File: asdf.py

package info (click to toggle)
python-asdf 2.7.2-1
  • links: PTS, VCS
  • area: main
  • in suites: bullseye, sid
  • size: 3,248 kB
  • sloc: python: 13,104; makefile: 125
file content (1524 lines) | stat: -rw-r--r-- 56,101 bytes parent folder | download
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
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
# Licensed under a 3-clause BSD style license - see LICENSE.rst
# -*- coding: utf-8 -*-


import io
import os
import time
import copy
import datetime
import warnings
from pkg_resources import parse_version

import numpy as np
from jsonschema import ValidationError

from . import block
from . import constants
from . import generic_io
from . import reference
from . import schema
from . import treeutil
from . import util
from . import version
from . import versioning
from . import yamlutil
from . import _display as display
from .exceptions import AsdfDeprecationWarning, AsdfWarning, AsdfConversionWarning
from .extension import AsdfExtensionList, default_extensions
from .util import NotSet
from .search import AsdfSearchResult

from .tags.core import AsdfObject, Software, HistoryEntry, ExtensionMetadata


def get_asdf_library_info():
    """
    Get information about asdf to include in the asdf_library entry
    in the Tree.
    """
    return Software({
        'name': 'asdf',
        'version': version.version,
        'homepage': 'http://github.com/spacetelescope/asdf',
        'author': 'Space Telescope Science Institute'
    })


class AsdfFile(versioning.VersionedMixin):
    """
    The main class that represents an ASDF file object.
    """
    def __init__(self, tree=None, uri=None, extensions=None, version=None,
                 ignore_version_mismatch=True, ignore_unrecognized_tag=False,
                 ignore_implicit_conversion=False, copy_arrays=False,
                 lazy_load=True, custom_schema=None, _readonly=False):
        """
        Parameters
        ----------
        tree : dict or AsdfFile, optional
            The main tree data in the ASDF file.  Must conform to the
            ASDF schema.

        uri : str, optional
            The URI for this ASDF file.  Used to resolve relative
            references against.  If not provided, will be
            automatically determined from the associated file object,
            if possible and if created from `AsdfFile.open`.

        extensions : list of AsdfExtension
            A list of extensions to use when reading and writing ASDF files.
            See `~asdf.types.AsdfExtension` for more information.

        version : str, optional
            The ASDF version to use when writing out.  If not
            provided, it will write out in the latest version
            supported by asdf.

        ignore_version_mismatch : bool, optional
            When `True`, do not raise warnings for mismatched schema versions.
            Set to `True` by default.

        ignore_unrecognized_tag : bool, optional
            When `True`, do not raise warnings for unrecognized tags. Set to
            `False` by default.

        ignore_implicit_conversion : bool
            When `True`, do not raise warnings when types in the tree are
            implicitly converted into a serializable object. The motivating
            case for this is currently `namedtuple`, which cannot be serialized
            as-is.

        copy_arrays : bool, optional
            When `False`, when reading files, attempt to memmap underlying data
            arrays when possible.

        lazy_load : bool, optional
            When `True` and the underlying file handle is seekable, data
            arrays will only be loaded lazily: i.e. when they are accessed
            for the first time. In this case the underlying file must stay
            open during the lifetime of the tree. Setting to False causes
            all data arrays to be loaded up front, which means that they
            can be accessed even after the underlying file is closed.
            Note: even if `lazy_load` is `False`, `copy_arrays` is still taken
            into account.

        custom_schema : str, optional
            Path to a custom schema file that will be used for a secondary
            validation pass. This can be used to ensure that particular ASDF
            files follow custom conventions beyond those enforced by the
            standard.

        """
        self._extensions = []
        self._extension_metadata = {}
        self._process_extensions(extensions)

        if custom_schema is not None:
            self._custom_schema = schema._load_schema_cached(custom_schema, self.resolver, True, False)
        else:
            self._custom_schema = None

        self._ignore_version_mismatch = ignore_version_mismatch
        self._ignore_unrecognized_tag = ignore_unrecognized_tag
        self._ignore_implicit_conversion = ignore_implicit_conversion

        # Set of (string, string) tuples representing tag version mismatches
        # that we've already warned about for this file.
        self._warned_tag_pairs = set()

        self._file_format_version = None

        # Context of a call to treeutil.walk_and_modify, needed in the AsdfFile
        # in case walk_and_modify is re-entered by extension code (via
        # custom_tree_to_tagged_tree or tagged_tree_to_custom_tree).
        self._tree_modification_context = treeutil._TreeModificationContext()

        self._fd = None
        self._closed = False
        self._external_asdf_by_uri = {}
        self._blocks = block.BlockManager(
            self, copy_arrays=copy_arrays, lazy_load=lazy_load,
            readonly=_readonly)
        self._uri = None
        if tree is None:
            # Bypassing the tree property here, to avoid validating
            # an empty tree.
            self._tree = AsdfObject()
        elif isinstance(tree, AsdfFile):
            if self._extensions != tree._extensions:
                raise ValueError(
                    "Can not copy AsdfFile and change active extensions")
            self._uri = tree.uri
            # Set directly to self._tree (bypassing property), since
            # we can assume the other AsdfFile is already valid.
            self._tree = tree.tree
            self.run_modifying_hook('copy_to_new_asdf', validate=False)
            self.find_references()
        else:
            self.tree = tree
            self.find_references()
        if uri is not None:
            self._uri = uri

        self._comments = []

        if version is not None:
            self.version = version

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        self.close()

    def _check_extensions(self, tree, strict=False):
        if 'history' not in tree or not isinstance(tree['history'], dict) or \
                'extensions' not in tree['history']:
            return

        for extension in tree['history']['extensions']:
            filename = "'{}' ".format(self._fname) if self._fname else ''
            if extension.extension_class not in self._extension_metadata:
                msg = "File {}was created with extension '{}', which is " \
                    "not currently installed"
                if extension.software:
                    msg += " (from package {}-{})".format(
                        extension.software['name'],
                        extension.software['version'])
                fmt_msg = msg.format(filename, extension.extension_class)
                if strict:
                    raise RuntimeError(fmt_msg)
                else:
                    warnings.warn(fmt_msg, AsdfWarning)

            elif extension.software:
                installed = self._extension_metadata[extension.extension_class]
                # Local extensions may not have a real version
                if not installed[1]:
                    continue
                # Compare version in file metadata with installed version
                if parse_version(installed[1]) < parse_version(extension.software['version']):
                    msg = "File {}was created with extension '{}' from " \
                    "package {}-{}, but older version {}-{} is installed"
                    fmt_msg = msg.format(
                        filename, extension.extension_class,
                        extension.software['name'],
                        extension.software['version'],
                        installed[0], installed[1])
                    if strict:
                        raise RuntimeError(fmt_msg)
                    else:
                        warnings.warn(fmt_msg, AsdfWarning)

    def _process_extensions(self, extensions):
        if extensions is None or extensions == []:
            self._extensions = default_extensions.extension_list
            self._extension_metadata = default_extensions.package_metadata
            return

        if isinstance(extensions, AsdfExtensionList):
            self._extensions = extensions
            return

        if not isinstance(extensions, list):
            extensions = [extensions]

        # Process metadata about custom extensions
        for extension in extensions:
            ext_name = util.get_class_name(extension)
            self._extension_metadata[ext_name] = ('', '')

        extensions = default_extensions.extensions + extensions
        self._extensions = AsdfExtensionList(extensions)
        self._extension_metadata.update(default_extensions.package_metadata)

    def _update_extension_history(self):
        if self.version < versioning.NEW_HISTORY_FORMAT_MIN_VERSION:
            return

        if 'history' not in self.tree:
            self.tree['history'] = dict(extensions=[])
        # Support clients who are still using the old history format
        elif isinstance(self.tree['history'], list):
            histlist = self.tree['history']
            self.tree['history'] = dict(entries=histlist, extensions=[])
            warnings.warn("The ASDF history format has changed in order to "
                          "support metadata about extensions. History entries "
                          "should now be stored under tree['history']['entries'].",
                          AsdfWarning)
        elif 'extensions' not in self.tree['history']:
            self.tree['history']['extensions'] = []

        for extension in self.type_index.get_extensions_used():
            ext_name = util.get_class_name(extension)
            ext_meta = ExtensionMetadata(extension_class=ext_name)
            metadata = self._extension_metadata.get(ext_name)
            if metadata is not None:
                ext_meta['software'] = Software(name=metadata[0], version=metadata[1])

            for i, entry in enumerate(self.tree['history']['extensions']):
                # Update metadata about this extension if it already exists
                if entry.extension_class == ext_meta.extension_class:
                    self.tree['history']['extensions'][i] = ext_meta
                    break
            else:
                self.tree['history']['extensions'].append(ext_meta)

    @property
    def file_format_version(self):
        if self._file_format_version is None:
            return versioning.AsdfVersion(self.version_map['FILE_FORMAT'])
        else:
            return self._file_format_version

    def close(self):
        """
        Close the file handles associated with the `AsdfFile`.
        """
        if self._fd and not self._closed:
            # This is ok to always do because GenericFile knows
            # whether it "owns" the file and should close it.
            self._fd.close()
            self._fd = None
            self._closed = True
        for external in self._external_asdf_by_uri.values():
            external.close()
        self._external_asdf_by_uri.clear()
        self._blocks.close()

    def copy(self):
        return self.__class__(
            copy.deepcopy(self._tree),
            self._uri,
            self._extensions
        )

    __copy__ = __deepcopy__ = copy

    @property
    def uri(self):
        """
        Get the URI associated with the `AsdfFile`.

        In many cases, it is automatically determined from the file
        handle used to read or write the file.
        """
        if self._uri is not None:
            return self._uri
        if self._fd is not None:
            return self._fd._uri
        return None

    @property
    def tag_to_schema_resolver(self):
        warnings.warn(
            "The 'tag_to_schema_resolver' property is deprecated. Use "
            "'tag_mapping' instead.",
            AsdfDeprecationWarning)
        return self._extensions.tag_mapping

    @property
    def tag_mapping(self):
        return self._extensions.tag_mapping

    @property
    def url_mapping(self):
        return self._extensions.url_mapping

    @property
    def resolver(self):
        return self._extensions.resolver

    @property
    def type_index(self):
        return self._extensions.type_index

    def resolve_uri(self, uri):
        """
        Resolve a (possibly relative) URI against the URI of this ASDF
        file.  May be overridden by base classes to change how URIs
        are resolved.  This does not apply any `uri_mapping` that was
        passed to the constructor.

        Parameters
        ----------
        uri : str
            An absolute or relative URI to resolve against the URI of
            this ASDF file.

        Returns
        -------
        uri : str
            The resolved URI.
        """
        return generic_io.resolve_uri(self.uri, uri)

    def open_external(self, uri, do_not_fill_defaults=False):
        """
        Open an external ASDF file, from the given (possibly relative)
        URI.  There is a cache (internal to this ASDF file) that ensures
        each external ASDF file is loaded only once.

        Parameters
        ----------
        uri : str
            An absolute or relative URI to resolve against the URI of
            this ASDF file.

        do_not_fill_defaults : bool, optional
            When `True`, do not fill in missing default values.

        Returns
        -------
        asdffile : AsdfFile
            The external ASDF file.
        """
        # For a cache key, we want to ignore the "fragment" part.
        base_uri = util.get_base_uri(uri)
        resolved_uri = self.resolve_uri(base_uri)

        # A uri like "#" should resolve back to ourself.  In that case,
        # just return `self`.
        if resolved_uri == '' or resolved_uri == self.uri:
            return self

        asdffile = self._external_asdf_by_uri.get(resolved_uri)
        if asdffile is None:
            asdffile = open_asdf(
                resolved_uri,
                mode='r', do_not_fill_defaults=do_not_fill_defaults)
            self._external_asdf_by_uri[resolved_uri] = asdffile
        return asdffile

    @property
    def tree(self):
        """
        Get/set the tree of data in the ASDF file.

        When set, the tree will be validated against the ASDF schema.
        """
        if self._closed:
            raise OSError("Cannot access data from closed ASDF file")
        return self._tree

    @tree.setter
    def tree(self, tree):
        asdf_object = AsdfObject(tree)
        # Only perform custom validation if the tree is not empty
        self._validate(asdf_object, custom=bool(tree))
        self._tree = asdf_object

    def keys(self):
        return self.tree.keys()

    def __getitem__(self, key):
        return self.tree[key]

    def __setitem__(self, key, value):
        self.tree[key] = value

    def __contains__(self, item):
        return item in self.tree

    @property
    def comments(self):
        """
        Get the comments after the header, before the tree.
        """
        return self._comments

    def _validate(self, tree, custom=True, reading=False):
        if reading:
            # If we're validating on read then the tree
            # is already guaranteed to be in tagged form.
            tagged_tree = tree
        else:
            tagged_tree = yamlutil.custom_tree_to_tagged_tree(
                tree, self)
        schema.validate(tagged_tree, self, reading=reading)
        # Perform secondary validation pass if requested
        if custom and self._custom_schema:
            schema.validate(tagged_tree, self, self._custom_schema,
                            reading=reading)

    def validate(self):
        """
        Validate the current state of the tree against the ASDF schema.
        """
        self._validate(self._tree)

    def make_reference(self, path=[]):
        """
        Make a new reference to a part of this file's tree, that can be
        assigned as a reference to another tree.

        Parameters
        ----------
        path : list of str and int, optional
            The parts of the path pointing to an item in this tree.
            If omitted, points to the root of the tree.

        Returns
        -------
        reference : reference.Reference
            A reference object.

        Examples
        --------
        For the given AsdfFile ``ff``, add an external reference to the data in
        an external file::

            >>> import asdf
            >>> flat = asdf.open("http://stsci.edu/reference_files/flat.asdf")  # doctest: +SKIP
            >>> ff.tree['flat_field'] = flat.make_reference(['data'])  # doctest: +SKIP
        """
        return reference.make_reference(self, path)

    @property
    def blocks(self):
        """
        Get the block manager associated with the `AsdfFile`.
        """
        return self._blocks

    def set_array_storage(self, arr, array_storage):
        """
        Set the block type to use for the given array data.

        Parameters
        ----------
        arr : numpy.ndarray
            The array to set.  If multiple views of the array are in
            the tree, only the most recent block type setting will be
            used, since all views share a single block.

        array_storage : str
            Must be one of:

            - ``internal``: The default.  The array data will be
              stored in a binary block in the same ASDF file.

            - ``external``: Store the data in a binary block in a
              separate ASDF file.

            - ``inline``: Store the data as YAML inline in the tree.
        """
        block = self.blocks[arr]
        self.blocks.set_array_storage(block, array_storage)

    def get_array_storage(self, arr):
        """
        Get the block type for the given array data.

        Parameters
        ----------
        arr : numpy.ndarray
        """
        return self.blocks[arr].array_storage

    def set_array_compression(self, arr, compression):
        """
        Set the compression to use for the given array data.

        Parameters
        ----------
        arr : numpy.ndarray
            The array to set.  If multiple views of the array are in
            the tree, only the most recent compression setting will be
            used, since all views share a single block.

        compression : str or None
            Must be one of:

            - ``''`` or `None`: no compression

            - ``zlib``: Use zlib compression

            - ``bzp2``: Use bzip2 compression

            - ``lz4``: Use lz4 compression

            - ``''`` or `None`: no compression

            - ``input``: Use the same compression as in the file read.
              If there is no prior file, acts as None.

        """
        self.blocks[arr].output_compression = compression

    def get_array_compression(self, arr):
        """
        Get the compression type for the given array data.

        Parameters
        ----------
        arr : numpy.ndarray

        Returns
        -------
        compression : str or None
        """
        return self.blocks[arr].output_compression

    @classmethod
    def _parse_header_line(cls, line):
        """
        Parses the header line in a ASDF file to obtain the ASDF version.
        """
        parts = line.split()
        if len(parts) != 2 or parts[0] != constants.ASDF_MAGIC:
            raise ValueError("Does not appear to be a ASDF file.")

        try:
            version = versioning.AsdfVersion(parts[1].decode('ascii'))
        except ValueError:
            raise ValueError(
                "Unparseable version in ASDF file: {0}".format(parts[1]))

        return version

    @classmethod
    def _parse_comment_section(cls, content):
        """
        Parses the comment section, between the header line and the
        Tree or first block.
        """
        comments = []

        lines = content.splitlines()
        for line in lines:
            if not line.startswith(b'#'):
                raise ValueError("Invalid content between header and tree")
            comments.append(line[1:].strip())

        return comments

    @classmethod
    def _find_asdf_version_in_comments(cls, comments):
        for comment in comments:
            parts = comment.split()
            if len(parts) == 2 and parts[0] == constants.ASDF_STANDARD_COMMENT:
                try:
                    version = versioning.AsdfVersion(parts[1].decode('ascii'))
                except ValueError:
                    pass
                else:
                    return version

        return None

    @classmethod
    def _open_asdf(cls, self, fd, uri=None, mode='r',
                   validate_checksums=False,
                   do_not_fill_defaults=False,
                   _get_yaml_content=False,
                   _force_raw_types=False,
                   strict_extension_check=False,
                   ignore_missing_extensions=False,
                   validate_on_read=True):
        """Attempt to populate AsdfFile data from file-like object"""

        if strict_extension_check and ignore_missing_extensions:
            raise ValueError(
                "'strict_extension_check' and 'ignore_missing_extensions' are "
                "incompatible options")

        self._mode = mode

        fd = generic_io.get_file(fd, mode=self._mode, uri=uri)
        self._fd = fd
        # The filename is currently only used for tracing warning information
        self._fname = self._fd._uri if self._fd._uri else ''
        header_line = fd.read_until(b'\r?\n', 2, "newline", include=True)
        self._file_format_version = cls._parse_header_line(header_line)
        self.version = self._file_format_version

        comment_section = fd.read_until(
            b'(%YAML)|(' + constants.BLOCK_MAGIC + b')', 5,
            "start of content", include=False, exception=False)
        self._comments = cls._parse_comment_section(comment_section)

        version = cls._find_asdf_version_in_comments(self._comments)
        if version is not None:
            self.version = version

        yaml_token = fd.read(4)
        has_blocks = False
        tree = None
        if yaml_token == b'%YAM':
            reader = fd.reader_until(
                constants.YAML_END_MARKER_REGEX, 7, 'End of YAML marker',
                include=True, initial_content=yaml_token)

            # For testing: just return the raw YAML content
            if _get_yaml_content:
                yaml_content = reader.read()
                fd.close()
                return yaml_content

            # We parse the YAML content into basic data structures
            # now, but we don't do anything special with it until
            # after the blocks have been read
            tree = yamlutil.load_tree(reader)
            has_blocks = fd.seek_until(constants.BLOCK_MAGIC, 4, include=True)
        elif yaml_token == constants.BLOCK_MAGIC:
            has_blocks = True
        elif yaml_token != b'':
            raise IOError("ASDF file appears to contain garbage after header.")

        if tree is None:
            # At this point the tree should be tagged, but we want it to be
            # tagged with the core/asdf version appropriate to this file's
            # ASDF Standard version.  We're using custom_tree_to_tagged_tree
            # to select the correct tag for us.
            tree = yamlutil.custom_tree_to_tagged_tree(AsdfObject(), self)

        if has_blocks:
            self._blocks.read_internal_blocks(
                fd, past_magic=True, validate_checksums=validate_checksums)
            self._blocks.read_block_index(fd, self)

        tree = reference.find_references(tree, self)
        if not do_not_fill_defaults:
            schema.fill_defaults(tree, self, reading=True)

        if validate_on_read:
            try:
                self._validate(tree, reading=True)
            except ValidationError:
                self.close()
                raise

        tree = yamlutil.tagged_tree_to_custom_tree(tree, self, _force_raw_types)

        if not (ignore_missing_extensions or _force_raw_types):
            self._check_extensions(tree, strict=strict_extension_check)

        self._tree = tree
        self.run_hook('post_read')

        return self

    @classmethod
    def _open_impl(cls, self, fd, uri=None, mode='r',
                   validate_checksums=False,
                   do_not_fill_defaults=False,
                   _get_yaml_content=False,
                   _force_raw_types=False,
                   strict_extension_check=False,
                   ignore_missing_extensions=False,
                   validate_on_read=True):
        """Attempt to open file-like object as either AsdfFile or AsdfInFits"""
        if not is_asdf_file(fd):
            try:
                # TODO: this feels a bit circular, try to clean up. Also
                # this introduces another dependency on astropy which may
                # not be desireable.
                from . import fits_embed
                return fits_embed.AsdfInFits._open_impl(fd, uri=uri,
                            validate_checksums=validate_checksums,
                            ignore_version_mismatch=self._ignore_version_mismatch,
                            extensions=self._extensions,
                            strict_extension_check=strict_extension_check,
                            ignore_missing_extensions=ignore_missing_extensions,
                            ignore_unrecognized_tag=self._ignore_unrecognized_tag,
                            _extension_metadata=self._extension_metadata,
                            validate_on_read=validate_on_read)
            except ValueError:
                raise ValueError(
                    "Input object does not appear to be an ASDF file or a FITS with " +
                    "ASDF extension") from None
            except ImportError:
                raise ValueError(
                    "Input object does not appear to be an ASDF file. Cannot check " +
                    "if it is a FITS with ASDF extension because 'astropy' is not " +
                    "installed") from None
        return cls._open_asdf(self, fd, uri=uri, mode=mode,
                validate_checksums=validate_checksums,
                do_not_fill_defaults=do_not_fill_defaults,
                _get_yaml_content=_get_yaml_content,
                _force_raw_types=_force_raw_types,
                strict_extension_check=strict_extension_check,
                ignore_missing_extensions=ignore_missing_extensions,
                validate_on_read=validate_on_read)

    @classmethod
    def open(cls, fd, uri=None, mode='r',
             validate_checksums=False,
             extensions=None,
             do_not_fill_defaults=False,
             ignore_version_mismatch=True,
             ignore_unrecognized_tag=False,
             _force_raw_types=False,
             copy_arrays=False,
             lazy_load=True,
             custom_schema=None,
             strict_extension_check=False,
             ignore_missing_extensions=False):
        """
        Open an existing ASDF file.

        .. deprecated:: 2.2
            Use `asdf.open` instead.
        """

        warnings.warn(
            "The method AsdfFile.open has been deprecated and will be removed "
            "in asdf-3.0. Use the top-level asdf.open function instead.",
            AsdfDeprecationWarning)

        return open_asdf(
            fd, uri=uri, mode=mode,
            validate_checksums=validate_checksums,
            extensions=extensions,
            do_not_fill_defaults=do_not_fill_defaults,
            ignore_version_mismatch=ignore_version_mismatch,
            ignore_unrecognized_tag=ignore_unrecognized_tag,
            _force_raw_types=_force_raw_types,
            copy_arrays=copy_arrays, lazy_load=lazy_load,
            custom_schema=custom_schema,
            strict_extension_check=strict_extension_check,
            ignore_missing_extensions=ignore_missing_extensions,
            _compat=True)

    def _write_tree(self, tree, fd, pad_blocks):
        fd.write(constants.ASDF_MAGIC)
        fd.write(b' ')
        fd.write(self.version_map['FILE_FORMAT'].encode('ascii'))
        fd.write(b'\n')

        fd.write(b'#')
        fd.write(constants.ASDF_STANDARD_COMMENT)
        fd.write(b' ')
        fd.write(self.version_string.encode('ascii'))
        fd.write(b'\n')

        if len(tree):
            yamlutil.dump_tree(tree, fd, self)

        if pad_blocks:
            padding = util.calculate_padding(
                fd.tell(), pad_blocks, fd.block_size)
            fd.fast_forward(padding)

    def _pre_write(self, fd, all_array_storage, all_array_compression,
                   auto_inline):
        if all_array_storage not in (None, 'internal', 'external', 'inline'):
            raise ValueError(
                "Invalid value for all_array_storage: '{0}'".format(
                    all_array_storage))
        self._all_array_storage = all_array_storage

        self._all_array_compression = all_array_compression

        if auto_inline in (True, False):
            raise ValueError(
                "Invalid value for auto_inline: '{0}'".format(auto_inline))
        if auto_inline is not None:
            try:
                self._auto_inline = int(auto_inline)
            except ValueError:
                raise ValueError(
                    "Invalid value for auto_inline: '{0}'".format(auto_inline))
        else:
            self._auto_inline = None

        if len(self._tree):
            self.run_hook('pre_write')

        # This is where we'd do some more sophisticated block
        # reorganization, if necessary
        self._blocks.finalize(self)

        self._tree['asdf_library'] = get_asdf_library_info()
        self._update_extension_history()

    def _serial_write(self, fd, pad_blocks, include_block_index):
        self._write_tree(self._tree, fd, pad_blocks)
        self.blocks.write_internal_blocks_serial(fd, pad_blocks)
        self.blocks.write_external_blocks(fd.uri, pad_blocks)
        if include_block_index:
            self.blocks.write_block_index(fd, self)

    def _random_write(self, fd, pad_blocks, include_block_index):
        self._write_tree(self._tree, fd, False)
        self.blocks.write_internal_blocks_random_access(fd)
        self.blocks.write_external_blocks(fd.uri, pad_blocks)
        if include_block_index:
            self.blocks.write_block_index(fd, self)
        fd.truncate()

    def _post_write(self, fd):
        if len(self._tree):
            self.run_hook('post_write')

        # TODO: there has got to be a better way to do this...
        if hasattr(self, '_all_array_storage'):
            del self._all_array_storage
        if hasattr(self, '_all_array_compression'):
            del self._all_array_compression
        if hasattr(self, '_auto_inline'):
            del self._auto_inline

    def update(self, all_array_storage=None, all_array_compression='input',
               auto_inline=None, pad_blocks=False, include_block_index=True,
               version=None):
        """
        Update the file on disk in place.

        Parameters
        ----------
        all_array_storage : string, optional
            If provided, override the array storage type of all blocks
            in the file immediately before writing.  Must be one of:

            - ``internal``: The default.  The array data will be
              stored in a binary block in the same ASDF file.

            - ``external``: Store the data in a binary block in a
              separate ASDF file.

            - ``inline``: Store the data as YAML inline in the tree.

        all_array_compression : string, optional
            If provided, set the compression type on all binary blocks
            in the file.  Must be one of:

            - ``''`` or `None`: No compression.

            - ``zlib``: Use zlib compression.

            - ``bzp2``: Use bzip2 compression.

            - ``lz4``: Use lz4 compression.

            - ``input``: Use the same compression as in the file read.
              If there is no prior file, acts as None

        auto_inline : int, optional
            When the number of elements in an array is less than this
            threshold, store the array as inline YAML, rather than a
            binary block.  This only works on arrays that do not share
            data with other arrays.  Default is 0.

        pad_blocks : float or bool, optional
            Add extra space between blocks to allow for updating of
            the file.  If `False` (default), add no padding (always
            return 0).  If `True`, add a default amount of padding of
            10% If a float, it is a factor to multiple content_size by
            to get the new total size.

        include_block_index : bool, optional
            If `False`, don't include a block index at the end of the
            file.  (Default: `True`)  A block index is never written
            if the file has a streamed block.

        version : str, optional
            The ASDF version to write out.  If not provided, it will
            write out in the latest version supported by asdf.
        """

        fd = self._fd

        if fd is None:
            raise ValueError(
                "Can not update, since there is no associated file")

        if not fd.writable():
            raise IOError(
                "Can not update, since associated file is read-only. Make "
                "sure that the AsdfFile was opened with mode='rw' and the "
                "underlying file handle is writable.")

        if version is not None:
            self.version = version

        if all_array_storage == 'external':
            # If the file is fully exploded, there's no benefit to
            # update, so just use write_to()
            self.write_to(fd, all_array_storage=all_array_storage)
            fd.truncate()
            return

        if not fd.seekable():
            raise IOError(
                "Can not update, since associated file is not seekable")

        self.blocks.finish_reading_internal_blocks()

        self._pre_write(fd, all_array_storage, all_array_compression,
                        auto_inline)

        try:
            fd.seek(0)

            if not self.blocks.has_blocks_with_offset():
                # If we don't have any blocks that are being reused, just
                # write out in a serial fashion.
                self._serial_write(fd, pad_blocks, include_block_index)
                fd.truncate()
                return

            # Estimate how big the tree will be on disk by writing the
            # YAML out in memory.  Since the block indices aren't yet
            # known, we have to count the number of block references and
            # add enough space to accommodate the largest block number
            # possible there.
            tree_serialized = io.BytesIO()
            self._write_tree(self._tree, tree_serialized, pad_blocks=False)
            array_ref_count = [0]
            from .tags.core.ndarray import NDArrayType

            for node in treeutil.iter_tree(self._tree):
                if (isinstance(node, (np.ndarray, NDArrayType)) and
                    self.blocks[node].array_storage == 'internal'):
                    array_ref_count[0] += 1

            serialized_tree_size = (
                tree_serialized.tell() +
                constants.MAX_BLOCKS_DIGITS * array_ref_count[0])

            if not block.calculate_updated_layout(
                    self.blocks, serialized_tree_size,
                    pad_blocks, fd.block_size):
                # If we don't have any blocks that are being reused, just
                # write out in a serial fashion.
                self._serial_write(fd, pad_blocks, include_block_index)
                fd.truncate()
                return

            fd.seek(0)
            self._random_write(fd, pad_blocks, include_block_index)
            fd.flush()
        finally:
            self._post_write(fd)

    def write_to(self, fd, all_array_storage=None, all_array_compression='input',
                 auto_inline=None, pad_blocks=False, include_block_index=True,
                 version=None):
        """
        Write the ASDF file to the given file-like object.

        `write_to` does not change the underlying file descriptor in
        the `AsdfFile` object, but merely copies the content to a new
        file.

        Parameters
        ----------
        fd : string or file-like object
            May be a string path to a file, or a Python file-like
            object.  If a string path, the file is automatically
            closed after writing.  If not a string path,

        all_array_storage : string, optional
            If provided, override the array storage type of all blocks
            in the file immediately before writing.  Must be one of:

            - ``internal``: The default.  The array data will be
              stored in a binary block in the same ASDF file.

            - ``external``: Store the data in a binary block in a
              separate ASDF file.

            - ``inline``: Store the data as YAML inline in the tree.

        all_array_compression : string, optional
            If provided, set the compression type on all binary blocks
            in the file.  Must be one of:

            - ``''`` or `None`: No compression.

            - ``zlib``: Use zlib compression.

            - ``bzp2``: Use bzip2 compression.

            - ``lz4``: Use lz4 compression.

            - ``input``: Use the same compression as in the file read.
              If there is no prior file, acts as None.

        auto_inline : int, optional
            When the number of elements in an array is less than this
            threshold, store the array as inline YAML, rather than a
            binary block.  This only works on arrays that do not share
            data with other arrays.  Default is 0.

        pad_blocks : float or bool, optional
            Add extra space between blocks to allow for updating of
            the file.  If `False` (default), add no padding (always
            return 0).  If `True`, add a default amount of padding of
            10% If a float, it is a factor to multiple content_size by
            to get the new total size.

        include_block_index : bool, optional
            If `False`, don't include a block index at the end of the
            file.  (Default: `True`)  A block index is never written
            if the file has a streamed block.

        version : str, optional
            The ASDF version to write out.  If not provided, it will
            write out in the latest version supported by asdf.
        """

        if version is not None:
            self.version = version

        with generic_io.get_file(fd, mode='w') as fd:
            # TODO: This is not ideal: we really should pass the URI through
            # explicitly to wherever it is required instead of making it an
            # attribute of the AsdfFile.
            if self._uri is None:
                self._uri = fd.uri
            self._pre_write(fd, all_array_storage, all_array_compression,
                            auto_inline)

            try:
                self._serial_write(fd, pad_blocks, include_block_index)
                fd.flush()
            finally:
                self._post_write(fd)

    def find_references(self):
        """
        Finds all external "JSON References" in the tree and converts
        them to `reference.Reference` objects.
        """
        # Set directly to self._tree, since it doesn't need to be re-validated.
        self._tree = reference.find_references(self._tree, self)

    def resolve_references(self, do_not_fill_defaults=False):
        """
        Finds all external "JSON References" in the tree, loads the
        external content, and places it directly in the tree.  Saving
        a ASDF file after this operation means it will have no
        external references, and will be completely self-contained.
        """
        # Set to the property self.tree so the resulting "complete"
        # tree will be validated.
        self.tree = reference.resolve_references(self._tree, self)

    def run_hook(self, hookname):
        """
        Run a "hook" for each custom type found in the tree.

        Parameters
        ----------
        hookname : str
            The name of the hook.  If a `AsdfType` is found with a method
            with this name, it will be called for every instance of the
            corresponding custom type in the tree.
        """
        type_index = self.type_index

        if not type_index.has_hook(hookname):
            return

        for node in treeutil.iter_tree(self._tree):
            hook = type_index.get_hook_for_type(hookname, type(node),
                                                self.version_string)
            if hook is not None:
                hook(node, self)

    def run_modifying_hook(self, hookname, validate=True):
        """
        Run a "hook" for each custom type found in the tree.  The hook
        is free to return a different object in order to modify the
        tree.

        Parameters
        ----------
        hookname : str
            The name of the hook.  If a `AsdfType` is found with a method
            with this name, it will be called for every instance of the
            corresponding custom type in the tree.

        validate : bool
            When `True` (default) validate the resulting tree.
        """
        type_index = self.type_index

        if not type_index.has_hook(hookname):
            return

        def walker(node):
            hook = type_index.get_hook_for_type(hookname, type(node),
                                                self.version_string)
            if hook is not None:
                return hook(node, self)
            return node
        tree = treeutil.walk_and_modify(self.tree, walker,
            ignore_implicit_conversion=self._ignore_implicit_conversion)

        if validate:
            self._validate(tree)
        self._tree = tree
        return self._tree

    def resolve_and_inline(self):
        """
        Resolves all external references and inlines all data.  This
        produces something that, when saved, is a 100% valid YAML
        file.
        """
        self.blocks.finish_reading_internal_blocks()
        self.resolve_references()
        for b in list(self.blocks.blocks):
            self.blocks.set_array_storage(b, 'inline')

    def fill_defaults(self):
        """
        Fill in any values that are missing in the tree using default
        values from the schema.
        """
        tree = yamlutil.custom_tree_to_tagged_tree(self._tree, self)
        schema.fill_defaults(tree, self)
        self._tree = yamlutil.tagged_tree_to_custom_tree(tree, self)

    def remove_defaults(self):
        """
        Remove any values in the tree that are the same as the default
        values in the schema
        """
        tree = yamlutil.custom_tree_to_tagged_tree(self._tree, self)
        schema.remove_defaults(tree, self)
        self._tree = yamlutil.tagged_tree_to_custom_tree(tree, self)

    def add_history_entry(self, description, software=None):
        """
        Add an entry to the history list.

        Parameters
        ----------
        description : str
            A description of the change.

        software : dict or list of dict
            A description of the software used.  It should not include
            asdf itself, as that is automatically notated in the
            `asdf_library` entry.

            Each dict must have the following keys:

            - ``name``: The name of the software
            - ``author``: The author or institution that produced the software
            - ``homepage``: A URI to the homepage of the software
            - ``version``: The version of the software
        """
        if isinstance(software, list):
            software = [Software(x) for x in software]
        elif software is not None:
            software = Software(software)

        time_ = datetime.datetime.utcfromtimestamp(
            int(os.environ.get('SOURCE_DATE_EPOCH', time.time())),
        )

        entry = HistoryEntry({
            'description': description,
            'time': time_,
        })

        if software is not None:
            entry['software'] = software

        if self.version >= versioning.NEW_HISTORY_FORMAT_MIN_VERSION:
            if 'history' not in self.tree:
                self.tree['history'] = dict(entries=[])
            elif 'entries' not in self.tree['history']:
                self.tree['history']['entries'] = []

            self.tree['history']['entries'].append(entry)

            try:
                self.validate()
            except Exception:
                self.tree['history']['entries'].pop()
                raise
        else:
            if 'history' not in self.tree:
                self.tree['history'] = []

            self.tree['history'].append(entry)

            try:
                self.validate()
            except Exception:
                self.tree['history'].pop()
                raise

    def get_history_entries(self):
        """
        Get a list of history entries from the file object.

        Returns
        -------
        entries : list
            A list of history entries.
        """

        if 'history' not in self.tree:
            return []

        if isinstance(self.tree['history'], list):
            return self.tree['history']

        if 'entries' in self.tree['history']:
            return self.tree['history']['entries']

        return []

    def info(self, max_rows=display.DEFAULT_MAX_ROWS, max_cols=display.DEFAULT_MAX_COLS, show_values=display.DEFAULT_SHOW_VALUES):
        """
        Print a rendering of this file's tree to stdout.

        Parameters
        ----------
        max_rows : int, tuple, or None, optional
            Maximum number of lines to print.  Nodes that cannot be
            displayed will be elided with a message.
            If int, constrain total number of displayed lines.
            If tuple, constrain lines per node at the depth corresponding \
                to the tuple index.
            If None, display all lines.

        max_cols : int or None, optional
            Maximum length of line to print.  Nodes that cannot
            be fully displayed will be truncated with a message.
            If int, constrain length of displayed lines.
            If None, line length is unconstrained.

        show_values : bool, optional
            Set to False to disable display of primitive values in
            the rendered tree.
        """
        lines = display.render_tree(self.tree, max_rows=max_rows, max_cols=max_cols, show_values=show_values, identifier="root.tree")
        print("\n".join(lines))

    def search(self, key=NotSet, type=NotSet, value=NotSet, filter=None):
        """
        Search this file's tree.

        Parameters
        ----------
        key : NotSet, str, or any other object
            Search query that selects nodes by dict key or list index.
            If NotSet, the node key is unconstrained.
            If str, the input is searched among keys/indexes as a regular
            expression pattern.
            If any other object, node's key or index must equal the queried key.

        type : NotSet, str, or builtins.type
            Search query that selects nodes by type.
            If NotSet, the node type is unconstrained.
            If str, the input is searched among (fully qualified) node type
            names as a regular expression pattern.
            If builtins.type, the node must be an instance of the input.

        value : NotSet, str, or any other object
            Search query that selects nodes by value.
            If NotSet, the node value is unconstrained.
            If str, the input is searched among values as a regular
            expression pattern.
            If any other object, node's value must equal the queried value.

        filter : callable
            Callable that filters nodes by arbitrary criteria.
            The callable accepts one or two arguments:

            - the node
            - the node's list index or dict key (optional)

            and returns True to retain the node, or False to remove it from
            the search results.

        Returns
        -------
        asdf.search.AsdfSearchResult
            the result of the search
        """
        result = AsdfSearchResult(["root.tree"], self.tree)
        return result.search(key=key, type=type, value=value, filter=filter)

    # This function is called from within TypeIndex when deserializing
    # the tree for this file.  It is kept here so that we can keep
    # state on the AsdfFile and prevent a flood of warnings for the
    # same tag.
    def _warn_tag_mismatch(self, tag, best_tag):
        if not self._ignore_version_mismatch and (tag, best_tag) not in self._warned_tag_pairs:
            message = (
                "No explicit ExtensionType support provided for tag '{}'. "
                "The ExtensionType subclass for tag '{}' will be used instead. "
                "This fallback behavior will be removed in asdf 3.0."
            ).format(tag, best_tag)
            warnings.warn(message, AsdfConversionWarning)
            self._warned_tag_pairs.add((tag, best_tag))


# Inherit docstring from dictionary
AsdfFile.keys.__doc__ = dict.keys.__doc__


def _check_and_set_mode(fileobj, asdf_mode):

    if asdf_mode is not None and asdf_mode not in ['r', 'rw']:
        msg = "Unrecognized asdf mode '{}'. Must be either 'r' or 'rw'"
        raise ValueError(msg.format(asdf_mode))

    if asdf_mode is None:
        if isinstance(fileobj, io.IOBase):
            return 'rw' if fileobj.writable() else 'r'

        if isinstance(fileobj, generic_io.GenericFile):
            return fileobj.mode

        # This is the safest assumption for the default fallback
        return 'r'

    return asdf_mode


def open_asdf(fd, uri=None, mode=None, validate_checksums=False,
              extensions=None, do_not_fill_defaults=False,
              ignore_version_mismatch=True, ignore_unrecognized_tag=False,
              _force_raw_types=False, copy_arrays=False, lazy_load=True,
              custom_schema=None, strict_extension_check=False,
              ignore_missing_extensions=False, validate_on_read=True,
              _compat=False):
    """
    Open an existing ASDF file.

    Parameters
    ----------
    fd : string or file-like object
        May be a string ``file`` or ``http`` URI, or a Python
        file-like object.

    uri : string, optional
        The URI of the file.  Only required if the URI can not be
        automatically determined from `fd`.

    mode : string, optional
        The mode to open the file in.  Must be ``r`` (default) or
        ``rw``.

    validate_checksums : bool, optional
        If `True`, validate the blocks against their checksums.
        Requires reading the entire file, so disabled by default.

    extensions : list of AsdfExtension
        A list of extensions to use when reading and writing ASDF files.
        See `~asdf.types.AsdfExtension` for more information.

    do_not_fill_defaults : bool, optional
        When `True`, do not fill in missing default values.

    ignore_version_mismatch : bool, optional
        When `True`, do not raise warnings for mismatched schema versions.
        Set to `True` by default.

    ignore_unrecognized_tag : bool, optional
        When `True`, do not raise warnings for unrecognized tags. Set to
        `False` by default.

    copy_arrays : bool, optional
        When `False`, when reading files, attempt to memmap underlying data
        arrays when possible.

    lazy_load : bool, optional
        When `True` and the underlying file handle is seekable, data
        arrays will only be loaded lazily: i.e. when they are accessed
        for the first time. In this case the underlying file must stay
        open during the lifetime of the tree. Setting to False causes
        all data arrays to be loaded up front, which means that they
        can be accessed even after the underlying file is closed.
        Note: even if `lazy_load` is `False`, `copy_arrays` is still taken
        into account.

    custom_schema : str, optional
        Path to a custom schema file that will be used for a secondary
        validation pass. This can be used to ensure that particular ASDF
        files follow custom conventions beyond those enforced by the
        standard.

    strict_extension_check : bool, optional
        When `True`, if the given ASDF file contains metadata about the
        extensions used to create it, and if those extensions are not
        installed, opening the file will fail. When `False`, opening a file
        under such conditions will cause only a warning. Defaults to
        `False`.

    ignore_missing_extensions : bool, optional
        When `True`, do not raise warnings when a file is read that
        contains metadata about extensions that are not available. Defaults
        to `False`.

    validate_on_read : bool, optional
        When `True`, validate the newly opened file against tag and custom
        schemas.  Recommended unless the file is already known to be valid.

    Returns
    -------
    asdffile : AsdfFile
        The new AsdfFile object.
    """

    readonly = False

    # For now retain backwards compatibility with the old API behavior,
    # specifically when being called from AsdfFile.open
    if not _compat:
        mode = _check_and_set_mode(fd, mode)
        readonly = (mode == 'r' and not copy_arrays)

    instance = AsdfFile(extensions=extensions,
                   ignore_version_mismatch=ignore_version_mismatch,
                   ignore_unrecognized_tag=ignore_unrecognized_tag,
                   copy_arrays=copy_arrays, lazy_load=lazy_load,
                   custom_schema=custom_schema, _readonly=readonly)

    return AsdfFile._open_impl(instance,
        fd, uri=uri, mode=mode,
        validate_checksums=validate_checksums,
        do_not_fill_defaults=do_not_fill_defaults,
        _force_raw_types=_force_raw_types,
        strict_extension_check=strict_extension_check,
        ignore_missing_extensions=ignore_missing_extensions,
        validate_on_read=validate_on_read)


def is_asdf_file(fd):
    """
    Determine if fd is an ASDF file.

    Reads the first five bytes and looks for the ``#ASDF`` string.

    Parameters
    ----------
    fd : str, `~asdf.generic_io.GenericFile`

    """
    if isinstance(fd, generic_io.InputStream):
        # If it's an InputStream let ASDF deal with it.
        return True

    to_close = False
    if isinstance(fd, AsdfFile):
        return True
    elif isinstance(fd, generic_io.GenericFile):
        pass
    else:
        try:
            fd = generic_io.get_file(fd, mode='r', uri=None)
            if not isinstance(fd, io.IOBase):
                to_close = True
        except ValueError:
            return False
    asdf_magic = fd.read(5)
    if fd.seekable():
        fd.seek(0)
    if to_close:
        fd.close()
    if asdf_magic == constants.ASDF_MAGIC:
        return True
    return False