summaryrefslogtreecommitdiffstats
path: root/Doc/whatsnew/3.5.rst
blob: 246d9c57cba1de5842ed6782b2fcf632f4a6d324 (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
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
****************************
  What's New In Python 3.5
****************************

:Editors: Elvis Pranskevichus <elvis@magic.io>, Yury Selivanov <yury@magic.io>

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

   XXX Describe the transmogrify() function added to the socket
   module.
   (Contributed by P.Y. Developer in :issue:`12345`.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

This article explains the new features in Python 3.5, compared to 3.4.
Python 3.5 was released on September 13, 2015.  See the
`changelog <https://docs.python.org/3.5/whatsnew/changelog.html>`_ for a full
list of changes.

.. seealso::

    :pep:`478` - Python 3.5 Release Schedule


Summary -- Release highlights
=============================

New syntax features:

* :ref:`PEP 492 <whatsnew-pep-492>`, coroutines with async and await syntax.
* :ref:`PEP 465 <whatsnew-pep-465>`, a new matrix multiplication operator: ``a @ b``.
* :ref:`PEP 448 <whatsnew-pep-448>`, additional unpacking generalizations.


New library modules:

* :mod:`typing`: :ref:`PEP 484 -- Type Hints <whatsnew-pep-484>`.
* :mod:`zipapp`: :ref:`PEP 441 Improving Python ZIP Application Support
  <whatsnew-zipapp>`.


New built-in features:

* ``bytes % args``, ``bytearray % args``: :ref:`PEP 461 <whatsnew-pep-461>` --
  Adding ``%`` formatting to bytes and bytearray.

* New :meth:`bytes.hex`, :meth:`bytearray.hex` and :meth:`memoryview.hex`
  methods. (Contributed by Arnon Yaari in :issue:`9951`.)

* :class:`memoryview` now supports tuple indexing (including multi-dimensional).
  (Contributed by Antoine Pitrou in :issue:`23632`.)

* Generators have a new ``gi_yieldfrom`` attribute, which returns the
  object being iterated by ``yield from`` expressions. (Contributed
  by Benno Leslie and Yury Selivanov in :issue:`24450`.)

* A new :exc:`RecursionError` exception is now raised when maximum
  recursion depth is reached.  (Contributed by Georg Brandl
  in :issue:`19235`.)


CPython implementation improvements:

* When the ``LC_TYPE`` locale is the POSIX locale (``C`` locale),
  :py:data:`sys.stdin` and :py:data:`sys.stdout` now use the
  ``surrogateescape`` error handler, instead of the ``strict`` error handler.
  (Contributed by Victor Stinner in :issue:`19977`.)

* ``.pyo`` files are no longer used and have been replaced by a more flexible
  scheme that includes the optimization level explicitly in ``.pyc`` name.
  (See :ref:`PEP 488 overview <whatsnew-pep-488>`.)

* Builtin and extension modules are now initialized in a multi-phase process,
  which is similar to how Python modules are loaded.
  (See :ref:`PEP 489 overview <whatsnew-pep-489>`.)


Significant improvements in the standard library:

* :class:`collections.OrderedDict` is now
  :ref:`implemented in C <whatsnew-ordereddict>`, which makes it
  4 to 100 times faster.

* The :mod:`ssl` module gained
  :ref:`support for Memory BIO <whatsnew-sslmemorybio>`, which decouples SSL
  protocol handling from network IO.

* The new :func:`os.scandir` function provides a
  :ref:`better and significantly faster way <whatsnew-pep-471>`
  of directory traversal.

* :func:`functools.lru_cache` has been mostly
  :ref:`reimplemented in C <whatsnew-lrucache>`, yielding much better
  performance.

* The new :func:`subprocess.run` function provides a
  :ref:`streamlined way to run subprocesses <whatsnew-subprocess>`.

* The :mod:`traceback` module has been significantly
  :ref:`enhanced <whatsnew-traceback>` for improved
  performance and developer convenience.


Security improvements:

* SSLv3 is now disabled throughout the standard library.
  It can still be enabled by instantiating a :class:`ssl.SSLContext`
  manually.  (See :issue:`22638` for more details; this change was
  backported to CPython 3.4 and 2.7.)

* HTTP cookie parsing is now stricter, in order to protect
  against potential injection attacks. (Contributed by Antoine Pitrou
  in :issue:`22796`.)


Windows improvements:

* A new installer for Windows has replaced the old MSI.
  See :ref:`using-on-windows` for more information.

* Windows builds now use Microsoft Visual C++ 14.0, and extension modules
  should use the same.


Please read on for a comprehensive list of user-facing changes, including many
other smaller improvements, CPython optimizations, deprecations, and potential
porting issues.


New Features
============

.. _whatsnew-pep-492:

PEP 492 - Coroutines with async and await syntax
------------------------------------------------

:pep:`492` greatly improves support for asynchronous programming in Python
by adding :term:`awaitable objects <awaitable>`,
:term:`coroutine functions <coroutine function>`,
:term:`asynchronous iteration <asynchronous iterable>`,
and :term:`asynchronous context managers <asynchronous context manager>`.

Coroutine functions are declared using the new :keyword:`async def` syntax::

    >>> async def coro():
    ...     return 'spam'

Inside a coroutine function, the new :keyword:`await` expression can be used
to suspend coroutine execution until the result is available.  Any object
can be *awaited*, as long as it implements the :term:`awaitable` protocol by
defining the :meth:`__await__` method.

PEP 492 also adds :keyword:`async for` statement for convenient iteration
over asynchronous iterables.

An example of a rudimentary HTTP client written using the new syntax::

    import asyncio

    async def http_get(domain):
        reader, writer = await asyncio.open_connection(domain, 80)

        writer.write(b'\r\n'.join([
            b'GET / HTTP/1.1',
            b'Host: %b' % domain.encode('latin-1'),
            b'Connection: close',
            b'', b''
        ]))

        async for line in reader:
            print('>>>', line)

        writer.close()

    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(http_get('example.com'))
    finally:
        loop.close()


Similarly to asynchronous iteration, there is a new syntax for asynchronous
context managers.  The following script::

    import asyncio

    async def coro(name, lock):
        print('coro {}: waiting for lock'.format(name))
        async with lock:
            print('coro {}: holding the lock'.format(name))
            await asyncio.sleep(1)
            print('coro {}: releasing the lock'.format(name))

    loop = asyncio.get_event_loop()
    lock = asyncio.Lock()
    coros = asyncio.gather(coro(1, lock), coro(2, lock))
    try:
        loop.run_until_complete(coros)
    finally:
        loop.close()

will output::

    coro 2: waiting for lock
    coro 2: holding the lock
    coro 1: waiting for lock
    coro 2: releasing the lock
    coro 1: holding the lock
    coro 1: releasing the lock

Note that both :keyword:`async for` and :keyword:`async with` can only
be used inside a coroutine function declared with :keyword:`async def`.

Coroutine functions are intended to be run inside a compatible event loop,
such as the :ref:`asyncio loop <asyncio-event-loop>`.


.. note::

   .. versionchanged:: 3.5.2
      Starting with CPython 3.5.2, ``__aiter__`` can directly return
      :term:`asynchronous iterators <asynchronous iterator>`.  Returning
      an :term:`awaitable` object will result in a
      :exc:`PendingDeprecationWarning`.

      See more details in the :ref:`async-iterators` documentation
      section.


.. seealso::

   :pep:`492` -- Coroutines with async and await syntax
      PEP written and implemented by Yury Selivanov.


.. _whatsnew-pep-465:

PEP 465 - A dedicated infix operator for matrix multiplication
--------------------------------------------------------------

:pep:`465` adds the ``@`` infix operator for matrix multiplication.
Currently, no builtin Python types implement the new operator, however, it
can be implemented by defining :meth:`__matmul__`, :meth:`__rmatmul__`,
and :meth:`__imatmul__` for regular, reflected, and in-place matrix
multiplication.  The semantics of these methods is similar to that of
methods defining other infix arithmetic operators.

Matrix multiplication is a notably common operation in many fields of
mathematics, science, engineering, and the addition of ``@`` allows writing
cleaner code::

    S = (H @ beta - r).T @ inv(H @ V @ H.T) @ (H @ beta - r)

instead of::

    S = dot((dot(H, beta) - r).T,
            dot(inv(dot(dot(H, V), H.T)), dot(H, beta) - r))

NumPy 1.10 has support for the new operator::

    >>> import numpy

    >>> x = numpy.ones(3)
    >>> x
    array([ 1., 1., 1.])

    >>> m = numpy.eye(3)
    >>> m
    array([[ 1., 0., 0.],
           [ 0., 1., 0.],
           [ 0., 0., 1.]])

    >>> x @ m
    array([ 1., 1., 1.])


.. seealso::

   :pep:`465` -- A dedicated infix operator for matrix multiplication
      PEP written by Nathaniel J. Smith; implemented by Benjamin Peterson.


.. _whatsnew-pep-448:

PEP 448 - Additional Unpacking Generalizations
----------------------------------------------

:pep:`448` extends the allowed uses of the ``*`` iterable unpacking
operator and ``**`` dictionary unpacking operator.  It is now possible
to use an arbitrary number of unpackings in :ref:`function calls <calls>`::

    >>> print(*[1], *[2], 3, *[4, 5])
    1 2 3 4 5

    >>> def fn(a, b, c, d):
    ...     print(a, b, c, d)
    ...

    >>> fn(**{'a': 1, 'c': 3}, **{'b': 2, 'd': 4})
    1 2 3 4

Similarly, tuple, list, set, and dictionary displays allow multiple
unpackings (see :ref:`exprlists` and :ref:`dict`)::

    >>> *range(4), 4
    (0, 1, 2, 3, 4)

    >>> [*range(4), 4]
    [0, 1, 2, 3, 4]

    >>> {*range(4), 4, *(5, 6, 7)}
    {0, 1, 2, 3, 4, 5, 6, 7}

    >>> {'x': 1, **{'y': 2}}
    {'x': 1, 'y': 2}

.. seealso::

   :pep:`448` -- Additional Unpacking Generalizations
      PEP written by Joshua Landau; implemented by Neil Girdhar,
      Thomas Wouters, and Joshua Landau.


.. _whatsnew-pep-461:

PEP 461 - percent formatting support for bytes and bytearray
------------------------------------------------------------

:pep:`461` adds support for the ``%``
:ref:`interpolation operator <bytes-formatting>` to :class:`bytes`
and :class:`bytearray`.

While interpolation is usually thought of as a string operation, there are
cases where interpolation on ``bytes`` or ``bytearrays`` makes sense, and the
work needed to make up for this missing functionality detracts from the
overall readability of the code.  This issue is particularly important when
dealing with wire format protocols, which are often a mixture of binary
and ASCII compatible text.

Examples::

    >>> b'Hello %b!' % b'World'
    b'Hello World!'

    >>> b'x=%i y=%f' % (1, 2.5)
    b'x=1 y=2.500000'

Unicode is not allowed for ``%b``, but it is accepted by ``%a`` (equivalent of
``repr(obj).encode('ascii', 'backslashreplace')``)::

    >>> b'Hello %b!' % 'World'
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires bytes, or an object that implements __bytes__, not 'str'

    >>> b'price: %a' % '10€'
    b"price: '10\\u20ac'"

Note that ``%s`` and ``%r`` conversion types, although supported, should
only be used in codebases that need compatibility with Python 2.

.. seealso::

   :pep:`461` -- Adding % formatting to bytes and bytearray
      PEP written by Ethan Furman; implemented by Neil Schemenauer and
      Ethan Furman.


.. _whatsnew-pep-484:

PEP 484 - Type Hints
--------------------

Function annotation syntax has been a Python feature since version 3.0
(:pep:`3107`), however the semantics of annotations has been left undefined.

Experience has shown that the majority of function annotation
uses were to provide type hints to function parameters and return values.  It
became evident that it would be beneficial for Python users, if the
standard library included the base definitions and tools for type annotations.

:pep:`484` introduces a :term:`provisional module <provisional api>` to
provide these standard definitions and tools, along with some conventions
for situations where annotations are not available.

For example, here is a simple function whose argument and return type
are declared in the annotations::

    def greeting(name: str) -> str:
        return 'Hello ' + name

While these annotations are available at runtime through the usual
:attr:`__annotations__` attribute, *no automatic type checking happens at
runtime*.  Instead, it is assumed that a separate off-line type checker
(e.g. `mypy <http://mypy-lang.org>`_) will be used for on-demand
source code analysis.

The type system supports unions, generic types, and a special type
named :class:`~typing.Any` which is consistent with (i.e. assignable to
and from) all types.

.. seealso::

   * :mod:`typing` module documentation
   * :pep:`484` -- Type Hints
        PEP written by Guido van Rossum, Jukka Lehtosalo, and Łukasz Langa;
        implemented by Guido van Rossum.
   * :pep:`483` -- The Theory of Type Hints
        PEP written by Guido van Rossum


.. _whatsnew-pep-471:

PEP 471 - os.scandir() function -- a better and faster directory iterator
-------------------------------------------------------------------------

:pep:`471` adds a new directory iteration function, :func:`os.scandir`,
to the standard library.  Additionally, :func:`os.walk` is now
implemented using ``scandir``, which makes it 3 to 5 times faster
on POSIX systems and 7 to 20 times faster on Windows systems.  This is
largely achieved by greatly reducing the number of calls to :func:`os.stat`
required to walk a directory tree.

Additionally, ``scandir`` returns an iterator, as opposed to returning
a list of file names, which improves memory efficiency when iterating
over very large directories.

The following example shows a simple use of :func:`os.scandir` to display all
the files (excluding directories) in the given *path* that don't start with
``'.'``. The :meth:`entry.is_file() <os.DirEntry.is_file>` call will generally
not make an additional system call::

    for entry in os.scandir(path):
        if not entry.name.startswith('.') and entry.is_file():
            print(entry.name)

.. seealso::

   :pep:`471` -- os.scandir() function -- a better and faster directory iterator
      PEP written and implemented by Ben Hoyt with the help of Victor Stinner.


.. _whatsnew-pep-475:

PEP 475: Retry system calls failing with EINTR
----------------------------------------------

An :py:data:`errno.EINTR` error code is returned whenever a system call, that
is waiting for I/O, is interrupted by a signal.  Previously, Python would
raise :exc:`InterruptedError` in such cases.  This meant that, when writing a
Python application, the developer had two choices:

#. Ignore the ``InterruptedError``.
#. Handle the ``InterruptedError`` and attempt to restart the interrupted
   system call at every call site.

The first option makes an application fail intermittently.
The second option adds a large amount of boilerplate that makes the
code nearly unreadable.  Compare::

    print("Hello World")

and::

    while True:
        try:
            print("Hello World")
            break
        except InterruptedError:
            continue

:pep:`475` implements automatic retry of system calls on
``EINTR``.  This removes the burden of dealing with ``EINTR``
or :exc:`InterruptedError` in user code in most situations and makes
Python programs, including the standard library, more robust.  Note that
the system call is only retried if the signal handler does not raise an
exception.

Below is a list of functions which are now retried when interrupted
by a signal:

* :func:`open` and :func:`io.open`;

* functions of the :mod:`faulthandler` module;

* :mod:`os` functions: :func:`~os.fchdir`, :func:`~os.fchmod`,
  :func:`~os.fchown`, :func:`~os.fdatasync`, :func:`~os.fstat`,
  :func:`~os.fstatvfs`, :func:`~os.fsync`, :func:`~os.ftruncate`,
  :func:`~os.mkfifo`, :func:`~os.mknod`, :func:`~os.open`,
  :func:`~os.posix_fadvise`, :func:`~os.posix_fallocate`, :func:`~os.pread`,
  :func:`~os.pwrite`, :func:`~os.read`, :func:`~os.readv`, :func:`~os.sendfile`,
  :func:`~os.wait3`, :func:`~os.wait4`, :func:`~os.wait`,
  :func:`~os.waitid`, :func:`~os.waitpid`, :func:`~os.write`,
  :func:`~os.writev`;

* special cases: :func:`os.close` and :func:`os.dup2` now ignore
  :py:data:`~errno.EINTR` errors; the syscall is not retried (see the PEP
  for the rationale);

* :mod:`select` functions: :func:`devpoll.poll() <select.devpoll.poll>`,
  :func:`epoll.poll() <select.epoll.poll>`,
  :func:`kqueue.control() <select.kqueue.control>`,
  :func:`poll.poll() <select.poll.poll>`, :func:`~select.select`;

* methods of the :class:`~socket.socket` class: :meth:`~socket.socket.accept`,
  :meth:`~socket.socket.connect` (except for non-blocking sockets),
  :meth:`~socket.socket.recv`, :meth:`~socket.socket.recvfrom`,
  :meth:`~socket.socket.recvmsg`, :meth:`~socket.socket.send`,
  :meth:`~socket.socket.sendall`, :meth:`~socket.socket.sendmsg`,
  :meth:`~socket.socket.sendto`;

* :func:`signal.sigtimedwait` and :func:`signal.sigwaitinfo`;

* :func:`time.sleep`.

.. seealso::

   :pep:`475` -- Retry system calls failing with EINTR
      PEP and implementation written by Charles-François Natali and
      Victor Stinner, with the help of Antoine Pitrou (the French connection).


.. _whatsnew-pep-479:

PEP 479: Change StopIteration handling inside generators
--------------------------------------------------------

The interaction of generators and :exc:`StopIteration` in Python 3.4 and
earlier was sometimes surprising, and could conceal obscure bugs.  Previously,
``StopIteration`` raised accidentally inside a generator function was
interpreted as the end of the iteration by the loop construct driving the
generator.

:pep:`479` changes the behavior of generators: when a ``StopIteration``
exception is raised inside a generator, it is replaced with a
:exc:`RuntimeError` before it exits the generator frame.  The main goal of
this change is to ease debugging in the situation where an unguarded
:func:`next` call raises ``StopIteration`` and causes the iteration controlled
by the generator to terminate silently. This is particularly pernicious in
combination with the ``yield from`` construct.

This is a backwards incompatible change, so to enable the new behavior,
a :term:`__future__` import is necessary::

    >>> from __future__ import generator_stop

    >>> def gen():
    ...     next(iter([]))
    ...     yield
    ...
    >>> next(gen())
    Traceback (most recent call last):
      File "<stdin>", line 2, in gen
    StopIteration

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    RuntimeError: generator raised StopIteration

Without a ``__future__`` import, a :exc:`PendingDeprecationWarning` will be
raised whenever a ``StopIteration`` exception is raised inside a generator.

.. seealso::

   :pep:`479` -- Change StopIteration handling inside generators
      PEP written by Chris Angelico and Guido van Rossum. Implemented by
      Chris Angelico, Yury Selivanov and Nick Coghlan.


.. _whatsnew-pep-485:

PEP 485: A function for testing approximate equality
----------------------------------------------------

:pep:`485` adds the :func:`math.isclose` and :func:`cmath.isclose`
functions which tell whether two values are approximately equal or
"close" to each other.  Whether or not two values are considered
close is determined according to given absolute and relative tolerances.
Relative tolerance is the maximum allowed difference between ``isclose``
arguments, relative to the larger absolute value::

    >>> import math
    >>> a = 5.0
    >>> b = 4.99998
    >>> math.isclose(a, b, rel_tol=1e-5)
    True
    >>> math.isclose(a, b, rel_tol=1e-6)
    False

It is also possible to compare two values using absolute tolerance, which
must be a non-negative value::

    >>> import math
    >>> a = 5.0
    >>> b = 4.99998
    >>> math.isclose(a, b, abs_tol=0.00003)
    True
    >>> math.isclose(a, b, abs_tol=0.00001)
    False

.. seealso::

   :pep:`485` -- A function for testing approximate equality
      PEP written by Christopher Barker; implemented by Chris Barker and
      Tal Einat.


.. _whatsnew-pep-486:

PEP 486: Make the Python Launcher aware of virtual environments
---------------------------------------------------------------

:pep:`486` makes the Windows launcher (see :pep:`397`) aware of an active
virtual environment. When the default interpreter would be used and the
``VIRTUAL_ENV`` environment variable is set, the interpreter in the virtual
environment will be used.

.. seealso::

    :pep:`486` -- Make the Python Launcher aware of virtual environments
        PEP written and implemented by Paul Moore.


.. _whatsnew-pep-488:

PEP 488: Elimination of PYO files
---------------------------------

:pep:`488` does away with the concept of ``.pyo`` files. This means that
``.pyc`` files represent both unoptimized and optimized bytecode. To prevent the
need to constantly regenerate bytecode files, ``.pyc`` files now have an
optional ``opt-`` tag in their name when the bytecode is optimized. This has the
side-effect of no more bytecode file name clashes when running under either
:option:`-O` or :option:`-OO`. Consequently, bytecode files generated from
:option:`-O`, and :option:`-OO` may now exist simultaneously.
:func:`importlib.util.cache_from_source` has an updated API to help with
this change.

.. seealso::

   :pep:`488` -- Elimination of PYO files
      PEP written and implemented by Brett Cannon.


.. _whatsnew-pep-489:

PEP 489: Multi-phase extension module initialization
----------------------------------------------------

:pep:`489` updates extension module initialization to take advantage of the
two step module loading mechanism introduced by :pep:`451` in Python 3.4.

This change brings the import semantics of extension modules that opt-in to
using the new mechanism much closer to those of Python source and bytecode
modules, including the ability to use any valid identifier as a module name,
rather than being restricted to ASCII.

.. seealso::

   :pep:`489` -- Multi-phase extension module initialization
      PEP written by Petr Viktorin, Stefan Behnel, and Nick Coghlan;
      implemented by Petr Viktorin.


Other Language Changes
======================

Some smaller changes made to the core Python language are:

* Added the ``"namereplace"`` error handlers.  The ``"backslashreplace"``
  error handlers now work with decoding and translating.
  (Contributed by Serhiy Storchaka in :issue:`19676` and :issue:`22286`.)

* The :option:`-b` option now affects comparisons of :class:`bytes` with
  :class:`int`.  (Contributed by Serhiy Storchaka in :issue:`23681`.)

* New Kazakh ``kz1048`` and Tajik ``koi8_t`` :ref:`codecs <standard-encodings>`.
  (Contributed by Serhiy Storchaka in :issue:`22682` and :issue:`22681`.)

* Property docstrings are now writable. This is especially useful for
  :func:`collections.namedtuple` docstrings.
  (Contributed by Berker Peksag in :issue:`24064`.)

* Circular imports involving relative imports are now supported.
  (Contributed by Brett Cannon and Antoine Pitrou in :issue:`17636`.)


New Modules
===========

typing
------

The new :mod:`typing` :term:`provisional <provisional api>` module
provides standard definitions and tools for function type annotations.
See :ref:`Type Hints <whatsnew-pep-484>` for more information.

.. _whatsnew-zipapp:

zipapp
------

The new :mod:`zipapp` module (specified in :pep:`441`) provides an API and
command line tool for creating executable Python Zip Applications, which
were introduced in Python 2.6 in :issue:`1739468`, but which were not well
publicized, either at the time or since.

With the new module, bundling your application is as simple as putting all
the files, including a ``__main__.py`` file, into a directory ``myapp``
and running:

.. code-block:: shell-session

    $ python -m zipapp myapp
    $ python myapp.pyz

The module implementation has been contributed by Paul Moore in
:issue:`23491`.

.. seealso::

   :pep:`441` -- Improving Python ZIP Application Support


Improved Modules
================

argparse
--------

The :class:`~argparse.ArgumentParser` class now allows disabling
:ref:`abbreviated usage <prefix-matching>` of long options by setting
:ref:`allow_abbrev` to ``False``.  (Contributed by Jonathan Paugh,
Steven Bethard, paul j3 and Daniel Eriksson in :issue:`14910`.)


asyncio
-------

Since the :mod:`asyncio` module is :term:`provisional <provisional api>`,
all changes introduced in Python 3.5 have also been backported to Python 3.4.x.

Notable changes in the :mod:`asyncio` module since Python 3.4.0:

* New debugging APIs: :meth:`loop.set_debug() <asyncio.BaseEventLoop.set_debug>`
  and :meth:`loop.get_debug() <asyncio.BaseEventLoop.get_debug>` methods.
  (Contributed by Victor Stinner.)

* The proactor event loop now supports SSL.
  (Contributed by Antoine Pitrou and Victor Stinner in :issue:`22560`.)

* A new :meth:`loop.is_closed() <asyncio.BaseEventLoop.is_closed>` method to
  check if the event loop is closed.
  (Contributed by Victor Stinner in :issue:`21326`.)

* A new :meth:`loop.create_task() <asyncio.BaseEventLoop.create_task>`
  to conveniently create and schedule a new :class:`~asyncio.Task`
  for a coroutine.  The ``create_task`` method is also used by all
  asyncio functions that wrap coroutines into tasks, such as
  :func:`asyncio.wait`, :func:`asyncio.gather`, etc.
  (Contributed by Victor Stinner.)

* A new :meth:`transport.get_write_buffer_limits() <asyncio.WriteTransport.get_write_buffer_limits>`
  method to inquire for *high-* and *low-* water limits of the flow
  control.
  (Contributed by Victor Stinner.)

* The :func:`~asyncio.async` function is deprecated in favor of
  :func:`~asyncio.ensure_future`.
  (Contributed by Yury Selivanov.)

* New :meth:`loop.set_task_factory()
  <asyncio.AbstractEventLoop.set_task_factory>` and
  :meth:`loop.get_task_factory() <asyncio.AbstractEventLoop.get_task_factory>`
  methods to customize the task factory that :meth:`loop.create_task()
  <asyncio.BaseEventLoop.create_task>` method uses.  (Contributed by Yury
  Selivanov.)

* New :meth:`Queue.join() <asyncio.Queue.join>` and
  :meth:`Queue.task_done() <asyncio.Queue.task_done>` queue methods.
  (Contributed by Victor Stinner.)

* The ``JoinableQueue`` class was removed, in favor of the
  :class:`asyncio.Queue` class.
  (Contributed by Victor Stinner.)

Updates in 3.5.1:

* The :func:`~asyncio.ensure_future` function and all functions that
  use it, such as :meth:`loop.run_until_complete() <asyncio.BaseEventLoop.run_until_complete>`,
  now accept all kinds of :term:`awaitable objects <awaitable>`.
  (Contributed by Yury Selivanov.)

* New :func:`~asyncio.run_coroutine_threadsafe` function to submit
  coroutines to event loops from other threads.
  (Contributed by Vincent Michel.)

* New :meth:`Transport.is_closing() <asyncio.BaseTransport.is_closing>`
  method to check if the transport is closing or closed.
  (Contributed by Yury Selivanov.)

* The :meth:`loop.create_server() <asyncio.BaseEventLoop.create_server>`
  method can now accept a list of hosts.
  (Contributed by Yann Sionneau.)

Updates in 3.5.2:

* New :meth:`loop.create_future() <asyncio.BaseEventLoop.create_future>`
  method to create Future objects.  This allows alternative event
  loop implementations, such as
  `uvloop <https://github.com/MagicStack/uvloop>`_, to provide a faster
  :class:`asyncio.Future` implementation.
  (Contributed by Yury Selivanov.)

* New :meth:`loop.get_exception_handler() <asyncio.BaseEventLoop.get_exception_handler>`
  method to get the current exception handler.
  (Contributed by Yury Selivanov.)

* New :meth:`StreamReader.readuntil() <asyncio.StreamReader.readuntil>`
  method to read data from the stream until a separator bytes
  sequence appears.
  (Contributed by Mark Korenberg.)

* The :meth:`loop.create_connection() <asyncio.BaseEventLoop.create_connection>`
  and :meth:`loop.create_server() <asyncio.BaseEventLoop.create_server>`
  methods are optimized to avoid calling the system ``getaddrinfo``
  function if the address is already resolved.
  (Contributed by A. Jesse Jiryu Davis.)

* The :meth:`loop.sock_connect(sock, address) <asyncio.BaseEventLoop.sock_connect>`
  no longer requires the *address* to be resolved prior to the call.
  (Contributed by A. Jesse Jiryu Davis.)


bz2
---

The :meth:`BZ2Decompressor.decompress <bz2.BZ2Decompressor.decompress>`
method now accepts an optional *max_length* argument to limit the maximum
size of decompressed data. (Contributed by Nikolaus Rath in :issue:`15955`.)


cgi
---

The :class:`~cgi.FieldStorage` class now supports the :term:`context manager`
protocol.  (Contributed by Berker Peksag in :issue:`20289`.)


cmath
-----

A new function :func:`~cmath.isclose` provides a way to test for approximate
equality.  (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)


code
----

The :func:`InteractiveInterpreter.showtraceback() <code.InteractiveInterpreter.showtraceback>`
method now prints the full chained traceback, just like the interactive
interpreter.  (Contributed by Claudiu Popa in :issue:`17442`.)


collections
-----------

.. _whatsnew-ordereddict:

The :class:`~collections.OrderedDict` class is now implemented in C, which
makes it 4 to 100 times faster.  (Contributed by Eric Snow in :issue:`16991`.)

:meth:`OrderedDict.items() <collections.OrderedDict.items>`,
:meth:`OrderedDict.keys() <collections.OrderedDict.keys>`,
:meth:`OrderedDict.values() <collections.OrderedDict.values>` views now support
:func:`reversed` iteration.
(Contributed by Serhiy Storchaka in :issue:`19505`.)

The :class:`~collections.deque` class now defines
:meth:`~collections.deque.index`, :meth:`~collections.deque.insert`, and
:meth:`~collections.deque.copy`, and supports the ``+`` and ``*`` operators.
This allows deques to be recognized as a :class:`~collections.abc.MutableSequence`
and improves their substitutability for lists.
(Contributed by Raymond Hettinger in :issue:`23704`.)

Docstrings produced by :func:`~collections.namedtuple` can now be updated::

    Point = namedtuple('Point', ['x', 'y'])
    Point.__doc__ += ': Cartesian coodinate'
    Point.x.__doc__ = 'abscissa'
    Point.y.__doc__ = 'ordinate'

(Contributed by Berker Peksag in :issue:`24064`.)

The :class:`~collections.UserString` class now implements the
:meth:`__getnewargs__`, :meth:`__rmod__`, :meth:`~str.casefold`,
:meth:`~str.format_map`, :meth:`~str.isprintable`, and :meth:`~str.maketrans`
methods to match the corresponding methods of :class:`str`.
(Contributed by Joe Jevnik in :issue:`22189`.)


collections.abc
---------------

The :meth:`Sequence.index() <collections.abc.Sequence.index>` method now
accepts *start* and *stop* arguments to match the corresponding methods
of :class:`tuple`, :class:`list`, etc.
(Contributed by Devin Jeanpierre in :issue:`23086`.)

A new :class:`~collections.abc.Generator` abstract base class. (Contributed
by Stefan Behnel in :issue:`24018`.)

New :class:`~collections.abc.Awaitable`, :class:`~collections.abc.Coroutine`,
:class:`~collections.abc.AsyncIterator`, and
:class:`~collections.abc.AsyncIterable` abstract base classes.
(Contributed by Yury Selivanov in :issue:`24184`.)

For earlier Python versions, a backport of the new ABCs is available in an
external `PyPI package <https://pypi.python.org/pypi/backports_abc>`_.


compileall
----------

A new :mod:`compileall` option, :samp:`-j {N}`, allows running *N* workers
simultaneously to perform parallel bytecode compilation.
The :func:`~compileall.compile_dir` function has a corresponding ``workers``
parameter.  (Contributed by Claudiu Popa in :issue:`16104`.)

Another new option, ``-r``, allows controlling the maximum recursion
level for subdirectories.  (Contributed by Claudiu Popa in :issue:`19628`.)

The ``-q`` command line option can now be specified more than once, in
which case all output, including errors, will be suppressed.  The corresponding
``quiet`` parameter in :func:`~compileall.compile_dir`,
:func:`~compileall.compile_file`, and :func:`~compileall.compile_path` can now
accept an integer value indicating the level of output suppression.
(Contributed by Thomas Kluyver in :issue:`21338`.)


concurrent.futures
------------------

The :meth:`Executor.map() <concurrent.futures.Executor.map>` method now accepts a
*chunksize* argument to allow batching of tasks to improve performance when
:meth:`~concurrent.futures.ProcessPoolExecutor` is used.
(Contributed by Dan O'Reilly in :issue:`11271`.)

The number of workers in the :class:`~concurrent.futures.ThreadPoolExecutor`
constructor is optional now.  The default value is 5 times the number of CPUs.
(Contributed by Claudiu Popa in :issue:`21527`.)


configparser
------------

:mod:`configparser` now provides a way to customize the conversion
of values by specifying a dictionary of converters in the
:class:`~configparser.ConfigParser` constructor, or by defining them
as methods in ``ConfigParser`` subclasses.  Converters defined in
a parser instance are inherited by its section proxies.

Example::

    >>> import configparser
    >>> conv = {}
    >>> conv['list'] = lambda v: [e.strip() for e in v.split() if e.strip()]
    >>> cfg = configparser.ConfigParser(converters=conv)
    >>> cfg.read_string("""
    ... [s]
    ... list = a b c d e f g
    ... """)
    >>> cfg.get('s', 'list')
    'a b c d e f g'
    >>> cfg.getlist('s', 'list')
    ['a', 'b', 'c', 'd', 'e', 'f', 'g']
    >>> section = cfg['s']
    >>> section.getlist('list')
    ['a', 'b', 'c', 'd', 'e', 'f', 'g']

(Contributed by Łukasz Langa in :issue:`18159`.)


contextlib
----------

The new :func:`~contextlib.redirect_stderr` :term:`context manager` (similar to
:func:`~contextlib.redirect_stdout`) makes it easier for utility scripts to
handle inflexible APIs that write their output to :data:`sys.stderr` and
don't provide any options to redirect it::

    >>> import contextlib, io, logging
    >>> f = io.StringIO()
    >>> with contextlib.redirect_stderr(f):
    ...     logging.warning('warning')
    ...
    >>> f.getvalue()
    'WARNING:root:warning\n'

(Contributed by Berker Peksag in :issue:`22389`.)


csv
---

The :meth:`~csv.csvwriter.writerow` method now supports arbitrary iterables,
not just sequences.  (Contributed by Serhiy Storchaka in :issue:`23171`.)


curses
------

The new :func:`~curses.update_lines_cols` function updates the :envvar:`LINES`
and :envvar:`COLS` environment variables.  This is useful for detecting
manual screen resizing.  (Contributed by Arnon Yaari in :issue:`4254`.)


dbm
---

:func:`dumb.open <dbm.dumb.open>` always creates a new database when the flag
has the value ``"n"``.  (Contributed by Claudiu Popa in :issue:`18039`.)


difflib
-------

The charset of HTML documents generated by
:meth:`HtmlDiff.make_file() <difflib.HtmlDiff.make_file>`
can now be customized by using a new *charset* keyword-only argument.
The default charset of HTML document changed from ``"ISO-8859-1"``
to ``"utf-8"``.
(Contributed by Berker Peksag in :issue:`2052`.)

The :func:`~difflib.diff_bytes` function can now compare lists of byte
strings.  This fixes a regression from Python 2.
(Contributed by Terry J. Reedy and Greg Ward in :issue:`17445`.)


distutils
---------

Both the ``build`` and ``build_ext`` commands now accept a ``-j`` option to
enable parallel building of extension modules.
(Contributed by Antoine Pitrou in :issue:`5309`.)

The :mod:`distutils` module now supports ``xz`` compression, and can be
enabled by passing ``xztar`` as an argument to ``bdist --format``.
(Contributed by Serhiy Storchaka in :issue:`16314`.)


doctest
-------

The :func:`~doctest.DocTestSuite` function returns an empty
:class:`unittest.TestSuite` if *module* contains no docstrings, instead of
raising :exc:`ValueError`.  (Contributed by Glenn Jones in :issue:`15916`.)


email
-----

A new policy option :attr:`Policy.mangle_from_ <email.policy.Policy.mangle_from_>`
controls whether or not lines that start with ``"From "`` in email bodies are
prefixed with a ``">"`` character by generators.  The default is ``True`` for
:attr:`~email.policy.compat32` and ``False`` for all other policies.
(Contributed by Milan Oberkirch in :issue:`20098`.)

A new
:meth:`Message.get_content_disposition() <email.message.Message.get_content_disposition>`
method provides easy access to a canonical value for the
:mailheader:`Content-Disposition` header.
(Contributed by Abhilash Raj in :issue:`21083`.)

A new policy option :attr:`EmailPolicy.utf8 <email.policy.EmailPolicy.utf8>`
can be set to ``True`` to encode email headers using the UTF-8 charset instead
of using encoded words.  This allows ``Messages`` to be formatted according to
:rfc:`6532` and used with an SMTP server that supports the :rfc:`6531`
``SMTPUTF8`` extension.  (Contributed by R. David Murray in
:issue:`24211`.)

The :class:`mime.text.MIMEText <email.mime.text.MIMEText>` constructor now
accepts a :class:`charset.Charset <email.charset.Charset>` instance.
(Contributed by Claude Paroz and Berker Peksag in :issue:`16324`.)


enum
----

The :class:`~enum.Enum` callable has a new parameter *start* to
specify the initial number of enum values if only *names* are provided::

    >>> Animal = enum.Enum('Animal', 'cat dog', start=10)
    >>> Animal.cat
    <Animal.cat: 10>
    >>> Animal.dog
    <Animal.dog: 11>

(Contributed by Ethan Furman in :issue:`21706`.)


faulthandler
------------

The :func:`~faulthandler.enable`, :func:`~faulthandler.register`,
:func:`~faulthandler.dump_traceback` and
:func:`~faulthandler.dump_traceback_later` functions now accept file
descriptors in addition to file-like objects.
(Contributed by Wei Wu in :issue:`23566`.)


functools
---------

.. _whatsnew-lrucache:

Most of the :func:`~functools.lru_cache` machinery is now implemented in C, making
it significantly faster.  (Contributed by Matt Joiner, Alexey Kachayev, and
Serhiy Storchaka in :issue:`14373`.)


glob
----

The :func:`~glob.iglob` and :func:`~glob.glob` functions now support recursive
search in subdirectories, using the ``"**"`` pattern.
(Contributed by Serhiy Storchaka in :issue:`13968`.)


gzip
----

The *mode* argument of the :class:`~gzip.GzipFile` constructor now
accepts ``"x"`` to request exclusive creation.
(Contributed by Tim Heaney in :issue:`19222`.)


heapq
-----

Element comparison in :func:`~heapq.merge` can now be customized by
passing a :term:`key function` in a new optional *key* keyword argument,
and a new optional *reverse* keyword argument can be used to reverse element
comparison::

    >>> import heapq
    >>> a = ['9', '777', '55555']
    >>> b = ['88', '6666']
    >>> list(heapq.merge(a, b, key=len))
    ['9', '88', '777', '6666', '55555']
    >>> list(heapq.merge(reversed(a), reversed(b), key=len, reverse=True))
    ['55555', '6666', '777', '88', '9']

(Contributed by Raymond Hettinger in :issue:`13742`.)


http
----

A new :class:`HTTPStatus <http.HTTPStatus>` enum that defines a set of
HTTP status codes, reason phrases and long descriptions written in English.
(Contributed by Demian Brecht in :issue:`21793`.)


http.client
-----------

:meth:`HTTPConnection.getresponse() <http.client.HTTPConnection.getresponse>`
now raises a :exc:`~http.client.RemoteDisconnected` exception when a
remote server connection is closed unexpectedly.  Additionally, if a
:exc:`ConnectionError` (of which ``RemoteDisconnected``
is a subclass) is raised, the client socket is now closed automatically,
and will reconnect on the next request::

    import http.client
    conn = http.client.HTTPConnection('www.python.org')
    for retries in range(3):
        try:
            conn.request('GET', '/')
            resp = conn.getresponse()
        except http.client.RemoteDisconnected:
            pass

(Contributed by Martin Panter in :issue:`3566`.)


idlelib and IDLE
----------------

Since idlelib implements the IDLE shell and editor and is not intended for
import by other programs, it gets improvements with every release.  See
:file:`Lib/idlelib/NEWS.txt` for a cumulative list of changes since 3.4.0,
as well as changes made in future 3.5.x releases. This file is also available
from the IDLE :menuselection:`Help --> About IDLE` dialog.


imaplib
-------

The :class:`~imaplib.IMAP4` class now supports the :term:`context manager` protocol.
When used in a :keyword:`with` statement, the IMAP4 ``LOGOUT``
command will be called automatically at the end of the block.
(Contributed by Tarek Ziadé and Serhiy Storchaka in :issue:`4972`.)

The :mod:`imaplib` module now supports :rfc:`5161` (ENABLE Extension)
and :rfc:`6855` (UTF-8 Support) via the :meth:`IMAP4.enable() <imaplib.IMAP4.enable>`
method.  A new :attr:`IMAP4.utf8_enabled <imaplib.IMAP4.utf8_enabled>`
attribute tracks whether or not :rfc:`6855` support is enabled.
(Contributed by Milan Oberkirch, R. David Murray, and Maciej Szulik in
:issue:`21800`.)

The :mod:`imaplib` module now automatically encodes non-ASCII string usernames
and passwords using UTF-8, as recommended by the RFCs.  (Contributed by Milan
Oberkirch in :issue:`21800`.)


imghdr
------

The :func:`~imghdr.what` function now recognizes the
`OpenEXR <http://www.openexr.com>`_ format
(contributed by Martin Vignali and Claudiu Popa in :issue:`20295`),
and the `WebP <https://en.wikipedia.org/wiki/WebP>`_ format
(contributed by Fabrice Aneche and Claudiu Popa in :issue:`20197`.)


importlib
---------

The :class:`util.LazyLoader <importlib.util.LazyLoader>` class allows for
lazy loading of modules in applications where startup time is important.
(Contributed by Brett Cannon in :issue:`17621`.)

The :func:`abc.InspectLoader.source_to_code() <importlib.abc.InspectLoader.source_to_code>`
method is now a static method.  This makes it easier to initialize a module
object with code compiled from a string by running
``exec(code, module.__dict__)``.
(Contributed by Brett Cannon in :issue:`21156`.)

The new :func:`util.module_from_spec() <importlib.util.module_from_spec>`
function is now the preferred way to create a new module.  As opposed to
creating a :class:`types.ModuleType` instance directly, this new function
will set the various import-controlled attributes based on the passed-in
spec object.  (Contributed by Brett Cannon in :issue:`20383`.)


inspect
-------

Both the :class:`~inspect.Signature` and :class:`~inspect.Parameter` classes are
now picklable and hashable.  (Contributed by Yury Selivanov in :issue:`20726`
and :issue:`20334`.)

A new
:meth:`BoundArguments.apply_defaults() <inspect.BoundArguments.apply_defaults>`
method provides a way to set default values for missing arguments::

    >>> def foo(a, b='ham', *args): pass
    >>> ba = inspect.signature(foo).bind('spam')
    >>> ba.apply_defaults()
    >>> ba.arguments
    OrderedDict([('a', 'spam'), ('b', 'ham'), ('args', ())])

(Contributed by Yury Selivanov in :issue:`24190`.)

A new class method
:meth:`Signature.from_callable() <inspect.Signature.from_callable>` makes
subclassing of :class:`~inspect.Signature` easier.  (Contributed
by Yury Selivanov and Eric Snow in :issue:`17373`.)

The :func:`~inspect.signature` function now accepts a *follow_wrapped*
optional keyword argument, which, when set to ``False``, disables automatic
following of ``__wrapped__`` links.
(Contributed by Yury Selivanov in :issue:`20691`.)

A set of new functions to inspect
:term:`coroutine functions <coroutine function>` and
:term:`coroutine objects <coroutine>` has been added:
:func:`~inspect.iscoroutine`, :func:`~inspect.iscoroutinefunction`,
:func:`~inspect.isawaitable`, :func:`~inspect.getcoroutinelocals`,
and :func:`~inspect.getcoroutinestate`.
(Contributed by Yury Selivanov in :issue:`24017` and :issue:`24400`.)

The :func:`~inspect.stack`, :func:`~inspect.trace`,
:func:`~inspect.getouterframes`, and :func:`~inspect.getinnerframes`
functions now return a list of named tuples.
(Contributed by Daniel Shahaf in :issue:`16808`.)


io
--

A new :meth:`BufferedIOBase.readinto1() <io.BufferedIOBase.readinto1>`
method, that uses at most one call to the underlying raw stream's
:meth:`RawIOBase.read() <io.RawIOBase.read>` or
:meth:`RawIOBase.readinto() <io.RawIOBase.readinto>` methods.
(Contributed by Nikolaus Rath in :issue:`20578`.)


ipaddress
---------

Both the :class:`~ipaddress.IPv4Network` and :class:`~ipaddress.IPv6Network` classes
now accept an ``(address, netmask)`` tuple argument, so as to easily construct
network objects from existing addresses::

    >>> import ipaddress
    >>> ipaddress.IPv4Network(('127.0.0.0', 8))
    IPv4Network('127.0.0.0/8')
    >>> ipaddress.IPv4Network(('127.0.0.0', '255.0.0.0'))
    IPv4Network('127.0.0.0/8')

(Contributed by Peter Moody and Antoine Pitrou in :issue:`16531`.)

A new :attr:`~ipaddress.IPv4Network.reverse_pointer` attribute for the
:class:`~ipaddress.IPv4Network` and :class:`~ipaddress.IPv6Network` classes
returns the name of the reverse DNS PTR record::

    >>> import ipaddress
    >>> addr = ipaddress.IPv4Address('127.0.0.1')
    >>> addr.reverse_pointer
    '1.0.0.127.in-addr.arpa'
    >>> addr6 = ipaddress.IPv6Address('::1')
    >>> addr6.reverse_pointer
    '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa'

(Contributed by Leon Weber in :issue:`20480`.)


json
----

The :mod:`json.tool` command line interface now preserves the order of keys in
JSON objects passed in input.  The new ``--sort-keys`` option can be used
to sort the keys alphabetically. (Contributed by Berker Peksag
in :issue:`21650`.)

JSON decoder now raises :exc:`~json.JSONDecodeError` instead of
:exc:`ValueError` to provide better context information about the error.
(Contributed by Serhiy Storchaka in :issue:`19361`.)


linecache
---------

A new :func:`~linecache.lazycache` function can be used to capture information
about a non-file-based module to permit getting its lines later via
:func:`~linecache.getline`. This avoids doing I/O until a line is actually
needed, without having to carry the module globals around indefinitely.
(Contributed by Robert Collins in :issue:`17911`.)


locale
------

A new :func:`~locale.delocalize` function can be used to convert a string into
a normalized number string, taking the ``LC_NUMERIC`` settings into account::

    >>> import locale
    >>> locale.setlocale(locale.LC_NUMERIC, 'de_DE.UTF-8')
    'de_DE.UTF-8'
    >>> locale.delocalize('1.234,56')
    '1234.56'
    >>> locale.setlocale(locale.LC_NUMERIC, 'en_US.UTF-8')
    'en_US.UTF-8'
    >>> locale.delocalize('1,234.56')
    '1234.56'

(Contributed by Cédric Krier in :issue:`13918`.)


logging
-------

All logging methods (:class:`~logging.Logger` :meth:`~logging.Logger.log`,
:meth:`~logging.Logger.exception`, :meth:`~logging.Logger.critical`,
:meth:`~logging.Logger.debug`, etc.), now accept exception instances
as an *exc_info* argument, in addition to boolean values and exception
tuples::

    >>> import logging
    >>> try:
    ...     1/0
    ... except ZeroDivisionError as ex:
    ...     logging.error('exception', exc_info=ex)
    ERROR:root:exception

(Contributed by Yury Selivanov in :issue:`20537`.)

The :class:`handlers.HTTPHandler <logging.handlers.HTTPHandler>` class now
accepts an optional :class:`ssl.SSLContext` instance to configure SSL
settings used in an HTTP connection.
(Contributed by Alex Gaynor in :issue:`22788`.)

The :class:`handlers.QueueListener <logging.handlers.QueueListener>` class now
takes a *respect_handler_level* keyword argument which, if set to ``True``,
will pass messages to handlers taking handler levels into account.
(Contributed by Vinay Sajip.)


lzma
----

The :meth:`LZMADecompressor.decompress() <lzma.LZMADecompressor.decompress>`
method now accepts an optional *max_length* argument to limit the maximum
size of decompressed data.
(Contributed by Martin Panter in :issue:`15955`.)


math
----

Two new constants have been added to the :mod:`math` module: :data:`~math.inf`
and :data:`~math.nan`.  (Contributed by Mark Dickinson in :issue:`23185`.)

A new function :func:`~math.isclose` provides a way to test for approximate
equality. (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)

A new :func:`~math.gcd` function has been added.  The :func:`fractions.gcd`
function is now deprecated. (Contributed by Mark Dickinson and Serhiy
Storchaka in :issue:`22486`.)


multiprocessing
---------------

:func:`sharedctypes.synchronized() <multiprocessing.sharedctypes.synchronized>`
objects now support the :term:`context manager` protocol.
(Contributed by Charles-François Natali in :issue:`21565`.)


operator
--------

:func:`~operator.attrgetter`, :func:`~operator.itemgetter`,
and :func:`~operator.methodcaller` objects now support pickling.
(Contributed by Josh Rosenberg and Serhiy Storchaka in :issue:`22955`.)

New :func:`~operator.matmul` and :func:`~operator.imatmul` functions
to perform matrix multiplication.
(Contributed by Benjamin Peterson in :issue:`21176`.)


os
--

The new :func:`~os.scandir` function returning an iterator of
:class:`~os.DirEntry` objects has been added.  If possible, :func:`~os.scandir`
extracts file attributes while scanning a directory, removing the need to
perform subsequent system calls to determine file type or attributes, which may
significantly improve performance.  (Contributed by Ben Hoyt with the help
of Victor Stinner in :issue:`22524`.)

On Windows, a new
:attr:`stat_result.st_file_attributes <os.stat_result.st_file_attributes>`
attribute is now available.  It corresponds to the ``dwFileAttributes`` member
of the ``BY_HANDLE_FILE_INFORMATION`` structure returned by
``GetFileInformationByHandle()``.  (Contributed by Ben Hoyt in :issue:`21719`.)

The :func:`~os.urandom` function now uses the ``getrandom()`` syscall on Linux 3.17
or newer, and ``getentropy()`` on OpenBSD 5.6 and newer, removing the need to
use ``/dev/urandom`` and avoiding failures due to potential file descriptor
exhaustion.  (Contributed by Victor Stinner in :issue:`22181`.)

New :func:`~os.get_blocking` and :func:`~os.set_blocking` functions allow
getting and setting a file descriptor's blocking mode (:data:`~os.O_NONBLOCK`.)
(Contributed by Victor Stinner in :issue:`22054`.)

The :func:`~os.truncate` and :func:`~os.ftruncate` functions are now supported
on Windows.  (Contributed by Steve Dower in :issue:`23668`.)

There is a new :func:`os.path.commonpath` function returning the longest
common sub-path of each passed pathname.  Unlike the
:func:`os.path.commonprefix` function, it always returns a valid
path::

    >>> os.path.commonprefix(['/usr/lib', '/usr/local/lib'])
    '/usr/l'

    >>> os.path.commonpath(['/usr/lib', '/usr/local/lib'])
    '/usr'

(Contributed by Rafik Draoui and Serhiy Storchaka in :issue:`10395`.)


pathlib
-------

The new :meth:`Path.samefile() <pathlib.Path.samefile>` method can be used
to check whether the path points to the same file as another path, which can
be either another :class:`~pathlib.Path` object, or a string::

    >>> import pathlib
    >>> p1 = pathlib.Path('/etc/hosts')
    >>> p2 = pathlib.Path('/etc/../etc/hosts')
    >>> p1.samefile(p2)
    True

(Contributed by Vajrasky Kok and Antoine Pitrou in :issue:`19775`.)

The :meth:`Path.mkdir() <pathlib.Path.mkdir>` method now accepts a new optional
*exist_ok* argument to match ``mkdir -p`` and :func:`os.makedirs`
functionality.  (Contributed by Berker Peksag in :issue:`21539`.)

There is a new :meth:`Path.expanduser() <pathlib.Path.expanduser>` method to
expand ``~`` and ``~user`` prefixes.  (Contributed by Serhiy Storchaka and
Claudiu Popa in :issue:`19776`.)

A new :meth:`Path.home() <pathlib.Path.home>` class method can be used to get
a :class:`~pathlib.Path` instance representing the user’s home
directory.
(Contributed by Victor Salgado and Mayank Tripathi in :issue:`19777`.)

New :meth:`Path.write_text() <pathlib.Path.write_text>`,
:meth:`Path.read_text() <pathlib.Path.read_text>`,
:meth:`Path.write_bytes() <pathlib.Path.write_bytes>`,
:meth:`Path.read_bytes() <pathlib.Path.read_bytes>` methods to simplify
read/write operations on files.

The following code snippet will create or rewrite existing file
``~/spam42``::

    >>> import pathlib
    >>> p = pathlib.Path('~/spam42')
    >>> p.expanduser().write_text('ham')
    3

(Contributed by Christopher Welborn in :issue:`20218`.)


pickle
------

Nested objects, such as unbound methods or nested classes, can now be pickled
using :ref:`pickle protocols <pickle-protocols>` older than protocol version 4.
Protocol version 4 already supports these cases.  (Contributed by Serhiy
Storchaka in :issue:`23611`.)


poplib
------

A new :meth:`POP3.utf8() <poplib.POP3.utf8>` command enables :rfc:`6856`
(Internationalized Email) support, if a POP server supports it.
(Contributed by Milan OberKirch in :issue:`21804`.)


re
--

References and conditional references to groups with fixed length are now
allowed in lookbehind assertions::

    >>> import re
    >>> pat = re.compile(r'(a|b).(?<=\1)c')
    >>> pat.match('aac')
    <_sre.SRE_Match object; span=(0, 3), match='aac'>
    >>> pat.match('bbc')
    <_sre.SRE_Match object; span=(0, 3), match='bbc'>

(Contributed by Serhiy Storchaka in :issue:`9179`.)

The number of capturing groups in regular expressions is no longer limited to
100.  (Contributed by Serhiy Storchaka in :issue:`22437`.)

The :func:`~re.sub` and :func:`~re.subn` functions now replace unmatched
groups with empty strings instead of raising an exception.
(Contributed by Serhiy Storchaka in :issue:`1519638`.)

The :class:`re.error` exceptions have new attributes,
:attr:`~re.error.msg`, :attr:`~re.error.pattern`,
:attr:`~re.error.pos`, :attr:`~re.error.lineno`,
and :attr:`~re.error.colno`, that provide better context
information about the error::

    >>> re.compile("""
    ...     (?x)
    ...     .++
    ... """)
    Traceback (most recent call last):
       ...
    sre_constants.error: multiple repeat at position 16 (line 3, column 7)

(Contributed by Serhiy Storchaka in :issue:`22578`.)


readline
--------

A new :func:`~readline.append_history_file` function can be used to append
the specified number of trailing elements in history to the given file.
(Contributed by Bruno Cauet in :issue:`22940`.)


selectors
---------

The new :class:`~selectors.DevpollSelector` supports efficient
``/dev/poll`` polling on Solaris.
(Contributed by Giampaolo Rodola' in :issue:`18931`.)


shutil
------

The :func:`~shutil.move` function now accepts a *copy_function* argument,
allowing, for example, the :func:`~shutil.copy` function to be used instead of
the default :func:`~shutil.copy2` if there is a need to ignore file metadata
when moving.
(Contributed by Claudiu Popa in :issue:`19840`.)

The :func:`~shutil.make_archive` function now supports the *xztar* format.
(Contributed by Serhiy Storchaka in :issue:`5411`.)


signal
------

On Windows, the :func:`~signal.set_wakeup_fd` function now also supports
socket handles.  (Contributed by Victor Stinner in :issue:`22018`.)

Various ``SIG*`` constants in the :mod:`signal` module have been converted into
:mod:`Enums <enum>`.  This allows meaningful names to be printed
during debugging, instead of integer "magic numbers".
(Contributed by Giampaolo Rodola' in :issue:`21076`.)


smtpd
-----

Both the :class:`~smtpd.SMTPServer` and :class:`~smtpd.SMTPChannel` classes now
accept a *decode_data* keyword argument to determine if the ``DATA`` portion of
the SMTP transaction is decoded using the ``"utf-8"`` codec or is instead
provided to the
:meth:`SMTPServer.process_message() <smtpd.SMTPServer.process_message>`
method as a byte string.  The default is ``True`` for backward compatibility
reasons, but will change to ``False`` in Python 3.6.  If *decode_data* is set
to ``False``, the ``process_message`` method must be prepared to accept keyword
arguments.
(Contributed by Maciej Szulik in :issue:`19662`.)

The :class:`~smtpd.SMTPServer` class now advertises the ``8BITMIME`` extension
(:rfc:`6152`) if *decode_data* has been set ``True``.  If the client
specifies ``BODY=8BITMIME`` on the ``MAIL`` command, it is passed to
:meth:`SMTPServer.process_message() <smtpd.SMTPServer.process_message>`
via the *mail_options* keyword.
(Contributed by Milan Oberkirch and R.  David Murray in :issue:`21795`.)

The :class:`~smtpd.SMTPServer` class now also supports the ``SMTPUTF8``
extension (:rfc:`6531`: Internationalized Email).  If the client specified
``SMTPUTF8 BODY=8BITMIME`` on the ``MAIL`` command, they are passed to
:meth:`SMTPServer.process_message() <smtpd.SMTPServer.process_message>`
via the *mail_options* keyword.  It is the responsibility of the
``process_message`` method to correctly handle the ``SMTPUTF8`` data.
(Contributed by Milan Oberkirch in :issue:`21725`.)

It is now possible to provide, directly or via name resolution, IPv6
addresses in the :class:`~smtpd.SMTPServer` constructor, and have it
successfully connect.  (Contributed by Milan Oberkirch in :issue:`14758`.)


smtplib
-------

A new :meth:`SMTP.auth() <smtplib.SMTP.auth>` method provides a convenient way to
implement custom authentication mechanisms. (Contributed by Milan
Oberkirch in :issue:`15014`.)

The :meth:`SMTP.set_debuglevel() <smtplib.SMTP.set_debuglevel>` method now
accepts an additional debuglevel (2), which enables timestamps in debug
messages. (Contributed by Gavin Chappell and Maciej Szulik in :issue:`16914`.)

Both the :meth:`SMTP.sendmail() <smtplib.SMTP.sendmail>` and
:meth:`SMTP.send_message() <smtplib.SMTP.send_message>` methods now
support :rfc:`6531` (SMTPUTF8).
(Contributed by Milan Oberkirch and R. David Murray in :issue:`22027`.)


sndhdr
------

The :func:`~sndhdr.what` and :func:`~sndhdr.whathdr` functions  now return
a :func:`~collections.namedtuple`.  (Contributed by Claudiu Popa in
:issue:`18615`.)


socket
------

Functions with timeouts now use a monotonic clock, instead of a system clock.
(Contributed by Victor Stinner in :issue:`22043`.)

A new :meth:`socket.sendfile() <socket.socket.sendfile>` method allows
sending a file over a socket by using the high-performance :func:`os.sendfile`
function on UNIX, resulting in uploads being from 2 to 3 times faster than when
using plain :meth:`socket.send() <socket.socket.send>`.
(Contributed by Giampaolo Rodola' in :issue:`17552`.)

The :meth:`socket.sendall() <socket.socket.sendall>` method no longer resets the
socket timeout every time bytes are received or sent.  The socket timeout is
now the maximum total duration to send all data.
(Contributed by Victor Stinner in :issue:`23853`.)

The *backlog* argument of the :meth:`socket.listen() <socket.socket.listen>`
method is now optional.  By default it is set to
:data:`SOMAXCONN <socket.SOMAXCONN>` or to ``128``, whichever is less.
(Contributed by Charles-François Natali in :issue:`21455`.)


ssl
---

.. _whatsnew-sslmemorybio:

Memory BIO Support
~~~~~~~~~~~~~~~~~~

(Contributed by Geert Jansen in :issue:`21965`.)

The new :class:`~ssl.SSLObject` class has been added to provide SSL protocol
support for cases when the network I/O capabilities of :class:`~ssl.SSLSocket`
are not necessary or are suboptimal.  ``SSLObject`` represents
an SSL protocol instance, but does not implement any network I/O methods, and
instead provides a memory buffer interface.  The new :class:`~ssl.MemoryBIO`
class can be used to pass data between Python and an SSL protocol instance.

The memory BIO SSL support is primarily intended to be used in frameworks
implementing asynchronous I/O for which :class:`~ssl.SSLSocket`'s readiness
model ("select/poll") is inefficient.

A new :meth:`SSLContext.wrap_bio() <ssl.SSLContext.wrap_bio>` method can be used
to create a new ``SSLObject`` instance.


Application-Layer Protocol Negotiation Support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(Contributed by Benjamin Peterson in :issue:`20188`.)

Where OpenSSL support is present, the :mod:`ssl` module now implements
the *Application-Layer Protocol Negotiation* TLS extension as described
in :rfc:`7301`.

The new :meth:`SSLContext.set_alpn_protocols() <ssl.SSLContext.set_alpn_protocols>`
can be used to specify which protocols a socket should advertise during
the TLS handshake.

The new
:meth:`SSLSocket.selected_alpn_protocol() <ssl.SSLSocket.selected_alpn_protocol>`
returns the protocol that was selected during the TLS handshake.
The :data:`~ssl.HAS_ALPN` flag indicates whether ALPN support is present.


Other Changes
~~~~~~~~~~~~~

There is a new :meth:`SSLSocket.version() <ssl.SSLSocket.version>` method to
query the actual protocol version in use.
(Contributed by Antoine Pitrou in :issue:`20421`.)

The :class:`~ssl.SSLSocket` class now implements
a :meth:`SSLSocket.sendfile() <ssl.SSLSocket.sendfile>` method.
(Contributed by Giampaolo Rodola' in :issue:`17552`.)

The :meth:`SSLSocket.send() <ssl.SSLSocket.send>` method now raises either
the :exc:`ssl.SSLWantReadError` or :exc:`ssl.SSLWantWriteError` exception on a
non-blocking socket if the operation would block. Previously, it would return
``0``.  (Contributed by Nikolaus Rath in :issue:`20951`.)

The :func:`~ssl.cert_time_to_seconds` function now interprets the input time
as UTC and not as local time, per :rfc:`5280`.  Additionally, the return
value is always an :class:`int`. (Contributed by Akira Li in :issue:`19940`.)

New :meth:`SSLObject.shared_ciphers() <ssl.SSLObject.shared_ciphers>` and
:meth:`SSLSocket.shared_ciphers() <ssl.SSLSocket.shared_ciphers>` methods return
the list of ciphers sent by the client during the handshake.
(Contributed by Benjamin Peterson in :issue:`23186`.)

The :meth:`SSLSocket.do_handshake() <ssl.SSLSocket.do_handshake>`,
:meth:`SSLSocket.read() <ssl.SSLSocket.read>`,
:meth:`SSLSocket.shutdown() <ssl.SSLSocket.shutdown>`, and
:meth:`SSLSocket.write() <ssl.SSLSocket.write>` methods of the :class:`~ssl.SSLSocket`
class no longer reset the socket timeout every time bytes are received or sent.
The socket timeout is now the maximum total duration of the method.
(Contributed by Victor Stinner in :issue:`23853`.)

The :func:`~ssl.match_hostname` function now supports matching of IP addresses.
(Contributed by Antoine Pitrou in :issue:`23239`.)


sqlite3
-------

The :class:`~sqlite3.Row` class now fully supports the sequence protocol,
in particular :func:`reversed` iteration and slice indexing.
(Contributed by Claudiu Popa in :issue:`10203`; by Lucas Sinclair,
Jessica McKellar, and  Serhiy Storchaka in :issue:`13583`.)


.. _whatsnew-subprocess:

subprocess
----------

The new :func:`~subprocess.run` function has been added.
It runs the specified command and returns a
:class:`~subprocess.CompletedProcess` object, which describes a finished
process.  The new API is more consistent and is the recommended approach
to invoking subprocesses in Python code that does not need to maintain
compatibility with earlier Python versions.
(Contributed by Thomas Kluyver in :issue:`23342`.)

Examples::

    >>> subprocess.run(["ls", "-l"])  # doesn't capture output
    CompletedProcess(args=['ls', '-l'], returncode=0)

    >>> subprocess.run("exit 1", shell=True, check=True)
    Traceback (most recent call last):
      ...
    subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

    >>> subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
    CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
    stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n')


sys
---

A new :func:`~sys.set_coroutine_wrapper` function allows setting a global
hook that will be called whenever a :term:`coroutine object <coroutine>`
is created by an :keyword:`async def` function.  A corresponding
:func:`~sys.get_coroutine_wrapper` can be used to obtain a currently set
wrapper.  Both functions are :term:`provisional <provisional api>`,
and are intended for debugging purposes only.  (Contributed by Yury Selivanov
in :issue:`24017`.)

A new :func:`~sys.is_finalizing` function can be used to check if the Python
interpreter is :term:`shutting down <interpreter shutdown>`.
(Contributed by Antoine Pitrou in :issue:`22696`.)


sysconfig
---------

The name of the user scripts directory on Windows now includes the first
two components of the Python version. (Contributed by Paul Moore
in :issue:`23437`.)


tarfile
-------

The *mode* argument of the :func:`~tarfile.open` function now accepts ``"x"``
to request exclusive creation.  (Contributed by Berker Peksag in :issue:`21717`.)

The :meth:`TarFile.extractall() <tarfile.TarFile.extractall>` and
:meth:`TarFile.extract() <tarfile.TarFile.extract>` methods now take a keyword
argument *numeric_only*.  If set to ``True``, the extracted files and
directories will be owned by the numeric ``uid`` and ``gid`` from the tarfile.
If set to ``False`` (the default, and the behavior in versions prior to 3.5),
they will be owned by the named user and group in the tarfile.
(Contributed by Michael Vogt and Eric Smith in :issue:`23193`.)

The :meth:`TarFile.list() <tarfile.TarFile.list>` now accepts an optional
*members* keyword argument that can be set to a subset of the list returned
by :meth:`TarFile.getmembers() <tarfile.TarFile.getmembers>`.
(Contributed by Serhiy Storchaka in :issue:`21549`.)


threading
---------

Both the :meth:`Lock.acquire() <threading.Lock.acquire>` and
:meth:`RLock.acquire() <threading.RLock.acquire>` methods
now use a monotonic clock for timeout management.
(Contributed by Victor Stinner in :issue:`22043`.)


time
----

The :func:`~time.monotonic` function is now always available.
(Contributed by Victor Stinner in :issue:`22043`.)


timeit
------

A new command line option ``-u`` or :samp:`--unit={U}` can be used to specify the time
unit for the timer output.  Supported options are ``usec``, ``msec``,
or ``sec``.  (Contributed by Julian Gindi in :issue:`18983`.)

The :func:`~timeit.timeit` function has a new *globals* parameter for
specifying the namespace in which the code will be running.
(Contributed by Ben Roberts in :issue:`2527`.)


tkinter
-------

The :mod:`tkinter._fix` module used for setting up the Tcl/Tk environment
on Windows has been replaced by a private function in the :mod:`_tkinter`
module which makes no permanent changes to environment variables.
(Contributed by Zachary Ware in :issue:`20035`.)


.. _whatsnew-traceback:

traceback
---------

New :func:`~traceback.walk_stack` and :func:`~traceback.walk_tb`
functions to conveniently traverse frame and traceback objects.
(Contributed by Robert Collins in :issue:`17911`.)

New lightweight classes: :class:`~traceback.TracebackException`,
:class:`~traceback.StackSummary`, and :class:`~traceback.FrameSummary`.
(Contributed by Robert Collins in :issue:`17911`.)

Both the :func:`~traceback.print_tb` and :func:`~traceback.print_stack` functions
now support negative values for the *limit* argument.
(Contributed by Dmitry Kazakov in :issue:`22619`.)


types
-----

A new :func:`~types.coroutine` function to transform
:term:`generator <generator iterator>` and
:class:`generator-like <collections.abc.Generator>` objects into
:term:`awaitables <awaitable>`.
(Contributed by Yury Selivanov in :issue:`24017`.)

A new type called :class:`~types.CoroutineType`, which is used for
:term:`coroutine` objects created by :keyword:`async def` functions.
(Contributed by Yury Selivanov in :issue:`24400`.)


unicodedata
-----------

The :mod:`unicodedata` module now uses data from `Unicode 8.0.0
<http://unicode.org/versions/Unicode8.0.0/>`_.


unittest
--------

The :meth:`TestLoader.loadTestsFromModule() <unittest.TestLoader.loadTestsFromModule>`
method now accepts a keyword-only argument *pattern* which is passed to
``load_tests`` as the third argument.  Found packages are now checked for
``load_tests`` regardless of whether their path matches *pattern*, because it
is impossible for a package name to match the default pattern.
(Contributed by Robert Collins and Barry A. Warsaw in :issue:`16662`.)

Unittest discovery errors now are exposed in the
:data:`TestLoader.errors <unittest.TestLoader.errors>` attribute of the
:class:`~unittest.TestLoader` instance.
(Contributed by Robert Collins in :issue:`19746`.)

A new command line option ``--locals`` to show local variables in
tracebacks.  (Contributed by Robert Collins in :issue:`22936`.)


unittest.mock
-------------

The :class:`~unittest.mock.Mock` class has the following improvements:

* The class constructor has a new *unsafe* parameter, which causes mock
  objects to raise :exc:`AttributeError` on attribute names starting
  with ``"assert"``.
  (Contributed by Kushal Das in :issue:`21238`.)

* A new :meth:`Mock.assert_not_called() <unittest.mock.Mock.assert_not_called>`
  method to check if the mock object was called.
  (Contributed by Kushal Das in :issue:`21262`.)

The :class:`~unittest.mock.MagicMock` class now supports :meth:`__truediv__`,
:meth:`__divmod__` and :meth:`__matmul__` operators.
(Contributed by Johannes Baiter in :issue:`20968`, and Håkan Lövdahl
in :issue:`23581` and :issue:`23568`.)

It is no longer necessary to explicitly pass ``create=True`` to the
:func:`~unittest.mock.patch` function when patching builtin names.
(Contributed by Kushal Das in :issue:`17660`.)


urllib
------

A new
:class:`request.HTTPPasswordMgrWithPriorAuth <urllib.request.HTTPPasswordMgrWithPriorAuth>`
class allows HTTP Basic Authentication credentials to be managed so as to
eliminate unnecessary ``401`` response handling, or to unconditionally send
credentials on the first request in order to communicate with servers that
return a ``404`` response instead of a ``401`` if the ``Authorization`` header
is not sent. (Contributed by Matej Cepl in :issue:`19494` and Akshit Khurana in
:issue:`7159`.)

A new *quote_via* argument for the
:func:`parse.urlencode() <urllib.parse.urlencode>`
function provides a way to control the encoding of query parts if needed.
(Contributed by Samwyse and Arnon Yaari in :issue:`13866`.)

The :func:`request.urlopen() <urllib.request.urlopen>` function accepts an
:class:`ssl.SSLContext` object as a *context* argument, which will be used for
the HTTPS connection.  (Contributed by Alex Gaynor in :issue:`22366`.)

The :func:`parse.urljoin() <urllib.parse.urljoin>` was updated to use the
:rfc:`3986` semantics for the resolution of relative URLs, rather than
:rfc:`1808` and :rfc:`2396`.
(Contributed by Demian Brecht and Senthil Kumaran in :issue:`22118`.)


wsgiref
-------

The *headers* argument of the :class:`headers.Headers <wsgiref.headers.Headers>`
class constructor is now optional.
(Contributed by Pablo Torres Navarrete and SilentGhost in :issue:`5800`.)


xmlrpc
------

The :class:`client.ServerProxy <xmlrpc.client.ServerProxy>` class now supports
the :term:`context manager` protocol.
(Contributed by Claudiu Popa in :issue:`20627`.)

The :class:`client.ServerProxy <xmlrpc.client.ServerProxy>` constructor now accepts
an optional :class:`ssl.SSLContext` instance.
(Contributed by Alex Gaynor in :issue:`22960`.)


xml.sax
-------

SAX parsers now support a character stream of the
:class:`xmlreader.InputSource <xml.sax.xmlreader.InputSource>` object.
(Contributed by Serhiy Storchaka in :issue:`2175`.)

:func:`~xml.sax.parseString` now accepts a :class:`str` instance.
(Contributed by Serhiy Storchaka in :issue:`10590`.)


zipfile
-------

ZIP output can now be written to unseekable streams.
(Contributed by Serhiy Storchaka in :issue:`23252`.)

The *mode* argument of :meth:`ZipFile.open() <zipfile.ZipFile.open>` method now
accepts ``"x"`` to request exclusive creation.
(Contributed by Serhiy Storchaka in :issue:`21717`.)


Other module-level changes
==========================

Many functions in the :mod:`mmap`, :mod:`ossaudiodev`, :mod:`socket`,
:mod:`ssl`, and :mod:`codecs` modules now accept writable
:term:`bytes-like objects <bytes-like object>`.
(Contributed by Serhiy Storchaka in :issue:`23001`.)


Optimizations
=============

The :func:`os.walk` function has been sped up by 3 to 5 times on POSIX systems,
and by 7 to 20 times on Windows.  This was done using the new :func:`os.scandir`
function, which exposes file information from the underlying ``readdir`` or
``FindFirstFile``/``FindNextFile`` system calls.  (Contributed by
Ben Hoyt with help from Victor Stinner in :issue:`23605`.)

Construction of ``bytes(int)`` (filled by zero bytes) is faster and uses less
memory for large objects. ``calloc()`` is used instead of ``malloc()`` to
allocate memory for these objects.
(Contributed by Victor Stinner in :issue:`21233`.)

Some operations on :mod:`ipaddress` :class:`~ipaddress.IPv4Network` and
:class:`~ipaddress.IPv6Network` have been massively sped up, such as
:meth:`~ipaddress.IPv4Network.subnets`, :meth:`~ipaddress.IPv4Network.supernet`,
:func:`~ipaddress.summarize_address_range`, :func:`~ipaddress.collapse_addresses`.
The speed up can range from 3 to 15 times.
(Contributed by Antoine Pitrou, Michel Albert, and Markus in
:issue:`21486`, :issue:`21487`, :issue:`20826`, :issue:`23266`.)

Pickling of :mod:`ipaddress` objects was optimized to produce significantly
smaller output.  (Contributed by Serhiy Storchaka in :issue:`23133`.)

Many operations on :class:`io.BytesIO` are now 50% to 100% faster.
(Contributed by Serhiy Storchaka in :issue:`15381` and David Wilson in
:issue:`22003`.)

The :func:`marshal.dumps` function is now faster: 65-85% with versions 3
and 4, 20-25% with versions 0 to 2 on typical data, and up to 5 times in
best cases.
(Contributed by Serhiy Storchaka in :issue:`20416` and :issue:`23344`.)

The UTF-32 encoder is now 3 to 7 times faster.
(Contributed by Serhiy Storchaka in :issue:`15027`.)

Regular expressions are now parsed up to 10% faster.
(Contributed by Serhiy Storchaka in :issue:`19380`.)

The :func:`json.dumps` function was optimized to run with
``ensure_ascii=False`` as fast as with ``ensure_ascii=True``.
(Contributed by Naoki Inada in :issue:`23206`.)

The :c:func:`PyObject_IsInstance` and :c:func:`PyObject_IsSubclass`
functions have been sped up in the common case that the second argument
has :class:`type` as its metaclass.
(Contributed Georg Brandl by in :issue:`22540`.)

Method caching was slightly improved, yielding up to 5% performance
improvement in some benchmarks.
(Contributed by Antoine Pitrou in :issue:`22847`.)

Objects from the :mod:`random` module now use 50% less memory on 64-bit
builds.  (Contributed by Serhiy Storchaka in :issue:`23488`.)

The :func:`property` getter calls are up to 25% faster.
(Contributed by Joe Jevnik in :issue:`23910`.)

Instantiation of :class:`fractions.Fraction` is now up to 30% faster.
(Contributed by Stefan Behnel in :issue:`22464`.)

String methods :meth:`~str.find`, :meth:`~str.rfind`, :meth:`~str.split`,
:meth:`~str.partition` and the :keyword:`in` string operator are now significantly
faster for searching 1-character substrings.
(Contributed by Serhiy Storchaka in :issue:`23573`.)


Build and C API Changes
=======================

New ``calloc`` functions were added:

* :c:func:`PyMem_RawCalloc`,
* :c:func:`PyMem_Calloc`,
* :c:func:`PyObject_Calloc`.

(Contributed by Victor Stinner in :issue:`21233`.)

New encoding/decoding helper functions:

* :c:func:`Py_DecodeLocale` (replaced ``_Py_char2wchar()``),
* :c:func:`Py_EncodeLocale` (replaced ``_Py_wchar2char()``).

(Contributed by Victor Stinner in :issue:`18395`.)

A new :c:func:`PyCodec_NameReplaceErrors` function to replace the unicode
encode error with ``\N{...}`` escapes.
(Contributed by Serhiy Storchaka in :issue:`19676`.)

A new :c:func:`PyErr_FormatV` function similar to :c:func:`PyErr_Format`,
but accepts a ``va_list`` argument.
(Contributed by Antoine Pitrou in :issue:`18711`.)

A new :c:data:`PyExc_RecursionError` exception.
(Contributed by Georg Brandl in :issue:`19235`.)

New :c:func:`PyModule_FromDefAndSpec`, :c:func:`PyModule_FromDefAndSpec2`,
and :c:func:`PyModule_ExecDef` functions introduced by :pep:`489` --
multi-phase extension module initialization.
(Contributed by Petr Viktorin in :issue:`24268`.)

New :c:func:`PyNumber_MatrixMultiply` and
:c:func:`PyNumber_InPlaceMatrixMultiply` functions to perform matrix
multiplication.
(Contributed by Benjamin Peterson in :issue:`21176`.  See also :pep:`465`
for details.)

The :c:member:`PyTypeObject.tp_finalize` slot is now part of the stable ABI.

Windows builds now require Microsoft Visual C++ 14.0, which
is available as part of `Visual Studio 2015 <https://www.visualstudio.com/>`_.

Extension modules now include a platform information tag in their filename on
some platforms (the tag is optional, and CPython will import extensions without
it, although if the tag is present and mismatched, the extension won't be
loaded):

* On Linux, extension module filenames end with
  ``.cpython-<major><minor>m-<architecture>-<os>.pyd``:

  * ``<major>`` is the major number of the Python version;
    for Python 3.5 this is ``3``.

  * ``<minor>`` is the minor number of the Python version;
    for Python 3.5 this is ``5``.

  * ``<architecture>`` is the hardware architecture the extension module
    was built to run on. It's most commonly either ``i386`` for 32-bit Intel
    platforms or ``x86_64`` for 64-bit Intel (and AMD) platforms.

  * ``<os>`` is always ``linux-gnu``, except for extensions built to
    talk to the 32-bit ABI on 64-bit platforms, in which case it is
    ``linux-gnu32`` (and ``<architecture>`` will be ``x86_64``).

* On Windows, extension module filenames end with
  ``<debug>.cp<major><minor>-<platform>.pyd``:

  * ``<major>`` is the major number of the Python version;
    for Python 3.5 this is ``3``.

  * ``<minor>`` is the minor number of the Python version;
    for Python 3.5 this is ``5``.

  * ``<platform>`` is the platform the extension module was built for,
    either ``win32`` for Win32, ``win_amd64`` for Win64, ``win_ia64`` for
    Windows Itanium 64, and ``win_arm`` for Windows on ARM.

  * If built in debug mode, ``<debug>`` will be ``_d``,
    otherwise it will be blank.

* On OS X platforms, extension module filenames now end with ``-darwin.so``.

* On all other platforms, extension module filenames are the same as they were
  with Python 3.4.


Deprecated
==========

New Keywords
------------

``async`` and ``await`` are not recommended to be used as variable, class,
function or module names.  Introduced by :pep:`492` in Python 3.5, they will
become proper keywords in Python 3.7.


Deprecated Python Behavior
--------------------------

Raising the :exc:`StopIteration` exception inside a generator will now generate a silent
:exc:`PendingDeprecationWarning`, which will become a non-silent deprecation
warning in Python 3.6 and will trigger a :exc:`RuntimeError` in Python 3.7.
See :ref:`PEP 479: Change StopIteration handling inside generators <whatsnew-pep-479>`
for details.


Unsupported Operating Systems
-----------------------------

Windows XP is no longer supported by Microsoft, thus, per :PEP:`11`, CPython
3.5 is no longer officially supported on this OS.


Deprecated Python modules, functions and methods
------------------------------------------------

The :mod:`formatter` module has now graduated to full deprecation and is still
slated for removal in Python 3.6.

The :func:`asyncio.async` function is deprecated in favor of
:func:`~asyncio.ensure_future`.

The :mod:`smtpd` module has in the past always decoded the DATA portion of
email messages using the ``utf-8`` codec.  This can now be controlled by the
new *decode_data* keyword to :class:`~smtpd.SMTPServer`.  The default value is
``True``, but this default is deprecated.  Specify the *decode_data* keyword
with an appropriate value to avoid the deprecation warning.

Directly assigning values to the :attr:`~http.cookies.Morsel.key`,
:attr:`~http.cookies.Morsel.value` and
:attr:`~http.cookies.Morsel.coded_value` of :class:`http.cookies.Morsel`
objects is deprecated.  Use the :meth:`~http.cookies.Morsel.set` method
instead.  In addition, the undocumented *LegalChars* parameter of
:meth:`~http.cookies.Morsel.set` is deprecated, and is now ignored.

Passing a format string as keyword argument *format_string* to the
:meth:`~string.Formatter.format` method of the :class:`string.Formatter`
class has been deprecated.
(Contributed by Serhiy Storchaka in :issue:`23671`.)

The :func:`platform.dist` and :func:`platform.linux_distribution` functions
are now deprecated.  Linux distributions use too many different ways of
describing themselves, so the functionality is left to a package.
(Contributed by Vajrasky Kok and Berker Peksag in :issue:`1322`.)

The previously undocumented ``from_function`` and ``from_builtin`` methods of
:class:`inspect.Signature` are deprecated.  Use the new
:meth:`Signature.from_callable() <inspect.Signature.from_callable>`
method instead. (Contributed by Yury Selivanov in :issue:`24248`.)

The :func:`inspect.getargspec` function is deprecated and scheduled to be
removed in Python 3.6.  (See :issue:`20438` for details.)

The :mod:`inspect` :func:`~inspect.getfullargspec`,
:func:`~inspect.getargvalues`, :func:`~inspect.getcallargs`,
:func:`~inspect.getargvalues`, :func:`~inspect.formatargspec`, and
:func:`~inspect.formatargvalues` functions are deprecated in favor of
the :func:`inspect.signature` API.
(Contributed by Yury Selivanov in :issue:`20438`.)

Use of :const:`re.LOCALE` flag with str patterns or :const:`re.ASCII` is now
deprecated.  (Contributed by Serhiy Storchaka in :issue:`22407`.)

Use of unrecognized special sequences consisting of ``'\'`` and an ASCII letter
in regular expression patterns and replacement patterns now raises a
deprecation warning and will be forbidden in Python 3.6.
(Contributed by Serhiy Storchaka in :issue:`23622`.)

The undocumented and unofficial *use_load_tests* default argument of the
:meth:`unittest.TestLoader.loadTestsFromModule` method now is
deprecated and ignored.
(Contributed by Robert Collins and Barry A. Warsaw in :issue:`16662`.)


Removed
=======

API and Feature Removals
------------------------

The following obsolete and previously deprecated APIs and features have been
removed:

* The ``__version__`` attribute has been dropped from the email package.  The
  email code hasn't been shipped separately from the stdlib for a long time,
  and the ``__version__`` string was not updated in the last few releases.

* The internal ``Netrc`` class in the :mod:`ftplib` module was deprecated in
  3.4, and has now been removed.
  (Contributed by Matt Chaput in :issue:`6623`.)

* The concept of ``.pyo`` files has been removed.

* The JoinableQueue class in the provisional :mod:`asyncio` module was
  deprecated in 3.4.4 and is now removed.
  (Contributed by A. Jesse Jiryu Davis in :issue:`23464`.)


Porting to Python 3.5
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.


Changes in Python behavior
--------------------------

* Due to an oversight, earlier Python versions erroneously accepted the
  following syntax::

      f(1 for x in [1], *args)
      f(1 for x in [1], **kwargs)

  Python 3.5 now correctly raises a :exc:`SyntaxError`, as generator
  expressions must be put in parentheses if not a sole argument to a function.


Changes in the Python API
-------------------------

* :pep:`475`: System calls are now retried when interrupted by a signal instead
  of raising :exc:`InterruptedError` if the Python signal handler does not
  raise an exception.

* Before Python 3.5, a :class:`datetime.time` object was considered to be false
  if it represented midnight in UTC.  This behavior was considered obscure and
  error-prone and has been removed in Python 3.5.  See :issue:`13936` for full
  details.

* The :meth:`ssl.SSLSocket.send()` method now raises either
  :exc:`ssl.SSLWantReadError` or :exc:`ssl.SSLWantWriteError`
  on a non-blocking socket if the operation would block.  Previously,
  it would return ``0``.  (Contributed by Nikolaus Rath in :issue:`20951`.)

* The ``__name__`` attribute of generators is now set from the function name,
  instead of being set from the code name. Use ``gen.gi_code.co_name`` to
  retrieve the code name. Generators also have a new ``__qualname__``
  attribute, the qualified name, which is now used for the representation
  of a generator (``repr(gen)``).
  (Contributed by Victor Stinner in :issue:`21205`.)

* The deprecated "strict" mode and argument of :class:`~html.parser.HTMLParser`,
  :meth:`HTMLParser.error`, and the :exc:`HTMLParserError` exception have been
  removed.  (Contributed by Ezio Melotti in :issue:`15114`.)
  The *convert_charrefs* argument of :class:`~html.parser.HTMLParser` is
  now ``True`` by default.  (Contributed by Berker Peksag in :issue:`21047`.)

* Although it is not formally part of the API, it is worth noting for porting
  purposes (ie: fixing tests) that error messages that were previously of the
  form "'sometype' does not support the buffer protocol" are now of the form "a
  :term:`bytes-like object` is required, not 'sometype'".
  (Contributed by Ezio Melotti in :issue:`16518`.)

* If the current directory is set to a directory that no longer exists then
  :exc:`FileNotFoundError` will no longer be raised and instead
  :meth:`~importlib.machinery.FileFinder.find_spec` will return ``None``
  **without** caching ``None`` in :data:`sys.path_importer_cache`, which is
  different than the typical case (:issue:`22834`).

* HTTP status code and messages from :mod:`http.client` and :mod:`http.server`
  were refactored into a common :class:`~http.HTTPStatus` enum.  The values in
  :mod:`http.client` and :mod:`http.server` remain available for backwards
  compatibility.  (Contributed by Demian Brecht in :issue:`21793`.)

* When an import loader defines :meth:`importlib.machinery.Loader.exec_module`
  it is now expected to also define
  :meth:`~importlib.machinery.Loader.create_module` (raises a
  :exc:`DeprecationWarning` now, will be an error in Python 3.6). If the loader
  inherits from :class:`importlib.abc.Loader` then there is nothing to do, else
  simply define :meth:`~importlib.machinery.Loader.create_module` to return
  ``None``.  (Contributed by Brett Cannon in :issue:`23014`.)

* The :func:`re.split` function always ignored empty pattern matches, so the
  ``"x*"`` pattern worked the same as ``"x+"``, and the ``"\b"`` pattern never
  worked.  Now :func:`re.split` raises a warning if the pattern could match
  an empty string.  For compatibility, use patterns that never match an empty
  string (e.g. ``"x+"`` instead of ``"x*"``).  Patterns that could only match
  an empty string (such as ``"\b"``) now raise an error.
  (Contributed by Serhiy Storchaka in :issue:`22818`.)

* The :class:`http.cookies.Morsel` dict-like interface has been made self
  consistent:  morsel comparison now takes the :attr:`~http.cookies.Morsel.key`
  and :attr:`~http.cookies.Morsel.value` into account,
  :meth:`~http.cookies.Morsel.copy` now results in a
  :class:`~http.cookies.Morsel` instance rather than a :class:`dict`, and
  :meth:`~http.cookies.Morsel.update` will now raise an exception if any of the
  keys in the update dictionary are invalid.  In addition, the undocumented
  *LegalChars* parameter of :func:`~http.cookies.Morsel.set` is deprecated and
  is now ignored.  (Contributed by Demian Brecht in :issue:`2211`.)

* :pep:`488` has removed ``.pyo`` files from Python and introduced the optional
  ``opt-`` tag in ``.pyc`` file names. The
  :func:`importlib.util.cache_from_source` has gained an *optimization*
  parameter to help control the ``opt-`` tag. Because of this, the
  *debug_override* parameter of the function is now deprecated. `.pyo` files
  are also no longer supported as a file argument to the Python interpreter and
  thus serve no purpose when distributed on their own (i.e. sourcless code
  distribution). Due to the fact that the magic number for bytecode has changed
  in Python 3.5, all old `.pyo` files from previous versions of Python are
  invalid regardless of this PEP.

* The :mod:`socket` module now exports the :data:`~socket.CAN_RAW_FD_FRAMES`
  constant on linux 3.6 and greater.

* The :func:`ssl.cert_time_to_seconds` function now interprets the input time
  as UTC and not as local time, per :rfc:`5280`.  Additionally, the return
  value is always an :class:`int`. (Contributed by Akira Li in :issue:`19940`.)

* The ``pygettext.py`` Tool now uses the standard +NNNN format for timezones in
  the POT-Creation-Date header.

* The :mod:`smtplib` module now uses :data:`sys.stderr` instead of the previous
  module-level :data:`stderr` variable for debug output.  If your (test)
  program depends on patching the module-level variable to capture the debug
  output, you will need to update it to capture sys.stderr instead.

* The :meth:`str.startswith` and :meth:`str.endswith` methods no longer return
  ``True`` when finding the empty string and the indexes are completely out of
  range.  (Contributed by Serhiy Storchaka in :issue:`24284`.)

* The :func:`inspect.getdoc` function now returns documentation strings
  inherited from base classes.  Documentation strings no longer need to be
  duplicated if the inherited documentation is appropriate.  To suppress an
  inherited string, an empty string must be specified (or the documentation
  may be filled in).  This change affects the output of the :mod:`pydoc`
  module and the :func:`help` function.
  (Contributed by Serhiy Storchaka in :issue:`15582`.)

* Nested :func:`functools.partial` calls are now flattened.  If you were
  relying on the previous behavior, you can now either add an attribute to a
  :func:`functools.partial` object or you can create a subclass of
  :func:`functools.partial`.
  (Contributed by Alexander Belopolsky in :issue:`7830`.)

Changes in the C API
--------------------

* The undocumented :c:member:`~PyMemoryViewObject.format` member of the
  (non-public) :c:type:`PyMemoryViewObject` structure has been removed.
  All extensions relying on the relevant parts in ``memoryobject.h``
  must be rebuilt.

* The :c:type:`PyMemAllocator` structure was renamed to
  :c:type:`PyMemAllocatorEx` and a new ``calloc`` field was added.

* Removed non-documented macro :c:macro:`PyObject_REPR` which leaked references.
  Use format character ``%R`` in :c:func:`PyUnicode_FromFormat`-like functions
  to format the :func:`repr` of the object.
  (Contributed by Serhiy Storchaka in :issue:`22453`.)

* Because the lack of the :attr:`__module__` attribute breaks pickling and
  introspection, a deprecation warning is now raised for builtin types without
  the :attr:`__module__` attribute.  This would be an AttributeError in
  the future.
  (Contributed by Serhiy Storchaka in :issue:`20204`.)

* As part of the :pep:`492` implementation, the ``tp_reserved`` slot of
  :c:type:`PyTypeObject` was replaced with a
  :c:member:`tp_as_async` slot.  Refer to :ref:`coro-objects` for
  new types, structures and functions.