', packing=packing.DATE, pack_pos=2),
        mp4 = StorageStyle("\xa9day", packing=packing.DATE, pack_pos=2),
        etc = StorageStyle('DATE', packing=packing.DATE, pack_pos=2),
        asf = StorageStyle('WM/Year', packing=packing.DATE, pack_pos=2),
    )
    date = CompositeDateField(year, month, day)

    # *Original* release date.
    original_year = MediaField(out_type=int,
        mp3 = StorageStyle('TDOR', packing=packing.DATE, pack_pos=0),
        mp4 = StorageStyle('----:com.apple.iTunes:ORIGINAL YEAR',
                           packing=packing.DATE, pack_pos=0),
        etc = StorageStyle('ORIGINALDATE', packing=packing.DATE, pack_pos=0),
        asf = StorageStyle('WM/OriginalReleaseYear', packing=packing.DATE,
                           pack_pos=0),
    )
    original_month = MediaField(out_type=int,
        mp3 = StorageStyle('TDOR', packing=packing.DATE, pack_pos=1),
        mp4 = StorageStyle('----:com.apple.iTunes:ORIGINAL YEAR',
                           packing=packing.DATE, pack_pos=1),
        etc = StorageStyle('ORIGINALDATE', packing=packing.DATE, pack_pos=1),
        asf = StorageStyle('WM/OriginalReleaseYear', packing=packing.DATE,
                           pack_pos=1),
    )
    original_day = MediaField(out_type=int,
        mp3 = StorageStyle('TDOR', packing=packing.DATE, pack_pos=2),
        mp4 = StorageStyle('----:com.apple.iTunes:ORIGINAL YEAR',
                           packing=packing.DATE, pack_pos=2),
        etc = StorageStyle('ORIGINALDATE', packing=packing.DATE, pack_pos=2),
        asf = StorageStyle('WM/OriginalReleaseYear', packing=packing.DATE,
                           pack_pos=2),
    )
    original_date = CompositeDateField(original_year, original_month,
                                       original_day)

    # Nonstandard metadata.
    artist_credit = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'Artist Credit'),
        mp4 = StorageStyle("----:com.apple.iTunes:Artist Credit"),
        etc = StorageStyle('ARTIST_CREDIT'),
        asf = StorageStyle('beets/Artist Credit'),
    )
    albumartist_credit = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'Album Artist Credit'),
        mp4 = StorageStyle("----:com.apple.iTunes:Album Artist Credit"),
        etc = StorageStyle('ALBUMARTIST_CREDIT'),
        asf = StorageStyle('beets/Album Artist Credit'),
    )

    # Album art.
    art = ImageField()

    # MusicBrainz IDs.
    mb_trackid = MediaField(
        mp3 = StorageStyle('UFID:http://musicbrainz.org',
                            list_elem = False,
                            id3_frame_field = 'data'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Track Id',
                           as_type=str),
        etc = StorageStyle('MUSICBRAINZ_TRACKID'),
        asf = StorageStyle('MusicBrainz/Track Id'),
    )
    mb_albumid = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'MusicBrainz Album Id'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Album Id',
                           as_type=str),
        etc = StorageStyle('MUSICBRAINZ_ALBUMID'),
        asf = StorageStyle('MusicBrainz/Album Id'),
    )
    mb_artistid = MediaField(
        mp3 = StorageStyle('TXXX', id3_desc=u'MusicBrainz Artist Id'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Artist Id',
                           as_type=str),
        etc = StorageStyle('MUSICBRAINZ_ARTISTID'),
        asf = StorageStyle('MusicBrainz/Artist Id'),
    )
    mb_albumartistid = MediaField(
        mp3 = StorageStyle('TXXX',
                            id3_desc=u'MusicBrainz Album Artist Id'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Album Artist Id',
                           as_type=str),
        etc = StorageStyle('MUSICBRAINZ_ALBUMARTISTID'),
        asf = StorageStyle('MusicBrainz/Album Artist Id'),
    )
    mb_releasegroupid = MediaField(
        mp3 = StorageStyle('TXXX',
                            id3_desc=u'MusicBrainz Release Group Id'),
        mp4 = StorageStyle('----:com.apple.iTunes:MusicBrainz Release Group Id',
                           as_type=str),
        etc = StorageStyle('MUSICBRAINZ_RELEASEGROUPID'),
        asf = StorageStyle('MusicBrainz/Release Group Id'),
    )

    # Acoustid fields.
    acoustid_fingerprint = MediaField(
        mp3 = StorageStyle('TXXX',
                            id3_desc=u'Acoustid Fingerprint'),
        mp4 = StorageStyle('----:com.apple.iTunes:Acoustid Fingerprint',
                           as_type=str),
        etc = StorageStyle('ACOUSTID_FINGERPRINT'),
        asf = StorageStyle('Acoustid/Fingerprint'),
    )
    acoustid_id = MediaField(
        mp3 = StorageStyle('TXXX',
                            id3_desc=u'Acoustid Id'),
        mp4 = StorageStyle('----:com.apple.iTunes:Acoustid Id',
                           as_type=str),
        etc = StorageStyle('ACOUSTID_ID'),
        asf = StorageStyle('Acoustid/Id'),
    )

    # ReplayGain fields.
    rg_track_gain = MediaField(out_type=float,
        mp3 = [StorageStyle('TXXX', id3_desc=u'REPLAYGAIN_TRACK_GAIN',
                            float_places=2, suffix=u' dB'),
               StorageStyle('COMM', id3_desc=u'iTunNORM', id3_lang='eng',
                            packing=packing.SC, pack_pos=0, pack_type=float)],
        mp4 = [StorageStyle('----:com.apple.iTunes:replaygain_track_gain',
                            as_type=str, float_places=2, suffix=b' dB'),
               StorageStyle('----:com.apple.iTunes:iTunNORM', 
                            packing=packing.SC, pack_pos=0, pack_type=float)],
        etc = StorageStyle(u'REPLAYGAIN_TRACK_GAIN',
                           float_places=2, suffix=u' dB'),
        asf = StorageStyle(u'replaygain_track_gain',
                           float_places=2, suffix=u' dB'),
    )
    rg_album_gain = MediaField(out_type=float,
        mp3 = StorageStyle('TXXX', id3_desc=u'REPLAYGAIN_ALBUM_GAIN',
                            float_places=2, suffix=u' dB'),
        mp4 = StorageStyle('----:com.apple.iTunes:replaygain_album_gain',
                           as_type=str, float_places=2, suffix=b' dB'),
        etc = StorageStyle(u'REPLAYGAIN_ALBUM_GAIN',
                           float_places=2, suffix=u' dB'),
        asf = StorageStyle(u'replaygain_album_gain',
                           float_places=2, suffix=u' dB'),
    )
    rg_track_peak = MediaField(out_type=float,
        mp3 = [StorageStyle('TXXX', id3_desc=u'REPLAYGAIN_TRACK_PEAK',
                            float_places=6),
               StorageStyle('COMM', id3_desc=u'iTunNORM', id3_lang='eng',
                            packing=packing.SC, pack_pos=1, pack_type=float)],
        mp4 = [StorageStyle('----:com.apple.iTunes:replaygain_track_peak',
                            as_type=str, float_places=6),
               StorageStyle('----:com.apple.iTunes:iTunNORM',
                            packing=packing.SC, pack_pos=1, pack_type=float)],
        etc = StorageStyle(u'REPLAYGAIN_TRACK_PEAK',
                           float_places=6),
        asf = StorageStyle(u'replaygain_track_peak',
                           float_places=6),
    )
    rg_album_peak = MediaField(out_type=float,
        mp3 = StorageStyle('TXXX', id3_desc=u'REPLAYGAIN_ALBUM_PEAK',
                            float_places=6),
        mp4 = StorageStyle('----:com.apple.iTunes:replaygain_album_peak',
                           as_type=str, float_places=6),
        etc = StorageStyle(u'REPLAYGAIN_ALBUM_PEAK',
                           float_places=6),
        asf = StorageStyle(u'replaygain_album_peak',
                           float_places=6),
    )

    @property
    def length(self):
        """The duration of the audio in seconds (a float)."""
        return self.mgfile.info.length

    @property
    def samplerate(self):
        """The audio's sample rate (an int)."""
        if hasattr(self.mgfile.info, 'sample_rate'):
            return self.mgfile.info.sample_rate
        return 0

    @property
    def bitdepth(self):
        """The number of bits per sample in the audio encoding (an int).
        Only available for certain file formats (zero where
        unavailable).
        """
        if hasattr(self.mgfile.info, 'bits_per_sample'):
            return self.mgfile.info.bits_per_sample
        return 0

    @property
    def channels(self):
        """The number of channels in the audio (an int)."""
        if isinstance(self.mgfile.info, mutagen.mp3.MPEGInfo):
            return {
                mutagen.mp3.STEREO: 2,
                mutagen.mp3.JOINTSTEREO: 2,
                mutagen.mp3.DUALCHANNEL: 2,
                mutagen.mp3.MONO: 1,
            }[self.mgfile.info.mode]
        if hasattr(self.mgfile.info, 'channels'):
            return self.mgfile.info.channels
        return 0

    @property
    def bitrate(self):
        """The number of bits per seconds used in the audio coding (an
        int). If this is provided explicitly by the compressed file
        format, this is a precise reflection of the encoding. Otherwise,
        it is estimated from the on-disk file size. In this case, some
        imprecision is possible because the file header is incorporated
        in the file size.
        """
        if hasattr(self.mgfile.info, 'bitrate') and self.mgfile.info.bitrate:
            # Many formats provide it explicitly.
            return self.mgfile.info.bitrate
        else:
            # Otherwise, we calculate bitrate from the file size. (This
            # is the case for all of the lossless formats.)
            if not self.length:
                # Avoid division by zero if length is not available.
                return 0
            size = os.path.getsize(self.path)
            return int(size * 8 / self.length)

    @property
    def format(self):
        """A string describing the file format/codec."""
        return TYPES[self.type]