summaryrefslogtreecommitdiffstats
path: root/Lib/importlib/_bootstrap_external.py
blob: cf75719c86f500e51ae95be6c5a43abc4c4a1553 (plain)
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
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
"""Core implementation of path-based import.

This module is NOT meant to be directly imported! It has been designed such
that it can be bootstrapped into Python as the implementation of import. As
such it requires the injection of specific modules and attributes in order to
work. One should use importlib as the public-facing version of this module.

"""
# IMPORTANT: Whenever making changes to this module, be sure to run a top-level
# `make regen-importlib` followed by `make` in order to get the frozen version
# of the module updated. Not doing so will result in the Makefile to fail for
# all others who don't have a ./python around to freeze the module in the early
# stages of compilation.
#

# See importlib._setup() for what is injected into the global namespace.

# When editing this code be aware that code executed at import time CANNOT
# reference any injected objects! This includes not only global code but also
# anything specified at the class level.

# Bootstrap-related code ######################################################
_CASE_INSENSITIVE_PLATFORMS_STR_KEY = 'win',
_CASE_INSENSITIVE_PLATFORMS_BYTES_KEY = 'cygwin', 'darwin'
_CASE_INSENSITIVE_PLATFORMS =  (_CASE_INSENSITIVE_PLATFORMS_BYTES_KEY
                                + _CASE_INSENSITIVE_PLATFORMS_STR_KEY)


def _make_relax_case():
    if sys.platform.startswith(_CASE_INSENSITIVE_PLATFORMS):
        if sys.platform.startswith(_CASE_INSENSITIVE_PLATFORMS_STR_KEY):
            key = 'PYTHONCASEOK'
        else:
            key = b'PYTHONCASEOK'

        def _relax_case():
            """True if filenames must be checked case-insensitively."""
            return key in _os.environ
    else:
        def _relax_case():
            """True if filenames must be checked case-insensitively."""
            return False
    return _relax_case


def _w_long(x):
    """Convert a 32-bit integer to little-endian."""
    return (int(x) & 0xFFFFFFFF).to_bytes(4, 'little')


def _r_long(int_bytes):
    """Convert 4 bytes in little-endian to an integer."""
    return int.from_bytes(int_bytes, 'little')


def _path_join(*path_parts):
    """Replacement for os.path.join()."""
    return path_sep.join([part.rstrip(path_separators)
                          for part in path_parts if part])


def _path_split(path):
    """Replacement for os.path.split()."""
    if len(path_separators) == 1:
        front, _, tail = path.rpartition(path_sep)
        return front, tail
    for x in reversed(path):
        if x in path_separators:
            front, tail = path.rsplit(x, maxsplit=1)
            return front, tail
    return '', path


def _path_stat(path):
    """Stat the path.

    Made a separate function to make it easier to override in experiments
    (e.g. cache stat results).

    """
    return _os.stat(path)


def _path_is_mode_type(path, mode):
    """Test whether the path is the specified mode type."""
    try:
        stat_info = _path_stat(path)
    except OSError:
        return False
    return (stat_info.st_mode & 0o170000) == mode


def _path_isfile(path):
    """Replacement for os.path.isfile."""
    return _path_is_mode_type(path, 0o100000)


def _path_isdir(path):
    """Replacement for os.path.isdir."""
    if not path:
        path = _os.getcwd()
    return _path_is_mode_type(path, 0o040000)


def _write_atomic(path, data, mode=0o666):
    """Best-effort function to write data to a path atomically.
    Be prepared to handle a FileExistsError if concurrent writing of the
    temporary file is attempted."""
    # id() is used to generate a pseudo-random filename.
    path_tmp = '{}.{}'.format(path, id(path))
    fd = _os.open(path_tmp,
                  _os.O_EXCL | _os.O_CREAT | _os.O_WRONLY, mode & 0o666)
    try:
        # We first write data to a temporary file, and then use os.replace() to
        # perform an atomic rename.
        with _io.FileIO(fd, 'wb') as file:
            file.write(data)
        _os.replace(path_tmp, path)
    except OSError:
        try:
            _os.unlink(path_tmp)
        except OSError:
            pass
        raise


_code_type = type(_write_atomic.__code__)


# Finder/loader utility code ###############################################

# Magic word to reject .pyc files generated by other Python versions.
# It should change for each incompatible change to the bytecode.
#
# The value of CR and LF is incorporated so if you ever read or write
# a .pyc file in text mode the magic number will be wrong; also, the
# Apple MPW compiler swaps their values, botching string constants.
#
# There were a variety of old schemes for setting the magic number.
# The current working scheme is to increment the previous value by
# 10.
#
# Starting with the adoption of PEP 3147 in Python 3.2, every bump in magic
# number also includes a new "magic tag", i.e. a human readable string used
# to represent the magic number in __pycache__ directories.  When you change
# the magic number, you must also set a new unique magic tag.  Generally this
# can be named after the Python major version of the magic number bump, but
# it can really be anything, as long as it's different than anything else
# that's come before.  The tags are included in the following table, starting
# with Python 3.2a0.
#
# Known values:
#  Python 1.5:   20121
#  Python 1.5.1: 20121
#     Python 1.5.2: 20121
#     Python 1.6:   50428
#     Python 2.0:   50823
#     Python 2.0.1: 50823
#     Python 2.1:   60202
#     Python 2.1.1: 60202
#     Python 2.1.2: 60202
#     Python 2.2:   60717
#     Python 2.3a0: 62011
#     Python 2.3a0: 62021
#     Python 2.3a0: 62011 (!)
#     Python 2.4a0: 62041
#     Python 2.4a3: 62051
#     Python 2.4b1: 62061
#     Python 2.5a0: 62071
#     Python 2.5a0: 62081 (ast-branch)
#     Python 2.5a0: 62091 (with)
#     Python 2.5a0: 62092 (changed WITH_CLEANUP opcode)
#     Python 2.5b3: 62101 (fix wrong code: for x, in ...)
#     Python 2.5b3: 62111 (fix wrong code: x += yield)
#     Python 2.5c1: 62121 (fix wrong lnotab with for loops and
#                          storing constants that should have been removed)
#     Python 2.5c2: 62131 (fix wrong code: for x, in ... in listcomp/genexp)
#     Python 2.6a0: 62151 (peephole optimizations and STORE_MAP opcode)
#     Python 2.6a1: 62161 (WITH_CLEANUP optimization)
#     Python 2.7a0: 62171 (optimize list comprehensions/change LIST_APPEND)
#     Python 2.7a0: 62181 (optimize conditional branches:
#                          introduce POP_JUMP_IF_FALSE and POP_JUMP_IF_TRUE)
#     Python 2.7a0  62191 (introduce SETUP_WITH)
#     Python 2.7a0  62201 (introduce BUILD_SET)
#     Python 2.7a0  62211 (introduce MAP_ADD and SET_ADD)
#     Python 3000:   3000
#                    3010 (removed UNARY_CONVERT)
#                    3020 (added BUILD_SET)
#                    3030 (added keyword-only parameters)
#                    3040 (added signature annotations)
#                    3050 (print becomes a function)
#                    3060 (PEP 3115 metaclass syntax)
#                    3061 (string literals become unicode)
#                    3071 (PEP 3109 raise changes)
#                    3081 (PEP 3137 make __file__ and __name__ unicode)
#                    3091 (kill str8 interning)
#                    3101 (merge from 2.6a0, see 62151)
#                    3103 (__file__ points to source file)
#     Python 3.0a4: 3111 (WITH_CLEANUP optimization).
#     Python 3.0a5: 3131 (lexical exception stacking, including POP_EXCEPT)
#     Python 3.1a0: 3141 (optimize list, set and dict comprehensions:
#             change LIST_APPEND and SET_ADD, add MAP_ADD)
#     Python 3.1a0: 3151 (optimize conditional branches:
#             introduce POP_JUMP_IF_FALSE and POP_JUMP_IF_TRUE)
#     Python 3.2a0: 3160 (add SETUP_WITH)
#                   tag: cpython-32
#     Python 3.2a1: 3170 (add DUP_TOP_TWO, remove DUP_TOPX and ROT_FOUR)
#                   tag: cpython-32
#     Python 3.2a2  3180 (add DELETE_DEREF)
#     Python 3.3a0  3190 __class__ super closure changed
#     Python 3.3a0  3200 (__qualname__ added)
#                      3210 (added size modulo 2**32 to the pyc header)
#     Python 3.3a1  3220 (changed PEP 380 implementation)
#     Python 3.3a4  3230 (revert changes to implicit __class__ closure)
#     Python 3.4a1  3250 (evaluate positional default arguments before
#                        keyword-only defaults)
#     Python 3.4a1  3260 (add LOAD_CLASSDEREF; allow locals of class to override
#                        free vars)
#     Python 3.4a1  3270 (various tweaks to the __class__ closure)
#     Python 3.4a1  3280 (remove implicit class argument)
#     Python 3.4a4  3290 (changes to __qualname__ computation)
#     Python 3.4a4  3300 (more changes to __qualname__ computation)
#     Python 3.4rc2 3310 (alter __qualname__ computation)
#     Python 3.5a0  3320 (matrix multiplication operator)
#     Python 3.5b1  3330 (PEP 448: Additional Unpacking Generalizations)
#     Python 3.5b2  3340 (fix dictionary display evaluation order #11205)
#     Python 3.5b2  3350 (add GET_YIELD_FROM_ITER opcode #24400)
#     Python 3.5.2  3351 (fix BUILD_MAP_UNPACK_WITH_CALL opcode #27286)
#     Python 3.6a0  3360 (add FORMAT_VALUE opcode #25483
#     Python 3.6a0  3361 (lineno delta of code.co_lnotab becomes signed)
#     Python 3.6a1  3370 (16 bit wordcode)
#     Python 3.6a1  3371 (add BUILD_CONST_KEY_MAP opcode #27140)
#     Python 3.6a1  3372 (MAKE_FUNCTION simplification, remove MAKE_CLOSURE
#                         #27095)
#     Python 3.6b1  3373 (add BUILD_STRING opcode #27078)
#     Python 3.6b1  3375 (add SETUP_ANNOTATIONS and STORE_ANNOTATION opcodes
#                         #27985)
#     Python 3.6b1  3376 (simplify CALL_FUNCTIONs & BUILD_MAP_UNPACK_WITH_CALL)
#     Python 3.6b1  3377 (set __class__ cell from type.__new__ #23722)
#     Python 3.6b2  3378 (add BUILD_TUPLE_UNPACK_WITH_CALL #28257)
#     Python 3.6rc1 3379 (more thorough __class__ validation #23722)
#     Python 3.7a0  3390 (add LOAD_METHOD and CALL_METHOD opcodes)
#     Python 3.7a0  3391 (update GET_AITER #31709)
#     Python 3.7a0  3392 (PEP 552: Deterministic pycs)
#
# MAGIC must change whenever the bytecode emitted by the compiler may no
# longer be understood by older implementations of the eval loop (usually
# due to the addition of new opcodes).
#
# Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
# in PC/launcher.c must also be updated.

MAGIC_NUMBER = (3392).to_bytes(2, 'little') + b'\r\n'
_RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c

_PYCACHE = '__pycache__'
_OPT = 'opt-'

SOURCE_SUFFIXES = ['.py']  # _setup() adds .pyw as needed.

BYTECODE_SUFFIXES = ['.pyc']
# Deprecated.
DEBUG_BYTECODE_SUFFIXES = OPTIMIZED_BYTECODE_SUFFIXES = BYTECODE_SUFFIXES

def cache_from_source(path, debug_override=None, *, optimization=None):
    """Given the path to a .py file, return the path to its .pyc file.

    The .py file does not need to exist; this simply returns the path to the
    .pyc file calculated as if the .py file were imported.

    The 'optimization' parameter controls the presumed optimization level of
    the bytecode file. If 'optimization' is not None, the string representation
    of the argument is taken and verified to be alphanumeric (else ValueError
    is raised).

    The debug_override parameter is deprecated. If debug_override is not None,
    a True value is the same as setting 'optimization' to the empty string
    while a False value is equivalent to setting 'optimization' to '1'.

    If sys.implementation.cache_tag is None then NotImplementedError is raised.

    """
    if debug_override is not None:
        _warnings.warn('the debug_override parameter is deprecated; use '
                       "'optimization' instead", DeprecationWarning)
        if optimization is not None:
            message = 'debug_override or optimization must be set to None'
            raise TypeError(message)
        optimization = '' if debug_override else 1
    path = _os.fspath(path)
    head, tail = _path_split(path)
    base, sep, rest = tail.rpartition('.')
    tag = sys.implementation.cache_tag
    if tag is None:
        raise NotImplementedError('sys.implementation.cache_tag is None')
    almost_filename = ''.join([(base if base else rest), sep, tag])
    if optimization is None:
        if sys.flags.optimize == 0:
            optimization = ''
        else:
            optimization = sys.flags.optimize
    optimization = str(optimization)
    if optimization != '':
        if not optimization.isalnum():
            raise ValueError('{!r} is not alphanumeric'.format(optimization))
        almost_filename = '{}.{}{}'.format(almost_filename, _OPT, optimization)
    return _path_join(head, _PYCACHE, almost_filename + BYTECODE_SUFFIXES[0])


def source_from_cache(path):
    """Given the path to a .pyc. file, return the path to its .py file.

    The .pyc file does not need to exist; this simply returns the path to
    the .py file calculated to correspond to the .pyc file.  If path does
    not conform to PEP 3147/488 format, ValueError will be raised. If
    sys.implementation.cache_tag is None then NotImplementedError is raised.

    """
    if sys.implementation.cache_tag is None:
        raise NotImplementedError('sys.implementation.cache_tag is None')
    path = _os.fspath(path)
    head, pycache_filename = _path_split(path)
    head, pycache = _path_split(head)
    if pycache != _PYCACHE:
        raise ValueError('{} not bottom-level directory in '
                         '{!r}'.format(_PYCACHE, path))
    dot_count = pycache_filename.count('.')
    if dot_count not in {2, 3}:
        raise ValueError('expected only 2 or 3 dots in '
                         '{!r}'.format(pycache_filename))
    elif dot_count == 3:
        optimization = pycache_filename.rsplit('.', 2)[-2]
        if not optimization.startswith(_OPT):
            raise ValueError("optimization portion of filename does not start "
                             "with {!r}".format(_OPT))
        opt_level = optimization[len(_OPT):]
        if not opt_level.isalnum():
            raise ValueError("optimization level {!r} is not an alphanumeric "
                             "value".format(optimization))
    base_filename = pycache_filename.partition('.')[0]
    return _path_join(head, base_filename + SOURCE_SUFFIXES[0])


def _get_sourcefile(bytecode_path):
    """Convert a bytecode file path to a source path (if possible).

    This function exists purely for backwards-compatibility for
    PyImport_ExecCodeModuleWithFilenames() in the C API.

    """
    if len(bytecode_path) == 0:
        return None
    rest, _, extension = bytecode_path.rpartition('.')
    if not rest or extension.lower()[-3:-1] != 'py':
        return bytecode_path
    try:
        source_path = source_from_cache(bytecode_path)
    except (NotImplementedError, ValueError):
        source_path = bytecode_path[:-1]
    return source_path if _path_isfile(source_path) else bytecode_path


def _get_cached(filename):
    if filename.endswith(tuple(SOURCE_SUFFIXES)):
        try:
            return cache_from_source(filename)
        except NotImplementedError:
            pass
    elif filename.endswith(tuple(BYTECODE_SUFFIXES)):
        return filename
    else:
        return None


def _calc_mode(path):
    """Calculate the mode permissions for a bytecode file."""
    try:
        mode = _path_stat(path).st_mode
    except OSError:
        mode = 0o666
    # We always ensure write access so we can update cached files
    # later even when the source files are read-only on Windows (#6074)
    mode |= 0o200
    return mode


def _check_name(method):
    """Decorator to verify that the module being requested matches the one the
    loader can handle.

    The first argument (self) must define _name which the second argument is
    compared against. If the comparison fails then ImportError is raised.

    """
    def _check_name_wrapper(self, name=None, *args, **kwargs):
        if name is None:
            name = self.name
        elif self.name != name:
            raise ImportError('loader for %s cannot handle %s' %
                                (self.name, name), name=name)
        return method(self, name, *args, **kwargs)
    try:
        _wrap = _bootstrap._wrap
    except NameError:
        # XXX yuck
        def _wrap(new, old):
            for replace in ['__module__', '__name__', '__qualname__', '__doc__']:
                if hasattr(old, replace):
                    setattr(new, replace, getattr(old, replace))
            new.__dict__.update(old.__dict__)
    _wrap(_check_name_wrapper, method)
    return _check_name_wrapper


def _find_module_shim(self, fullname):
    """Try to find a loader for the specified module by delegating to
    self.find_loader().

    This method is deprecated in favor of finder.find_spec().

    """
    # Call find_loader(). If it returns a string (indicating this
    # is a namespace package portion), generate a warning and
    # return None.
    loader, portions = self.find_loader(fullname)
    if loader is None and len(portions):
        msg = 'Not importing directory {}: missing __init__'
        _warnings.warn(msg.format(portions[0]), ImportWarning)
    return loader


def _classify_pyc(data, name, exc_details):
    """Perform basic validity checking of a pyc header and return the flags field,
    which determines how the pyc should be further validated against the source.

    *data* is the contents of the pyc file. (Only the first 16 bytes are
    required, though.)

    *name* is the name of the module being imported. It is used for logging.

    *exc_details* is a dictionary passed to ImportError if it raised for
    improved debugging.

    ImportError is raised when the magic number is incorrect or when the flags
    field is invalid. EOFError is raised when the data is found to be truncated.

    """
    magic = data[:4]
    if magic != MAGIC_NUMBER:
        message = f'bad magic number in {name!r}: {magic!r}'
        _bootstrap._verbose_message('{}', message)
        raise ImportError(message, **exc_details)
    if len(data) < 16:
        message = f'reached EOF while reading pyc header of {name!r}'
        _bootstrap._verbose_message('{}', message)
        raise EOFError(message)
    flags = _r_long(data[4:8])
    # Only the first two flags are defined.
    if flags & ~0b11:
        message = f'invalid flags {flags!r} in {name!r}'
        raise ImportError(message, **exc_details)
    return flags


def _validate_timestamp_pyc(data, source_mtime, source_size, name,
                            exc_details):
    """Validate a pyc against the source last-modified time.

    *data* is the contents of the pyc file. (Only the first 16 bytes are
    required.)

    *source_mtime* is the last modified timestamp of the source file.

    *source_size* is None or the size of the source file in bytes.

    *name* is the name of the module being imported. It is used for logging.

    *exc_details* is a dictionary passed to ImportError if it raised for
    improved debugging.

    An ImportError is raised if the bytecode is stale.

    """
    if _r_long(data[8:12]) != (source_mtime & 0xFFFFFFFF):
        message = f'bytecode is stale for {name!r}'
        _bootstrap._verbose_message('{}', message)
        raise ImportError(message, **exc_details)
    if (source_size is not None and
        _r_long(data[12:16]) != (source_size & 0xFFFFFFFF)):
        raise ImportError(f'bytecode is stale for {name!r}', **exc_details)


def _validate_hash_pyc(data, source_hash, name, exc_details):
    """Validate a hash-based pyc by checking the real source hash against the one in
    the pyc header.

    *data* is the contents of the pyc file. (Only the first 16 bytes are
    required.)

    *source_hash* is the importlib.util.source_hash() of the source file.

    *name* is the name of the module being imported. It is used for logging.

    *exc_details* is a dictionary passed to ImportError if it raised for
    improved debugging.

    An ImportError is raised if the bytecode is stale.

    """
    if data[8:16] != source_hash:
        raise ImportError(
            f'hash in bytecode doesn\'t match hash of source {name!r}',
            **exc_details,
        )


def _compile_bytecode(data, name=None, bytecode_path=None, source_path=None):
    """Compile bytecode as found in a pyc."""
    code = marshal.loads(data)
    if isinstance(code, _code_type):
        _bootstrap._verbose_message('code object from {!r}', bytecode_path)
        if source_path is not None:
            _imp._fix_co_filename(code, source_path)
        return code
    else:
        raise ImportError('Non-code object in {!r}'.format(bytecode_path),
                          name=name, path=bytecode_path)


def _code_to_timestamp_pyc(code, mtime=0, source_size=0):
    "Produce the data for a timestamp-based pyc."
    data = bytearray(MAGIC_NUMBER)
    data.extend(_w_long(0))
    data.extend(_w_long(mtime))
    data.extend(_w_long(source_size))
    data.extend(marshal.dumps(code))
    return data


def _code_to_hash_pyc(code, source_hash, checked=True):
    "Produce the data for a hash-based pyc."
    data = bytearray(MAGIC_NUMBER)
    flags = 0b1 | checked << 1
    data.extend(_w_long(flags))
    assert len(source_hash) == 8
    data.extend(source_hash)
    data.extend(marshal.dumps(code))
    return data


def decode_source(source_bytes):
    """Decode bytes representing source code and return the string.

    Universal newline support is used in the decoding.
    """
    import tokenize  # To avoid bootstrap issues.
    source_bytes_readline = _io.BytesIO(source_bytes).readline
    encoding = tokenize.detect_encoding(source_bytes_readline)
    newline_decoder = _io.IncrementalNewlineDecoder(None, True)
    return newline_decoder.decode(source_bytes.decode(encoding[0]))


# Module specifications #######################################################

_POPULATE = object()


def spec_from_file_location(name, location=None, *, loader=None,
                            submodule_search_locations=_POPULATE):
    """Return a module spec based on a file location.

    To indicate that the module is a package, set
    submodule_search_locations to a list of directory paths.  An
    empty list is sufficient, though its not otherwise useful to the
    import system.

    The loader must take a spec as its only __init__() arg.

    """
    if location is None:
        # The caller may simply want a partially populated location-
        # oriented spec.  So we set the location to a bogus value and
        # fill in as much as we can.
        location = '<unknown>'
        if hasattr(loader, 'get_filename'):
            # ExecutionLoader
            try:
                location = loader.get_filename(name)
            except ImportError:
                pass
    else:
        location = _os.fspath(location)

    # If the location is on the filesystem, but doesn't actually exist,
    # we could return None here, indicating that the location is not
    # valid.  However, we don't have a good way of testing since an
    # indirect location (e.g. a zip file or URL) will look like a
    # non-existent file relative to the filesystem.

    spec = _bootstrap.ModuleSpec(name, loader, origin=location)
    spec._set_fileattr = True

    # Pick a loader if one wasn't provided.
    if loader is None:
        for loader_class, suffixes in _get_supported_file_loaders():
            if location.endswith(tuple(suffixes)):
                loader = loader_class(name, location)
                spec.loader = loader
                break
        else:
            return None

    # Set submodule_search_paths appropriately.
    if submodule_search_locations is _POPULATE:
        # Check the loader.
        if hasattr(loader, 'is_package'):
            try:
                is_package = loader.is_package(name)
            except ImportError:
                pass
            else:
                if is_package:
                    spec.submodule_search_locations = []
    else:
        spec.submodule_search_locations = submodule_search_locations
    if spec.submodule_search_locations == []:
        if location:
            dirname = _path_split(location)[0]
            spec.submodule_search_locations.append(dirname)

    return spec


# Loaders #####################################################################

class WindowsRegistryFinder:

    """Meta path finder for modules declared in the Windows registry."""

    REGISTRY_KEY = (
        'Software\\Python\\PythonCore\\{sys_version}'
        '\\Modules\\{fullname}')
    REGISTRY_KEY_DEBUG = (
        'Software\\Python\\PythonCore\\{sys_version}'
        '\\Modules\\{fullname}\\Debug')
    DEBUG_BUILD = False  # Changed in _setup()

    @classmethod
    def _open_registry(cls, key):
        try:
            return _winreg.OpenKey(_winreg.HKEY_CURRENT_USER, key)
        except OSError:
            return _winreg.OpenKey(_winreg.HKEY_LOCAL_MACHINE, key)

    @classmethod
    def _search_registry(cls, fullname):
        if cls.DEBUG_BUILD:
            registry_key = cls.REGISTRY_KEY_DEBUG
        else:
            registry_key = cls.REGISTRY_KEY
        key = registry_key.format(fullname=fullname,
                                  sys_version='%d.%d' % sys.version_info[:2])
        try:
            with cls._open_registry(key) as hkey:
                filepath = _winreg.QueryValue(hkey, '')
        except OSError:
            return None
        return filepath

    @classmethod
    def find_spec(cls, fullname, path=None, target=None):
        filepath = cls._search_registry(fullname)
        if filepath is None:
            return None
        try:
            _path_stat(filepath)
        except OSError:
            return None
        for loader, suffixes in _get_supported_file_loaders():
            if filepath.endswith(tuple(suffixes)):
                spec = _bootstrap.spec_from_loader(fullname,
                                                   loader(fullname, filepath),
                                                   origin=filepath)
                return spec

    @classmethod
    def find_module(cls, fullname, path=None):
        """Find module named in the registry.

        This method is deprecated.  Use exec_module() instead.

        """
        spec = cls.find_spec(fullname, path)
        if spec is not None:
            return spec.loader
        else:
            return None


class _LoaderBasics:

    """Base class of common code needed by both SourceLoader and
    SourcelessFileLoader."""

    def is_package(self, fullname):
        """Concrete implementation of InspectLoader.is_package by checking if
        the path returned by get_filename has a filename of '__init__.py'."""
        filename = _path_split(self.get_filename(fullname))[1]
        filename_base = filename.rsplit('.', 1)[0]
        tail_name = fullname.rpartition('.')[2]
        return filename_base == '__init__' and tail_name != '__init__'

    def create_module(self, spec):
        """Use default semantics for module creation."""

    def exec_module(self, module):
        """Execute the module."""
        code = self.get_code(module.__name__)
        if code is None:
            raise ImportError('cannot load module {!r} when get_code() '
                              'returns None'.format(module.__name__))
        _bootstrap._call_with_frames_removed(exec, code, module.__dict__)

    def load_module(self, fullname):
        """This module is deprecated."""
        return _bootstrap._load_module_shim(self, fullname)


class SourceLoader(_LoaderBasics):

    def path_mtime(self, path):
        """Optional method that returns the modification time (an int) for the
        specified path, where path is a str.

        Raises OSError when the path cannot be handled.
        """
        raise OSError

    def path_stats(self, path):
        """Optional method returning a metadata dict for the specified path
        to by the path (str).
        Possible keys:
        - 'mtime' (mandatory) is the numeric timestamp of last source
          code modification;
        - 'size' (optional) is the size in bytes of the source code.

        Implementing this method allows the loader to read bytecode files.
        Raises OSError when the path cannot be handled.
        """
        return {'mtime': self.path_mtime(path)}

    def _cache_bytecode(self, source_path, cache_path, data):
        """Optional method which writes data (bytes) to a file path (a str).

        Implementing this method allows for the writing of bytecode files.

        The source path is needed in order to correctly transfer permissions
        """
        # For backwards compatibility, we delegate to set_data()
        return self.set_data(cache_path, data)

    def set_data(self, path, data):
        """Optional method which writes data (bytes) to a file path (a str).

        Implementing this method allows for the writing of bytecode files.
        """


    def get_source(self, fullname):
        """Concrete implementation of InspectLoader.get_source."""
        path = self.get_filename(fullname)
        try:
            source_bytes = self.get_data(path)
        except OSError as exc:
            raise ImportError('source not available through get_data()',
                              name=fullname) from exc
        return decode_source(source_bytes)

    def source_to_code(self, data, path, *, _optimize=-1):
        """Return the code object compiled from source.

        The 'data' argument can be any object type that compile() supports.
        """
        return _bootstrap._call_with_frames_removed(compile, data, path, 'exec',
                                        dont_inherit=True, optimize=_optimize)

    def get_code(self, fullname):
        """Concrete implementation of InspectLoader.get_code.

        Reading of bytecode requires path_stats to be implemented. To write
        bytecode, set_data must also be implemented.

        """
        source_path = self.get_filename(fullname)
        source_mtime = None
        source_bytes = None
        source_hash = None
        hash_based = False
        check_source = True
        try:
            bytecode_path = cache_from_source(source_path)
        except NotImplementedError:
            bytecode_path = None
        else:
            try:
                st = self.path_stats(source_path)
            except OSError:
                pass
            else:
                source_mtime = int(st['mtime'])
                try:
                    data = self.get_data(bytecode_path)
                except OSError:
                    pass
                else:
                    exc_details = {
                        'name': fullname,
                        'path': bytecode_path,
                    }
                    try:
                        flags = _classify_pyc(data, fullname, exc_details)
                        bytes_data = memoryview(data)[16:]
                        hash_based = flags & 0b1 != 0
                        if hash_based:
                            check_source = flags & 0b10 != 0
                            if (_imp.check_hash_based_pycs != 'never' and
                                (check_source or
                                 _imp.check_hash_based_pycs == 'always')):
                                source_bytes = self.get_data(source_path)
                                source_hash = _imp.source_hash(
                                    _RAW_MAGIC_NUMBER,
                                    source_bytes,
                                )
                                _validate_hash_pyc(data, source_hash, fullname,
                                                   exc_details)
                        else:
                            _validate_timestamp_pyc(
                                data,
                                source_mtime,
                                st['size'],
                                fullname,
                                exc_details,
                            )
                    except (ImportError, EOFError):
                        pass
                    else:
                        _bootstrap._verbose_message('{} matches {}', bytecode_path,
                                                    source_path)
                        return _compile_bytecode(bytes_data, name=fullname,
                                                 bytecode_path=bytecode_path,
                                                 source_path=source_path)
        if source_bytes is None:
            source_bytes = self.get_data(source_path)
        code_object = self.source_to_code(source_bytes, source_path)
        _bootstrap._verbose_message('code object from {}', source_path)
        if (not sys.dont_write_bytecode and bytecode_path is not None and
                source_mtime is not None):
            if hash_based:
                if source_hash is None:
                    source_hash = _imp.source_hash(source_bytes)
                data = _code_to_hash_pyc(code_object, source_hash, check_source)
            else:
                data = _code_to_timestamp_pyc(code_object, source_mtime,
                                              len(source_bytes))
            try:
                self._cache_bytecode(source_path, bytecode_path, data)
                _bootstrap._verbose_message('wrote {!r}', bytecode_path)
            except NotImplementedError:
                pass
        return code_object


class FileLoader:

    """Base file loader class which implements the loader protocol methods that
    require file system usage."""

    def __init__(self, fullname, path):
        """Cache the module name and the path to the file found by the
        finder."""
        self.name = fullname
        self.path = path

    def __eq__(self, other):
        return (self.__class__ == other.__class__ and
                self.__dict__ == other.__dict__)

    def __hash__(self):
        return hash(self.name) ^ hash(self.path)

    @_check_name
    def load_module(self, fullname):
        """Load a module from a file.

        This method is deprecated.  Use exec_module() instead.

        """
        # The only reason for this method is for the name check.
        # Issue #14857: Avoid the zero-argument form of super so the implementation
        # of that form can be updated without breaking the frozen module
        return super(FileLoader, self).load_module(fullname)

    @_check_name
    def get_filename(self, fullname):
        """Return the path to the source file as found by the finder."""
        return self.path

    def get_data(self, path):
        """Return the data from path as raw bytes."""
        with _io.FileIO(path, 'r') as file:
            return file.read()

    # ResourceReader ABC API.

    @_check_name
    def get_resource_reader(self, module):
        if self.is_package(module):
            return self
        return None

    def open_resource(self, resource):
        path = _path_join(_path_split(self.path)[0], resource)
        return _io.FileIO(path, 'r')

    def resource_path(self, resource):
        if not self.is_resource(resource):
            raise FileNotFoundError
        path = _path_join(_path_split(self.path)[0], resource)
        return path

    def is_resource(self, name):
        if path_sep in name:
            return False
        path = _path_join(_path_split(self.path)[0], name)
        return _path_isfile(path)

    def contents(self):
        return iter(_os.listdir(_path_split(self.path)[0]))


class SourceFileLoader(FileLoader, SourceLoader):

    """Concrete implementation of SourceLoader using the file system."""

    def path_stats(self, path):
        """Return the metadata for the path."""
        st = _path_stat(path)
        return {'mtime': st.st_mtime, 'size': st.st_size}

    def _cache_bytecode(self, source_path, bytecode_path, data):
        # Adapt between the two APIs
        mode = _calc_mode(source_path)
        return self.set_data(bytecode_path, data, _mode=mode)

    def set_data(self, path, data, *, _mode=0o666):
        """Write bytes data to a file."""
        parent, filename = _path_split(path)
        path_parts = []
        # Figure out what directories are missing.
        while parent and not _path_isdir(parent):
            parent, part = _path_split(parent)
            path_parts.append(part)
        # Create needed directories.
        for part in reversed(path_parts):
            parent = _path_join(parent, part)
            try:
                _os.mkdir(parent)
            except FileExistsError:
                # Probably another Python process already created the dir.
                continue
            except OSError as exc:
                # Could be a permission error, read-only filesystem: just forget
                # about writing the data.
                _bootstrap._verbose_message('could not create {!r}: {!r}',
                                            parent, exc)
                return
        try:
            _write_atomic(path, data, _mode)
            _bootstrap._verbose_message('created {!r}', path)
        except OSError as exc:
            # Same as above: just don't write the bytecode.
            _bootstrap._verbose_message('could not create {!r}: {!r}', path,
                                        exc)


class SourcelessFileLoader(FileLoader, _LoaderBasics):

    """Loader which handles sourceless file imports."""

    def get_code(self, fullname):
        path = self.get_filename(fullname)
        data = self.get_data(path)
        # Call _classify_pyc to do basic validation of the pyc but ignore the
        # result. There's no source to check against.
        exc_details = {
            'name': fullname,
            'path': path,
        }
        _classify_pyc(data, fullname, exc_details)
        return _compile_bytecode(
            memoryview(data)[16:],
            name=fullname,
            bytecode_path=path,
        )

    def get_source(self, fullname):
        """Return None as there is no source code."""
        return None


# Filled in by _setup().
EXTENSION_SUFFIXES = []


class ExtensionFileLoader(FileLoader, _LoaderBasics):

    """Loader for extension modules.

    The constructor is designed to work with FileFinder.

    """

    def __init__(self, name, path):
        self.name = name
        self.path = path

    def __eq__(self, other):
        return (self.__class__ == other.__class__ and
                self.__dict__ == other.__dict__)

    def __hash__(self):
        return hash(self.name) ^ hash(self.path)

    def create_module(self, spec):
        """Create an unitialized extension module"""
        module = _bootstrap._call_with_frames_removed(
            _imp.create_dynamic, spec)
        _bootstrap._verbose_message('extension module {!r} loaded from {!r}',
                         spec.name, self.path)
        return module

    def exec_module(self, module):
        """Initialize an extension module"""
        _bootstrap._call_with_frames_removed(_imp.exec_dynamic, module)
        _bootstrap._verbose_message('extension module {!r} executed from {!r}',
                         self.name, self.path)

    def is_package(self, fullname):
        """Return True if the extension module is a package."""
        file_name = _path_split(self.path)[1]
        return any(file_name == '__init__' + suffix
                   for suffix in EXTENSION_SUFFIXES)

    def get_code(self, fullname):
        """Return None as an extension module cannot create a code object."""
        return None

    def get_source(self, fullname):
        """Return None as extension modules have no source code."""
        return None

    @_check_name
    def get_filename(self, fullname):
        """Return the path to the source file as found by the finder."""
        return self.path


class _NamespacePath:
    """Represents a namespace package's path.  It uses the module name
    to find its parent module, and from there it looks up the parent's
    __path__.  When this changes, the module's own path is recomputed,
    using path_finder.  For top-level modules, the parent module's path
    is sys.path."""

    def __init__(self, name, path, path_finder):
        self._name = name
        self._path = path
        self._last_parent_path = tuple(self._get_parent_path())
        self._path_finder = path_finder

    def _find_parent_path_names(self):
        """Returns a tuple of (parent-module-name, parent-path-attr-name)"""
        parent, dot, me = self._name.rpartition('.')
        if dot == '':
            # This is a top-level module. sys.path contains the parent path.
            return 'sys', 'path'
        # Not a top-level module. parent-module.__path__ contains the
        #  parent path.
        return parent, '__path__'

    def _get_parent_path(self):
        parent_module_name, path_attr_name = self._find_parent_path_names()
        return getattr(sys.modules[parent_module_name], path_attr_name)

    def _recalculate(self):
        # If the parent's path has changed, recalculate _path
        parent_path = tuple(self._get_parent_path()) # Make a copy
        if parent_path != self._last_parent_path:
            spec = self._path_finder(self._name, parent_path)
            # Note that no changes are made if a loader is returned, but we
            #  do remember the new parent path
            if spec is not None and spec.loader is None:
                if spec.submodule_search_locations:
                    self._path = spec.submodule_search_locations
            self._last_parent_path = parent_path     # Save the copy
        return self._path

    def __iter__(self):
        return iter(self._recalculate())

    def __setitem__(self, index, path):
        self._path[index] = path

    def __len__(self):
        return len(self._recalculate())

    def __repr__(self):
        return '_NamespacePath({!r})'.format(self._path)

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

    def append(self, item):
        self._path.append(item)


# We use this exclusively in module_from_spec() for backward-compatibility.
class _NamespaceLoader:
    def __init__(self, name, path, path_finder):
        self._path = _NamespacePath(name, path, path_finder)

    @classmethod
    def module_repr(cls, module):
        """Return repr for the module.

        The method is deprecated.  The import machinery does the job itself.

        """
        return '<module {!r} (namespace)>'.format(module.__name__)

    def is_package(self, fullname):
        return True

    def get_source(self, fullname):
        return ''

    def get_code(self, fullname):
        return compile('', '<string>', 'exec', dont_inherit=True)

    def create_module(self, spec):
        """Use default semantics for module creation."""

    def exec_module(self, module):
        pass

    def load_module(self, fullname):
        """Load a namespace module.

        This method is deprecated.  Use exec_module() instead.

        """
        # The import system never calls this method.
        _bootstrap._verbose_message('namespace module loaded with path {!r}',
                                    self._path)
        return _bootstrap._load_module_shim(self, fullname)


# Finders #####################################################################

class PathFinder:

    """Meta path finder for sys.path and package __path__ attributes."""

    @classmethod
    def invalidate_caches(cls):
        """Call the invalidate_caches() method on all path entry finders
        stored in sys.path_importer_caches (where implemented)."""
        for finder in sys.path_importer_cache.values():
            if hasattr(finder, 'invalidate_caches'):
                finder.invalidate_caches()

    @classmethod
    def _path_hooks(cls, path):
        """Search sys.path_hooks for a finder for 'path'."""
        if sys.path_hooks is not None and not sys.path_hooks:
            _warnings.warn('sys.path_hooks is empty', ImportWarning)
        for hook in sys.path_hooks:
            try:
                return hook(path)
            except ImportError:
                continue
        else:
            return None

    @classmethod
    def _path_importer_cache(cls, path):
        """Get the finder for the path entry from sys.path_importer_cache.

        If the path entry is not in the cache, find the appropriate finder
        and cache it. If no finder is available, store None.

        """
        if path == '':
            try:
                path = _os.getcwd()
            except FileNotFoundError:
                # Don't cache the failure as the cwd can easily change to
                # a valid directory later on.
                return None
        try:
            finder = sys.path_importer_cache[path]
        except KeyError:
            finder = cls._path_hooks(path)
            sys.path_importer_cache[path] = finder
        return finder

    @classmethod
    def _legacy_get_spec(cls, fullname, finder):
        # This would be a good place for a DeprecationWarning if
        # we ended up going that route.
        if hasattr(finder, 'find_loader'):
            loader, portions = finder.find_loader(fullname)
        else:
            loader = finder.find_module(fullname)
            portions = []
        if loader is not None:
            return _bootstrap.spec_from_loader(fullname, loader)
        spec = _bootstrap.ModuleSpec(fullname, None)
        spec.submodule_search_locations = portions
        return spec

    @classmethod
    def _get_spec(cls, fullname, path, target=None):
        """Find the loader or namespace_path for this module/package name."""
        # If this ends up being a namespace package, namespace_path is
        #  the list of paths that will become its __path__
        namespace_path = []
        for entry in path:
            if not isinstance(entry, (str, bytes)):
                continue
            finder = cls._path_importer_cache(entry)
            if finder is not None:
                if hasattr(finder, 'find_spec'):
                    spec = finder.find_spec(fullname, target)
                else:
                    spec = cls._legacy_get_spec(fullname, finder)
                if spec is None:
                    continue
                if spec.loader is not None:
                    return spec
                portions = spec.submodule_search_locations
                if portions is None:
                    raise ImportError('spec missing loader')
                # This is possibly part of a namespace package.
                #  Remember these path entries (if any) for when we
                #  create a namespace package, and continue iterating
                #  on path.
                namespace_path.extend(portions)
        else:
            spec = _bootstrap.ModuleSpec(fullname, None)
            spec.submodule_search_locations = namespace_path
            return spec

    @classmethod
    def find_spec(cls, fullname, path=None, target=None):
        """Try to find a spec for 'fullname' on sys.path or 'path'.

        The search is based on sys.path_hooks and sys.path_importer_cache.
        """
        if path is None:
            path = sys.path
        spec = cls._get_spec(fullname, path, target)
        if spec is None:
            return None
        elif spec.loader is None:
            namespace_path = spec.submodule_search_locations
            if namespace_path:
                # We found at least one namespace path.  Return a
                #  spec which can create the namespace package.
                spec.origin = 'namespace'
                spec.submodule_search_locations = _NamespacePath(fullname, namespace_path, cls._get_spec)
                return spec
            else:
                return None
        else:
            return spec

    @classmethod
    def find_module(cls, fullname, path=None):
        """find the module on sys.path or 'path' based on sys.path_hooks and
        sys.path_importer_cache.

        This method is deprecated.  Use find_spec() instead.

        """
        spec = cls.find_spec(fullname, path)
        if spec is None:
            return None
        return spec.loader


class FileFinder:

    """File-based finder.

    Interactions with the file system are cached for performance, being
    refreshed when the directory the finder is handling has been modified.

    """

    def __init__(self, path, *loader_details):
        """Initialize with the path to search on and a variable number of
        2-tuples containing the loader and the file suffixes the loader
        recognizes."""
        loaders = []
        for loader, suffixes in loader_details:
            loaders.extend((suffix, loader) for suffix in suffixes)
        self._loaders = loaders
        # Base (directory) path
        self.path = path or '.'
        self._path_mtime = -1
        self._path_cache = set()
        self._relaxed_path_cache = set()

    def invalidate_caches(self):
        """Invalidate the directory mtime."""
        self._path_mtime = -1

    find_module = _find_module_shim

    def find_loader(self, fullname):
        """Try to find a loader for the specified module, or the namespace
        package portions. Returns (loader, list-of-portions).

        This method is deprecated.  Use find_spec() instead.

        """
        spec = self.find_spec(fullname)
        if spec is None:
            return None, []
        return spec.loader, spec.submodule_search_locations or []

    def _get_spec(self, loader_class, fullname, path, smsl, target):
        loader = loader_class(fullname, path)
        return spec_from_file_location(fullname, path, loader=loader,
                                       submodule_search_locations=smsl)

    def find_spec(self, fullname, target=None):
        """Try to find a spec for the specified module.

        Returns the matching spec, or None if not found.
        """
        is_namespace = False
        tail_module = fullname.rpartition('.')[2]
        try:
            mtime = _path_stat(self.path or _os.getcwd()).st_mtime
        except OSError:
            mtime = -1
        if mtime != self._path_mtime:
            self._fill_cache()
            self._path_mtime = mtime
        # tail_module keeps the original casing, for __file__ and friends
        if _relax_case():
            cache = self._relaxed_path_cache
            cache_module = tail_module.lower()
        else:
            cache = self._path_cache
            cache_module = tail_module
        # Check if the module is the name of a directory (and thus a package).
        if cache_module in cache:
            base_path = _path_join(self.path, tail_module)
            for suffix, loader_class in self._loaders:
                init_filename = '__init__' + suffix
                full_path = _path_join(base_path, init_filename)
                if _path_isfile(full_path):
                    return self._get_spec(loader_class, fullname, full_path, [base_path], target)
            else:
                # If a namespace package, return the path if we don't
                #  find a module in the next section.
                is_namespace = _path_isdir(base_path)
        # Check for a file w/ a proper suffix exists.
        for suffix, loader_class in self._loaders:
            full_path = _path_join(self.path, tail_module + suffix)
            _bootstrap._verbose_message('trying {}', full_path, verbosity=2)
            if cache_module + suffix in cache:
                if _path_isfile(full_path):
                    return self._get_spec(loader_class, fullname, full_path,
                                          None, target)
        if is_namespace:
            _bootstrap._verbose_message('possible namespace for {}', base_path)
            spec = _bootstrap.ModuleSpec(fullname, None)
            spec.submodule_search_locations = [base_path]
            return spec
        return None

    def _fill_cache(self):
        """Fill the cache of potential modules and packages for this directory."""
        path = self.path
        try:
            contents = _os.listdir(path or _os.getcwd())
        except (FileNotFoundError, PermissionError, NotADirectoryError):
            # Directory has either been removed, turned into a file, or made
            # unreadable.
            contents = []
        # We store two cached versions, to handle runtime changes of the
        # PYTHONCASEOK environment variable.
        if not sys.platform.startswith('win'):
            self._path_cache = set(contents)
        else:
            # Windows users can import modules with case-insensitive file
            # suffixes (for legacy reasons). Make the suffix lowercase here
            # so it's done once instead of for every import. This is safe as
            # the specified suffixes to check against are always specified in a
            # case-sensitive manner.
            lower_suffix_contents = set()
            for item in contents:
                name, dot, suffix = item.partition('.')
                if dot:
                    new_name = '{}.{}'.format(name, suffix.lower())
                else:
                    new_name = name
                lower_suffix_contents.add(new_name)
            self._path_cache = lower_suffix_contents
        if sys.platform.startswith(_CASE_INSENSITIVE_PLATFORMS):
            self._relaxed_path_cache = {fn.lower() for fn in contents}

    @classmethod
    def path_hook(cls, *loader_details):
        """A class method which returns a closure to use on sys.path_hook
        which will return an instance using the specified loaders and the path
        called on the closure.

        If the path called on the closure is not a directory, ImportError is
        raised.

        """
        def path_hook_for_FileFinder(path):
            """Path hook for importlib.machinery.FileFinder."""
            if not _path_isdir(path):
                raise ImportError('only directories are supported', path=path)
            return cls(path, *loader_details)

        return path_hook_for_FileFinder

    def __repr__(self):
        return 'FileFinder({!r})'.format(self.path)


# Import setup ###############################################################

def _fix_up_module(ns, name, pathname, cpathname=None):
    # This function is used by PyImport_ExecCodeModuleObject().
    loader = ns.get('__loader__')
    spec = ns.get('__spec__')
    if not loader:
        if spec:
            loader = spec.loader
        elif pathname == cpathname:
            loader = SourcelessFileLoader(name, pathname)
        else:
            loader = SourceFileLoader(name, pathname)
    if not spec:
        spec = spec_from_file_location(name, pathname, loader=loader)
    try:
        ns['__spec__'] = spec
        ns['__loader__'] = loader
        ns['__file__'] = pathname
        ns['__cached__'] = cpathname
    except Exception:
        # Not important enough to report.
        pass


def _get_supported_file_loaders():
    """Returns a list of file-based module loaders.

    Each item is a tuple (loader, suffixes).
    """
    extensions = ExtensionFileLoader, _imp.extension_suffixes()
    source = SourceFileLoader, SOURCE_SUFFIXES
    bytecode = SourcelessFileLoader, BYTECODE_SUFFIXES
    return [extensions, source, bytecode]


def _setup(_bootstrap_module):
    """Setup the path-based importers for importlib by importing needed
    built-in modules and injecting them into the global namespace.

    Other components are extracted from the core bootstrap module.

    """
    global sys, _imp, _bootstrap
    _bootstrap = _bootstrap_module
    sys = _bootstrap.sys
    _imp = _bootstrap._imp

    # Directly load built-in modules needed during bootstrap.
    self_module = sys.modules[__name__]
    for builtin_name in ('_io', '_warnings', 'builtins', 'marshal'):
        if builtin_name not in sys.modules:
            builtin_module = _bootstrap._builtin_from_name(builtin_name)
        else:
            builtin_module = sys.modules[builtin_name]
        setattr(self_module, builtin_name, builtin_module)

    # Directly load the os module (needed during bootstrap).
    os_details = ('posix', ['/']), ('nt', ['\\', '/'])
    for builtin_os, path_separators in os_details:
        # Assumption made in _path_join()
        assert all(len(sep) == 1 for sep in path_separators)
        path_sep = path_separators[0]
        if builtin_os in sys.modules:
            os_module = sys.modules[builtin_os]
            break
        else:
            try:
                os_module = _bootstrap._builtin_from_name(builtin_os)
                break
            except ImportError:
                continue
    else:
        raise ImportError('importlib requires posix or nt')
    setattr(self_module, '_os', os_module)
    setattr(self_module, 'path_sep', path_sep)
    setattr(self_module, 'path_separators', ''.join(path_separators))

    # Directly load the _thread module (needed during bootstrap).
    thread_module = _bootstrap._builtin_from_name('_thread')
    setattr(self_module, '_thread', thread_module)

    # Directly load the _weakref module (needed during bootstrap).
    weakref_module = _bootstrap._builtin_from_name('_weakref')
    setattr(self_module, '_weakref', weakref_module)

    # Directly load the winreg module (needed during bootstrap).
    if builtin_os == 'nt':
        winreg_module = _bootstrap._builtin_from_name('winreg')
        setattr(self_module, '_winreg', winreg_module)

    # Constants
    setattr(self_module, '_relax_case', _make_relax_case())
    EXTENSION_SUFFIXES.extend(_imp.extension_suffixes())
    if builtin_os == 'nt':
        SOURCE_SUFFIXES.append('.pyw')
        if '_d.pyd' in EXTENSION_SUFFIXES:
            WindowsRegistryFinder.DEBUG_BUILD = True


def _install(_bootstrap_module):
    """Install the path-based import components."""
    _setup(_bootstrap_module)
    supported_loaders = _get_supported_file_loaders()
    sys.path_hooks.extend([FileFinder.path_hook(*supported_loaders)])
    sys.meta_path.append(PathFinder)