summaryrefslogtreecommitdiffstats
path: root/Doc/whatsnew/2.7.rst
blob: 8f02d1c723ee3fafb16c44b851fcbdd5860cefe1 (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
****************************
  What's New in Python 2.7
****************************

:Author: A.M. Kuchling (amk at amk.ca)
:Release: |release|
:Date: |today|

.. Fix accents on Kristjan Valur Jonsson, Fuerstenau

.. Big jobs: argparse, ElementTree 1.3, pep 391, 3106, sysconfig
..  unittest test discovery
..  hyperlink all the methods & functions.

.. $Id$
   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 in a parenthetical comment.

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

   This saves the maintainer some effort going through the SVN logs
   when researching a change.

This article explains the new features in Python 2.7.  The final
release of 2.7 is currently scheduled for June 2010; the detailed
schedule is described in :pep:`373`.

Python 2.7 is planned to be the last major release in the 2.x series.
Though more major releases have not been absolutely ruled out, the
Python maintainers are planning to focus more on Python 3.x.  Despite
that, it's likely that the 2.7 release will have a longer period of
maintenance compared to earlier 2.x versions.

.. Compare with previous release in 2 - 3 sentences here.
   add hyperlink when the documentation becomes available online.

.. _whatsnew27-python31:

Python 3.1 Features
=======================

Much as Python 2.6 incorporated features from Python 3.0,
version 2.7 incorporates some of the new features
in Python 3.1.  The 2.x series continues to provide tools
for migrating to the 3.x series.

A partial list of 3.1 features that were backported to 2.7:

* A version of the :mod:`io` library, rewritten in C for performance.
* The ordered-dictionary type described in :ref:`pep-0372`.
* The new format specifier described in :ref:`pep-0378`.
* The :class:`memoryview` object.
* A small subset of the :mod:`importlib` module `described below <#importlib-section>`__.
* Float-to-string and string-to-float conversions now round their
  results more correctly.  And :func:`repr` of a floating-point
  number *x* returns a result that's guaranteed to round back to the
  same number when converted back to a string.
* The :cfunc:`PyLong_AsLongAndOverflow` C API function.

One porting change: the :option:`-3` switch now automatically
enables the :option:`-Qwarn` switch that causes warnings
about using classic division with integers and long integers.

Other new Python3-mode warnings include:

* :func:`operator.isCallable` and :func:`operator.sequenceIncludes`,
  which are not supported in 3.x.

.. ========================================================================
.. Large, PEP-level features and changes should be described here.
.. ========================================================================

.. _pep-0372:

PEP 372: Adding an ordered dictionary to collections
====================================================

Regular Python dictionaries iterate over key/value pairs in arbitrary order.
Over the years, a number of authors have written alternative implementations
that remember the order that the keys were originally inserted.  Based on
the experiences from those implementations, a new
:class:`~collections.OrderedDict` class has been introduced in the
:mod:`collections` module.

The :class:`~collections.OrderedDict` API is substantially the same as regular
dictionaries but will iterate over keys and values in a guaranteed order
depending on when a key was first inserted::

    >>> from collections import OrderedDict
    >>> d = OrderedDict([('first', 1), ('second', 2),
    ...                  ('third', 3)])
    >>> d.items()
    [('first', 1), ('second', 2), ('third', 3)]

If a new entry overwrites an existing entry, the original insertion
position is left unchanged::

    >>> d['second'] = 4
    >>> d.items()
    [('first', 1), ('second', 4), ('third', 3)]

Deleting an entry and reinserting it will move it to the end::

    >>> del d['second']
    >>> d['second'] = 5
    >>> d.items()
    [('first', 1), ('third', 3), ('second', 5)]

The :meth:`~collections.OrderedDict.popitem` method has an optional *last*
argument that defaults to True.  If *last* is True, the most recently
added key is returned and removed; if it's False, the
oldest key is selected::

    >>> od = OrderedDict([(x,0) for x in range(20)])
    >>> od.popitem()
    (19, 0)
    >>> od.popitem()
    (18, 0)
    >>> od.popitem(last=False)
    (0, 0)
    >>> od.popitem(last=False)
    (1, 0)

Comparing two ordered dictionaries checks both the keys and values,
and requires that the insertion order was the same::

    >>> od1 = OrderedDict([('first', 1), ('second', 2),
    ...                    ('third', 3)])
    >>> od2 = OrderedDict([('third', 3), ('first', 1),
    ...                    ('second', 2)])
    >>> od1 == od2
    False
    >>> # Move 'third' key to the end
    >>> del od2['third']; od2['third'] = 3
    >>> od1 == od2
    True

Comparing an :class:`~collections.OrderedDict` with a regular dictionary
ignores the insertion order and just compares the keys and values.

How does the :class:`~collections.OrderedDict` work?  It maintains a
doubly-linked list of keys, appending new keys to the list as they're inserted.
A secondary dictionary maps keys to their corresponding list node, so
deletion doesn't have to traverse the entire linked list and therefore
remains O(1).

.. XXX check O(1)-ness with Raymond
..     Also check if the 'somenamedtuple' in the collection module should
..     be replaced/removed in order to use
..     :meth:`~collections.namedtuple._asdict()` (see below)

The standard library now supports use of ordered dictionaries in several
modules.  The :mod:`ConfigParser` module uses them by default.  This lets
configuration files be read, modified, and then written back in their original
order.  The :meth:`~collections.somenamedtuple._asdict()` method for
:func:`collections.namedtuple` now returns an ordered dictionary with the
values appearing in the same order as the underlying tuple indices.
The :mod:`json` module is being built-out with an *object_pairs_hook* to allow
OrderedDicts to be built by the decoder.
Support was also added for third-party tools like `PyYAML <http://pyyaml.org/>`_.

.. seealso::

   :pep:`372` - Adding an ordered dictionary to collections
     PEP written by Armin Ronacher and Raymond Hettinger;
     implemented by Raymond Hettinger.

.. _pep-0378:

PEP 378: Format Specifier for Thousands Separator
=================================================

To make program output more readable, it can be useful to add
separators to large numbers and render them as
18,446,744,073,709,551,616 instead of 18446744073709551616.

The fully general solution for doing this is the :mod:`locale` module,
which can use different separators ("," in North America, "." in
Europe) and different grouping sizes, but :mod:`locale` is complicated
to use and unsuitable for multi-threaded applications where different
threads are producing output for different locales.

Therefore, a simple comma-grouping mechanism has been added to the
mini-language used by the :meth:`str.format` method.  When
formatting a floating-point number, simply include a comma between the
width and the precision::

   >>> '{:20,.2f}'.format(18446744073709551616.0)
   '18,446,744,073,709,551,616.00'

When formatting an integer, include the comma after the width:

   >>> '{:20,d}'.format(18446744073709551616)
   '18,446,744,073,709,551,616'

This mechanism is not adaptable at all; commas are always used as the
separator and the grouping is always into three-digit groups.  The
comma-formatting mechanism isn't as general as the :mod:`locale`
module, but it's easier to use.

.. XXX "Format String Syntax" in string.rst could use many more examples.

.. seealso::

   :pep:`378` - Format Specifier for Thousands Separator
     PEP written by Raymond Hettinger; implemented by Eric Smith.

PEP 389: The argparse Module for Parsing Command Lines
======================================================

The :mod:`argparse` module for parsing command-line arguments was
added, intended as a more powerful replacement for the
:mod:`optparse` module.

This means Python now supports three different modules for parsing
command-line arguments: :mod:`getopt`, :mod:`optparse`, and
:mod:`argparse`.  The :mod:`getopt` module closely resembles the C
:cfunc:`getopt` function, so it remains useful if you're writing a
Python prototype that will eventually be rewritten in C.
:mod:`optparse` becomes redundant, but there are no plans to remove it
because there are many scripts still using it, and there's no
automated way to update these scripts.  (Making the :mod:`argparse`
API consistent with :mod:`optparse`'s interface was discussed but
rejected as too messy and difficult.)

In short, if you're writing a new script and don't need to worry
about compatibility with earlier versions of Python, use
:mod:`argparse` instead of :mod:`optparse`.

Here's an example::

    import argparse

    parser = argparse.ArgumentParser(description='Command-line example.')

    # Add optional switches
    parser.add_argument('-v', action='store_true', dest='is_verbose',
                        help='produce verbose output')
    parser.add_argument('-o', action='store', dest='output',
                        metavar='FILE',
                        help='direct output to FILE instead of stdout')
    parser.add_argument('-C', action='store', type=int, dest='context',
                        metavar='NUM', default=0,
                        help='display NUM lines of added context')

    # Allow any number of additional arguments.
    parser.add_argument(nargs='*', action='store', dest='inputs',
                        help='input filenames (default is stdin)')

    args = parser.parse_args()
    print args.__dict__

Unless you override it, :option:`-h` and :option:`--help` switches
are automatically added, and produce neatly formatted output::

    -> ./python.exe argparse-example.py --help
    usage: parse.py [-h] [-v] [-o FILE] [-C NUM]

    Command-line example.

    positional arguments:
      inputs      input filenames (default is stdin)

    optional arguments:
      -h, --help  show this help message and exit
      -v          produce verbose output
      -o FILE     direct output to FILE instead of stdout
      -C NUM      display NUM lines of added context

Similarly to :mod:`optparse`, the command-line switches and arguments
are returned as an object with attributes named by the *dest* parameters::

    -> ./python.exe argparse-example.py -v
    {'output': None, 'is_verbose': True, 'context': 0, 'inputs': []}

    -> ./python.exe argparse-example.py -v -o /tmp/output -C 4 file1 file2
    {'output': '/tmp/output', 'is_verbose': True, 'context': 4,
     'inputs': ['file1', 'file2']}

:mod:`argparse` has much fancier validation than :mod:`optparse`; you
can specify an exact number of arguments as an integer, 0 or more
arguments by passing ``'*'``, 1 or more by passing ``'+'``, or an
optional argument with ``'?'``.  A top-level parser can contain
sub-parsers, so you can define subcommands that have different sets of
switches, as in ``svn commit``, ``svn checkout``, etc.  You can
specify an argument type as :class:`~argparse.FileType`, which will
automatically open files for you and understands that ``'-'`` means
standard input or output.

.. seealso::

   `argparse module documentation <http://docs.python.org/dev/library/argparse.html>`__

   `Upgrading optparse code to use argparse <http://docs.python.org/dev/library/argparse.html#upgrading-optparse-code>`__

   :pep:`389` - argparse - New Command Line Parsing Module
     PEP written and implemented by Steven Bethard.

PEP 391: Dictionary-Based Configuration For Logging
====================================================

.. not documented in library reference yet.

The :mod:`logging` module is very flexible; an application can define
a tree of logging subsystems, and each logger in this tree can filter
out certain messages, format them differently, and direct messages to
a varying number of handlers.

All this flexibility can require a lot of configuration.  You can
write Python statements to create objects and set their properties,
but a complex set-up would require verbose but boring code.
:mod:`logging` also supports a :func:`~logging.config.fileConfig`
function that parses a file, but the file format doesn't support
configuring filters, and it's messier to generate programmatically.

Python 2.7 adds a :func:`~logging.config.dictConfig` function that
uses a dictionary, and there are many ways to produce a dictionary
from different sources.  You can construct one with code, of course.
Python's standard library now includes a JSON parser, so you could
parse a file containing JSON, or you could use a YAML parsing library
if one is installed.

XXX describe an example.

Two smaller enhancements to the logging module are:

.. rev79293

* :class:`Logger` instances gained a :meth:`getChild` that retrieves a
  descendant logger using a relative path.  For example,
  once you retrieve a logger by doing ``log = getLogger('app')``,
  calling ``log.getChild('network.listen')`` is equivalent to
  ``getLogger('app.network.listen')``.

* The :class:`LoggerAdapter` class gained a :meth:`isEnabledFor` method
  that takes a *level* and returns whether the underlying logger would
  process a message of that level of importance.

.. seealso::

   :pep:`391` - Dictionary-Based Configuration For Logging
     PEP written and implemented by Vinay Sajip.

PEP 3106: Dictionary Views
====================================================

XXX write this section.

.. seealso::

   :pep:`3106` - Revamping dict.keys(), .values() and .items()
     PEP written by Guido van Rossum.
     Backported to 2.7 by Alexandre Vassalotti; :issue:`1967`.


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

Some smaller changes made to the core Python language are:

* The syntax for set literals has been backported from Python 3.x.
  Curly brackets are used to surround the contents of the resulting
  mutable set; set literals are
  distinguished from dictionaries by not containing colons and values.
  ``{}`` continues to represent an empty dictionary; use
  ``set()`` for an empty set.

    >>> {1,2,3,4,5}
    set([1, 2, 3, 4, 5])
    >>> set() # empty set
    set([])
    >>> {}    # empty dict
    {}

  Backported by Alexandre Vassalotti; :issue:`2335`.

* Dictionary and set comprehensions are another feature backported from
  3.x, generalizing list/generator comprehensions to use
  the literal syntax for sets and dictionaries.

    >>> {x:x*x for x in range(6)}
    {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
    >>> {'a'*x for x in range(6)}
    set(['', 'a', 'aa', 'aaa', 'aaaa', 'aaaaa'])

  Backported by Alexandre Vassalotti; :issue:`2333`.

* The :keyword:`with` statement can now use multiple context managers
  in one statement.  Context managers are processed from left to right
  and each one is treated as beginning a new :keyword:`with` statement.
  This means that::

   with A() as a, B() as b:
       ... suite of statements ...

  is equivalent to::

   with A() as a:
       with B() as b:
           ... suite of statements ...

  The :func:`contextlib.nested` function provides a very similar
  function, so it's no longer necessary and has been deprecated.

  (Proposed in http://codereview.appspot.com/53094; implemented by
  Georg Brandl.)

* Conversions between floating-point numbers and strings are
  now correctly rounded on most platforms.  These conversions occur
  in many different places: :func:`str` on
  floats and complex numbers; the :class:`float` and :class:`complex`
  constructors;
  numeric formatting; serialization and
  deserialization of floats and complex numbers using the
  :mod:`marshal`, :mod:`pickle`
  and :mod:`json` modules;
  parsing of float and imaginary literals in Python code;
  and :class:`~decimal.Decimal`-to-float conversion.

  Related to this, the :func:`repr` of a floating-point number *x*
  now returns a result based on the shortest decimal string that's
  guaranteed to round back to *x* under correct rounding (with
  round-half-to-even rounding mode).  Previously it gave a string
  based on rounding x to 17 decimal digits.

  .. maybe add an example?

  The rounding library responsible for this improvement works on
  Windows, and on Unix platforms using the gcc, icc, or suncc
  compilers.  There may be a small number of platforms where correct
  operation of this code cannot be guaranteed, so the code is not
  used on such systems.  You can find out which code is being used
  by checking :data:`sys.float_repr_style`,  which will be ``short``
  if the new code is in use and ``legacy`` if it isn't.

  Implemented by Eric Smith and Mark Dickinson, using David Gay's
  :file:`dtoa.c` library; :issue:`7117`.

* The :meth:`str.format` method now supports automatic numbering of the replacement
  fields.  This makes using :meth:`str.format` more closely resemble using
  ``%s`` formatting::

    >>> '{}:{}:{}'.format(2009, 04, 'Sunday')
    '2009:4:Sunday'
    >>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
    '2009:4:Sunday'

  The auto-numbering takes the fields from left to right, so the first ``{...}``
  specifier will use the first argument to :meth:`str.format`, the next
  specifier will use the next argument, and so on.  You can't mix auto-numbering
  and explicit numbering -- either number all of your specifier fields or none
  of them -- but you can mix auto-numbering and named fields, as in the second
  example above.  (Contributed by Eric Smith; :issue:`5237`.)

  Complex numbers now correctly support usage with :func:`format`,
  and default to being right-aligned.
  Specifying a precision or comma-separation applies to both the real
  and imaginary parts of the number, but a specified field width and
  alignment is applied to the whole of the resulting ``1.5+3j``
  output.  (Contributed by Eric Smith; :issue:`1588` and :issue:`7988`.)

  The 'F' format code now always formats its output using uppercase characters,
  so it will now produce 'INF' and 'NAN'.
  (Contributed by Eric Smith; :issue:`3382`.)

* The :func:`int` and :func:`long` types gained a ``bit_length``
  method that returns the number of bits necessary to represent
  its argument in binary::

      >>> n = 37
      >>> bin(n)
      '0b100101'
      >>> n.bit_length()
      6
      >>> n = 2**123-1
      >>> n.bit_length()
      123
      >>> (n+1).bit_length()
      124

  (Contributed by Fredrik Johansson and Victor Stinner; :issue:`3439`.)

* Conversions from long integers and regular integers to floating
  point now round differently, returning the floating-point number
  closest to the number.  This doesn't matter for small integers that
  can be converted exactly, but for large numbers that will
  unavoidably lose precision, Python 2.7 now approximates more
  closely.  For example, Python 2.6 computed the following::

    >>> n = 295147905179352891391
    >>> float(n)
    2.9514790517935283e+20
    >>> n - long(float(n))
    65535L

  Python 2.7's floating-point result is larger, but much closer to the
  true value::

    >>> n = 295147905179352891391
    >>> float(n)
    2.9514790517935289e+20
    >>> n - long(float(n))
    -1L

  (Implemented by Mark Dickinson; :issue:`3166`.)

  Integer division is also more accurate in its rounding behaviours.  (Also
  implemented by Mark Dickinson; :issue:`1811`.)

* It's now possible for a subclass of the built-in :class:`unicode` type
  to override the :meth:`__unicode__` method.  (Implemented by
  Victor Stinner; :issue:`1583863`.)

* The :class:`bytearray` type's :meth:`~bytearray.translate` method now accepts
  ``None`` as its first argument.  (Fixed by Georg Brandl;
  :issue:`4759`.)

  .. bytearray doesn't seem to be documented

* When using ``@classmethod`` and ``@staticmethod`` to wrap
  methods as class or static methods, the wrapper object now
  exposes the wrapped function as their :attr:`__func__` attribute.
  (Contributed by Amaury Forgeot d'Arc, after a suggestion by
  George Sakkis; :issue:`5982`.)

* A new encoding named "cp720", used primarily for Arabic text, is now
  supported.  (Contributed by Alexander Belchenko and Amaury Forgeot
  d'Arc; :issue:`1616979`.)

* The :class:`file` object will now set the :attr:`filename` attribute
  on the :exc:`IOError` exception when trying to open a directory
  on POSIX platforms (noted by Jan Kaliszewski; :issue:`4764`), and
  now explicitly checks for and forbids writing to read-only file objects
  instead of trusting the C library to catch and report the error
  (fixed by Stefan Krah; :issue:`5677`).

* The Python tokenizer now translates line endings itself, so the
  :func:`compile` built-in function can now accept code using any
  line-ending convention.  Additionally, it no longer requires that the
  code end in a newline.

* Extra parentheses in function definitions are illegal in Python 3.x,
  meaning that you get a syntax error from ``def f((x)): pass``.  In
  Python3-warning mode, Python 2.7 will now warn about this odd usage.
  (Noted by James Lingard; :issue:`7362`.)

* It's now possible to create weak references to old-style class
  objects.  New-style classes were always weak-referenceable.  (Fixed
  by Antoine Pitrou; :issue:`8268`.)

* When a module object is garbage-collected, the module's dictionary is
  now only cleared if no one else is holding a reference to the
  dictionary (:issue:`7140`).

.. ======================================================================

.. _new-27-interpreter:

Interpreter Changes
-------------------------------

A new environment variable, :envvar:`PYTHONWARNINGS`,
allows controlling warnings.  It should be set to a string
containing warning settings, equivalent to those
used with the :option:`-W` switch, separated by commas.
(Contributed by Brian Curtin; :issue:`7301`.)

For example, the following setting will print warnings every time
they occur, but turn warnings from the :mod:`Cookie` module into an
error.  (The exact syntax for setting an environment variable varies
across operating systems and shells, so it may be different for you.)

::

  export PYTHONWARNINGS=all,error:::Cookie:0


.. ======================================================================


Optimizations
-------------

Several performance enhancements have been added:

.. * A new :program:`configure` option, :option:`--with-computed-gotos`,
   compiles the main bytecode interpreter loop using a new dispatch
   mechanism that gives speedups of up to 20%, depending on the system
   and benchmark.  The new mechanism is only supported on certain
   compilers, such as gcc, SunPro, and icc.

* A new opcode was added to perform the initial setup for
  :keyword:`with` statements, looking up the :meth:`__enter__` and
  :meth:`__exit__` methods.  (Contributed by Benjamin Peterson.)

* The garbage collector now performs better for one common usage
  pattern: when many objects are being allocated without deallocating
  any of them.  This would previously take quadratic
  time for garbage collection, but now the number of full garbage collections
  is reduced as the number of objects on the heap grows.
  The new logic is to only perform a full garbage collection pass when
  the middle generation has been collected 10 times and when the
  number of survivor objects from the middle generation exceeds 10% of
  the number of objects in the oldest generation.  (Suggested by Martin
  von Löwis and implemented by Antoine Pitrou; :issue:`4074`.)

* The garbage collector tries to avoid tracking simple containers
  which can't be part of a cycle. In Python 2.7, this is now true for
  tuples and dicts containing atomic types (such as ints, strings,
  etc.). Transitively, a dict containing tuples of atomic types won't
  be tracked either. This helps reduce the cost of each
  garbage collection by decreasing the number of objects to be
  considered and traversed by the collector.
  (Contributed by Antoine Pitrou; :issue:`4688`.)

* Long integers are now stored internally either in base 2**15 or in base
  2**30, the base being determined at build time.  Previously, they
  were always stored in base 2**15.  Using base 2**30 gives
  significant performance improvements on 64-bit machines, but
  benchmark results on 32-bit machines have been mixed.  Therefore,
  the default is to use base 2**30 on 64-bit machines and base 2**15
  on 32-bit machines; on Unix, there's a new configure option
  :option:`--enable-big-digits` that can be used to override this default.

  Apart from the performance improvements this change should be
  invisible to end users, with one exception: for testing and
  debugging purposes there's a new structseq :data:`sys.long_info` that
  provides information about the internal format, giving the number of
  bits per digit and the size in bytes of the C type used to store
  each digit::

     >>> import sys
     >>> sys.long_info
     sys.long_info(bits_per_digit=30, sizeof_digit=4)

  (Contributed by Mark Dickinson; :issue:`4258`.)

  Another set of changes made long objects a few bytes smaller: 2 bytes
  smaller on 32-bit systems and 6 bytes on 64-bit.
  (Contributed by Mark Dickinson; :issue:`5260`.)

* The division algorithm for long integers has been made faster
  by tightening the inner loop, doing shifts instead of multiplications,
  and fixing an unnecessary extra iteration.
  Various benchmarks show speedups of between 50% and 150% for long
  integer divisions and modulo operations.
  (Contributed by Mark Dickinson; :issue:`5512`.)
  Bitwise operations are also significantly faster (initial patch by
  Gregory Smith; :issue:`1087418`).

* The implementation of ``%`` checks for the left-side operand being
  a Python string and special-cases it; this results in a 1-3%
  performance increase for applications that frequently use ``%``
  with strings, such as templating libraries.
  (Implemented by Collin Winter; :issue:`5176`.)

* List comprehensions with an ``if`` condition are compiled into
  faster bytecode.  (Patch by Antoine Pitrou, back-ported to 2.7
  by Jeffrey Yasskin; :issue:`4715`.)

* Converting an integer or long integer to a decimal string was made
  faster by special-casing base 10 instead of using a generalized
  conversion function that supports arbitrary bases.
  (Patch by Gawain Bolton; :issue:`6713`.)

* The :meth:`split`, :meth:`replace`, :meth:`rindex`,
  :meth:`rpartition`, and :meth:`rsplit` methods of string-like types
  (strings, Unicode strings, and :class:`bytearray` objects) now use a
  fast reverse-search algorithm instead of a character-by-character
  scan.  This is sometimes faster by a factor of 10.  (Added by
  Florent Xicluna; :issue:`7462` and :issue:`7622`.)

* The :mod:`pickle` and :mod:`cPickle` modules now automatically
  intern the strings used for attribute names, reducing memory usage
  of the objects resulting from unpickling.  (Contributed by Jake
  McGuire; :issue:`5084`.)

* The :mod:`cPickle` module now special-cases dictionaries,
  nearly halving the time required to pickle them.
  (Contributed by Collin Winter; :issue:`5670`.)

.. ======================================================================

New and Improved Modules
========================

As in every release, Python's standard library received a number of
enhancements and bug fixes.  Here's a partial list of the most notable
changes, sorted alphabetically by module name. Consult the
:file:`Misc/NEWS` file in the source tree for a more complete list of
changes, or look through the Subversion logs for all the details.

* The :mod:`bdb` module's base debugging class :class:`~bdb.Bdb`
  gained a feature for skipping modules.  The constructor
  now takes an iterable containing glob-style patterns such as
  ``django.*``; the debugger will not step into stack frames
  from a module that matches one of these patterns.
  (Contributed by Maru Newby after a suggestion by
  Senthil Kumaran; :issue:`5142`.)

* The :mod:`binascii` module now supports the buffer API, so it can be
  used with :class:`memoryview` instances and other similar buffer objects.
  (Backported from 3.x by Florent Xicluna; :issue:`7703`.)

* Updated module: the :mod:`bsddb` module has been updated from 4.7.2devel9
  to version 4.8.4 of
  `the pybsddb package <http://www.jcea.es/programacion/pybsddb.htm>`__.
  The new version features better Python 3.x compatibility, various bug fixes,
  and adds several new BerkeleyDB flags and methods.
  (Updated by Jesús Cea Avión; :issue:`8156`.  The pybsddb
  changelog can be browsed at http://hg.jcea.es/pybsddb/file/tip/ChangeLog.)

* The :mod:`bz2` module's :class:`~bz2.BZ2File` now supports the context
  management protocol, so you can write ``with bz2.BZ2File(...) as f: ...``.
  (Contributed by Hagen Fuerstenau; :issue:`3860`.)

* New class: the :class:`~collections.Counter` class in the :mod:`collections`
  module is useful for tallying data.  :class:`~collections.Counter` instances
  behave mostly like dictionaries but return zero for missing keys instead of
  raising a :exc:`KeyError`:

  .. doctest::
     :options: +NORMALIZE_WHITESPACE

     >>> from collections import Counter
     >>> c = Counter()
     >>> for letter in 'here is a sample of english text':
     ...   c[letter] += 1
     ...
     >>> c
     Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
     'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
     'p': 1, 'r': 1, 'x': 1})
     >>> c['e']
     5
     >>> c['z']
     0

  There are three additional :class:`~collections.Counter` methods:
  :meth:`~collections.Counter.most_common` returns the N most common
  elements and their counts.  :meth:`~collections.Counter.elements`
  returns an iterator over the contained elements, repeating each
  element as many times as its count.
  :meth:`~collections.Counter.subtract` takes an iterable and
  subtracts one for each element instead of adding; if the argument is
  a dictionary or another :class:`Counter`, the counts are
  subtracted. ::

    >>> c.most_common(5)
    [(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
    >>> c.elements() ->
       'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
       'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
       'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
       's', 's', 'r', 't', 't', 'x'
    >>> c['e']
    5
    >>> c.subtract('very heavy on the letter e')
    >>> c['e']    # Count is now lower
    -1

  Contributed by Raymond Hettinger; :issue:`1696199`.

  .. revision 79660

  The new :class:`~collections.OrderedDict` class is described in the earlier
  section :ref:`pep-0372`.

  The :class:`~collections.namedtuple` class now has an optional *rename* parameter.
  If *rename* is true, field names that are invalid because they've
  been repeated or that aren't legal Python identifiers will be
  renamed to legal names that are derived from the field's
  position within the list of fields:

     >>> from collections import namedtuple
     >>> T = namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True)
     >>> T._fields
     ('field1', '_1', '_2', 'field2')

  (Added by Raymond Hettinger; :issue:`1818`.)

  The :class:`~collections.deque` data type now has a
  :meth:`~collections.deque.count` method that returns the number of
  contained elements equal to the supplied argument *x*, and a
  :meth:`~collections.deque.reverse` method that reverses the elements
  of the deque in-place.  :class:`deque` also exposes its maximum
  length as the read-only :attr:`~collections.deque.maxlen` attribute.
  (Both features added by Raymond Hettinger.)

* The :mod:`copy` module's :func:`~copy.deepcopy` function will now
  correctly copy bound instance methods.  (Implemented by
  Robert Collins; :issue:`1515`.)

* The :mod:`ctypes` module now always converts ``None`` to a C NULL
  pointer for arguments declared as pointers.  (Changed by Thomas
  Heller; :issue:`4606`.)  The underlying `libffi library
  <http://sourceware.org/libffi/>`__ has been updated to version
  3.0.9, containing various fixes for different platforms.  (Updated
  by Matthias Klose; :issue:`8142`.)

* New method: the :mod:`datetime` module's :class:`~datetime.timedelta` class
  gained a :meth:`~datetime.timedelta.total_seconds` method that returns the
  number of seconds in the duration.  (Contributed by Brian Quinlan; :issue:`5788`.)

* New method: the :class:`~decimal.Decimal` class gained a
  :meth:`~decimal.Decimal.from_float` class method that performs an exact
  conversion of a floating-point number to a :class:`~decimal.Decimal`.
  Note that this is an **exact** conversion that strives for the
  closest decimal approximation to the floating-point representation's value;
  the resulting decimal value will therefore still include the inaccuracy,
  if any.
  For example, ``Decimal.from_float(0.1)`` returns
  ``Decimal('0.1000000000000000055511151231257827021181583404541015625')``.
  (Implemented by Raymond Hettinger; :issue:`4796`.)

  Most of the methods of the :class:`~decimal.Context` class now accept integers
  as well as :class:`~decimal.Decimal` instances; the only exceptions are the
  :meth:`~decimal.Context.canonical` and :meth:`~decimal.Context.is_canonical`
  methods.  (Patch by Juan José Conti; :issue:`7633`.)

  The constructor for :class:`~decimal.Decimal` now accepts
  floating-point numbers (added by Raymond Hettinger; :issue:`8257`)
  and non-European Unicode characters such as Arabic-Indic digits
  (contributed by Mark Dickinson; :issue:`6595`).

  When using :class:`~decimal.Decimal` instances with a string's
  :meth:`~str.format` method, the default alignment was previously
  left-alignment.  This has been changed to right-alignment, which seems
  more sensible for numeric types.  (Changed by Mark Dickinson; :issue:`6857`.)

* The :class:`~fractions.Fraction` class now accepts a single float or
  :class:`~decimal.Decimal` instance, or two rational numbers, as
  arguments to its constructor.  (Implemented by Mark Dickinson;
  rationals added in :issue:`5812`, and float/decimal in
  :issue:`8294`.)

  An oversight was fixed, making the :class:`Fraction` match the other
  numeric types; ordering comparisons (``<``, ``<=``, ``>``, ``>=``) between
  fractions and complex numbers now raise a :exc:`TypeError`.

  .. revision 79455

* New class: a new :class:`~ftplib.FTP_TLS` class in
  the :mod:`ftplib` module provides secure FTP
  connections using TLS encapsulation of authentication as well as
  subsequent control and data transfers.
  (Contributed by Giampaolo Rodola', :issue:`2054`.)

  The :meth:`~ftplib.FTP.storbinary` method for binary uploads can now restart
  uploads thanks to an added *rest* parameter (patch by Pablo Mouzo;
  :issue:`6845`.)

* New class decorator: :func:`total_ordering` in the :mod:`functools`
  module takes a class that defines an :meth:`__eq__` method and one of
  :meth:`__lt__`, :meth:`__le__`, :meth:`__gt__`, or :meth:`__ge__`,
  and generates the missing comparison methods.  Since the
  :meth:`__cmp__` method is being deprecated in Python 3.x,
  this decorator makes it easier to define ordered classes.
  (Added by Raymond Hettinger; :issue:`5479`.)

  New function: :func:`cmp_to_key` will take an old-style comparison
  function that expects two arguments and return a new callable that
  can be used as the *key* parameter to functions such as
  :func:`sorted`, :func:`min` and :func:`max`, etc.  The primary
  intended use is to help with making code compatible with Python 3.x.
  (Added by Raymond Hettinger.)

* New function: the :mod:`gc` module's :func:`~gc.is_tracked` returns
  true if a given instance is tracked by the garbage collector, false
  otherwise. (Contributed by Antoine Pitrou; :issue:`4688`.)

* The :mod:`gzip` module's :class:`~gzip.GzipFile` now supports the context
  management protocol, so you can write ``with gzip.GzipFile(...) as f: ...``
  (contributed by Hagen Fuerstenau; :issue:`3860`), and it now implements
  the :class:`io.BufferedIOBase` ABC, so you can wrap it with
  :class:`io.BufferedReader` for faster processing
  (contributed by Nir Aides; :issue:`7471`).
  It's also now possible to override the modification time
  recorded in a gzipped file by providing an optional timestamp to
  the constructor.  (Contributed by Jacques Frechet; :issue:`4272`.)

  Files in gzip format can be padded with trailing zero bytes; the
  :mod:`gzip` module will now consume these trailing bytes.  (Fixed by
  Tadek Pietraszek and Brian Curtin; :issue:`2846`.)

* New attribute: the :mod:`hashlib` module now has an :attr:`~hashlib.hashlib.algorithms`
  attribute containing a tuple naming the supported algorithms.
  In Python 2.7, ``hashlib.algorithms`` contains
  ``('md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512')``
  (Contributed by Carl Chenet; :issue:`7418`.)

* The default :class:`~httplib.HTTPResponse` class used by the :mod:`httplib` module now
  supports buffering, resulting in much faster reading of HTTP responses.
  (Contributed by Kristjan Valur Jonsson; :issue:`4879`.)

  The :class:`~httplib.HTTPConnection` and :class:`~httplib.HTTPSConnection` classes
  now support a *source_address* parameter, a ``(host, port)`` 2-tuple
  giving the source address that will be used for the connection.
  (Contributed by Eldon Ziegler; :issue:`3972`.)

* The :mod:`imaplib` module now supports IPv6 addresses.
  (Contributed by Derek Morr; :issue:`1655`.)

* New function: the :mod:`inspect` module's :func:`~inspect.getcallargs`
  takes a callable and its positional and keyword arguments,
  and figures out which of the callable's parameters will receive each argument,
  returning a dictionary mapping argument names to their values.  For example::

    >>> from inspect import getcallargs
    >>> def f(a, b=1, *pos, **named):
    ...     pass
    >>> getcallargs(f, 1, 2, 3)
    {'a': 1, 'named': {}, 'b': 2, 'pos': (3,)}
    >>> getcallargs(f, a=2, x=4)
    {'a': 2, 'named': {'x': 4}, 'b': 1, 'pos': ()}
    >>> getcallargs(f)
    Traceback (most recent call last):
    ...
    TypeError: f() takes at least 1 argument (0 given)

  Contributed by George Sakkis; :issue:`3135`.

* Updated module: The :mod:`io` library has been upgraded to the version shipped with
  Python 3.1.  For 3.1, the I/O library was entirely rewritten in C
  and is 2 to 20 times faster depending on the task being performed.  The
  original Python version was renamed to the :mod:`_pyio` module.

  One minor resulting change: the :class:`io.TextIOBase` class now
  has an :attr:`errors` attribute giving the error setting
  used for encoding and decoding errors (one of ``'strict'``, ``'replace'``,
  ``'ignore'``).

  The :class:`io.FileIO` class now raises an :exc:`OSError` when passed
  an invalid file descriptor.  (Implemented by Benjamin Peterson;
  :issue:`4991`.)  The :meth:`~io.IOBase.truncate` method now preserves the
  file position; previously it would change the file position to the
  end of the new file.  (Fixed by Pascal Chambon; :issue:`6939`.)

* New function: ``itertools.compress(data, selectors)`` takes two
  iterators.  Elements of *data* are returned if the corresponding
  value in *selectors* is true::

    itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
      A, C, E, F

  .. maybe here is better to use >>> list(itertools.compress(...)) instead

  New function: ``itertools.combinations_with_replacement(iter, r)``
  returns all the possible *r*-length combinations of elements from the
  iterable *iter*.  Unlike :func:`~itertools.combinations`, individual elements
  can be repeated in the generated combinations::

    itertools.combinations_with_replacement('abc', 2) =>
      ('a', 'a'), ('a', 'b'), ('a', 'c'),
      ('b', 'b'), ('b', 'c'), ('c', 'c')

  Note that elements are treated as unique depending on their position
  in the input, not their actual values.

  The :func:`itertools.count` function now has a *step* argument that
  allows incrementing by values other than 1.  :func:`~itertools.count` also
  now allows keyword arguments, and using non-integer values such as
  floats or :class:`~decimal.Decimal` instances.  (Implemented by Raymond
  Hettinger; :issue:`5032`.)

  :func:`itertools.combinations` and :func:`itertools.product` were
  previously raising :exc:`ValueError` for values of *r* larger than
  the input iterable.  This was deemed a specification error, so they
  now return an empty iterator.  (Fixed by Raymond Hettinger; :issue:`4816`.)

* Updated module: The :mod:`json` module was upgraded to version 2.0.9 of the
  simplejson package, which includes a C extension that makes
  encoding and decoding faster.
  (Contributed by Bob Ippolito; :issue:`4136`.)

  To support the new :class:`collections.OrderedDict` type, :func:`json.load`
  now has an optional *object_pairs_hook* parameter that will be called
  with any object literal that decodes to a list of pairs.
  (Contributed by Raymond Hettinger; :issue:`5381`.)

* New functions: the :mod:`math` module gained
  :func:`~math.erf` and :func:`~math.erfc` for the error function and the complementary error function,
  :func:`~math.expm1` which computes ``e**x - 1`` with more precision than
  using :func:`~math.exp` and subtracting 1,
  :func:`~math.gamma` for the Gamma function, and
  :func:`~math.lgamma` for the natural log of the Gamma function.
  (Contributed by Mark Dickinson and nirinA raseliarison; :issue:`3366`.)

* The :mod:`multiprocessing` module's :class:`Manager*` classes
  can now be passed a callable that will be called whenever
  a subprocess is started, along with a set of arguments that will be
  passed to the callable.
  (Contributed by lekma; :issue:`5585`.)

  The :class:`~multiprocessing.Pool` class, which controls a pool of worker processes,
  now has an optional *maxtasksperchild* parameter.  Worker processes
  will perform the specified number of tasks and then exit, causing the
  :class:`~multiprocessing.Pool` to start a new worker.  This is useful if tasks may leak
  memory or other resources, or if some tasks will cause the worker to
  become very large.
  (Contributed by Charles Cazabon; :issue:`6963`.)

* The :mod:`nntplib` module now supports IPv6 addresses.
  (Contributed by Derek Morr; :issue:`1664`.)

* New functions: the :mod:`os` module wraps the following POSIX system
  calls: :func:`~os.getresgid` and :func:`~os.getresuid`, which return the
  real, effective, and saved GIDs and UIDs;
  :func:`~os.setresgid` and :func:`~os.setresuid`, which set
  real, effective, and saved GIDs and UIDs to new values;
  :func:`~os.initgroups`.  (GID/UID functions
  contributed by Travis H.; :issue:`6508`.  Support for initgroups added
  by Jean-Paul Calderone; :issue:`7333`.)

  The :func:`os.fork` function now re-initializes the import lock in
  the child process; this fixes problems on Solaris when :func:`~os.fork`
  is called from a thread.  (Fixed by Zsolt Cserna; :issue:`7242`.)

* In the :mod:`os.path` module, the :func:`~os.path.normpath` and
  :func:`~os.path.abspath` functions now preserve Unicode; if their input path
  is a Unicode string, the return value is also a Unicode string.
  (:meth:`~os.path.normpath` fixed by Matt Giuca in :issue:`5827`;
  :meth:`~os.path.abspath` fixed by Ezio Melotti in :issue:`3426`.)

* The :mod:`pydoc` module now has help for the various symbols that Python
  uses.  You can now do ``help('<<')`` or ``help('@')``, for example.
  (Contributed by David Laban; :issue:`4739`.)

* The :mod:`re` module's :func:`~re.split`, :func:`~re.sub`, and :func:`~re.subn`
  now accept an optional *flags* argument, for consistency with the
  other functions in the module.  (Added by Gregory P. Smith.)

* New function: in the :mod:`shutil` module, :func:`~shutil.make_archive`
  takes a filename, archive type (zip or tar-format), and a directory
  path, and creates an archive containing the directory's contents.
  (Added by Tarek Ziadé.)

  :mod:`shutil`'s :func:`~shutil.copyfile` and :func:`~shutil.copytree`
  functions now raise a :exc:`~shutil.SpecialFileError` exception when
  asked to copy a named pipe.  Previously the code would treat
  named pipes like a regular file by opening them for reading, and
  this would block indefinitely.  (Fixed by Antoine Pitrou; :issue:`3002`.)

* New functions: in the :mod:`site` module, three new functions
  return various site- and user-specific paths.
  :func:`~site.getsitepackages` returns a list containing all
  global site-packages directories, and
  :func:`~site.getusersitepackages` returns the path of the user's
  site-packages directory.
  :func:`~site.getuserbase` returns the value of the :envvar:`USER_BASE`
  environment variable, giving the path to a directory that can be used
  to store data.
  (Contributed by Tarek Ziadé; :issue:`6693`.)

  The :mod:`site` module now reports exceptions occurring
  when the :mod:`sitecustomize` module is imported, and will no longer
  catch and swallow the :exc:`KeyboardInterrupt` exception.  (Fixed by
  Victor Stinner; :issue:`3137`.)

* The :mod:`socket` module's :class:`~ssl.SSL` objects now support the
  buffer API, which fixed a test suite failure (fix by Antoine Pitrou;
  :issue:`7133`).  :class:`SSL` objects also now automatically set
  OpenSSL's :cmacro:`SSL_MODE_AUTO_RETRY`, which will prevent an error
  code being returned from :meth:`recv` operations that trigger an SSL
  renegotiation (fix by Antoine Pitrou; :issue:`8222`).

  The version of OpenSSL being used is now available as the module
  attributes :attr:`OPENSSL_VERSION` (a string),
  :attr:`OPENSSL_VERSION_INFO` (a 5-tuple), and
  :attr:`OPENSSL_VERSION_NUMBER` (an integer).  (Added by Antoine
  Pitrou; :issue:`8321`.)

  The :func:`~socket.create_connection` function
  gained a *source_address* parameter, a ``(host, port)`` 2-tuple
  giving the source address that will be used for the connection.
  (Contributed by Eldon Ziegler; :issue:`3972`.)

  The :meth:`~socket.socket.recv_into` and :meth:`~socket.socket.recvfrom_into`
  methods will now write into objects that support the buffer API, most usefully
  the :class:`bytearray` and :class:`memoryview` objects.  (Implemented by
  Antoine Pitrou; :issue:`8104`.)

* The :mod:`SocketServer` module's :class:`~SocketServer.TCPServer` class now
  has a :attr:`~SocketServer.TCPServer.disable_nagle_algorithm` class attribute.
  The default value is False; if overridden to be True,
  new request connections will have the TCP_NODELAY option set to
  prevent buffering many small sends into a single TCP packet.
  (Contributed by Kristjan Valur Jonsson; :issue:`6192`.)

* Updated module: the :mod:`sqlite3` module has been updated to
  version 2.6.0 of the `pysqlite package <http://code.google.com/p/pysqlite/>`__. Version 2.6.0 includes a number of bugfixes, and adds
  the ability to load SQLite extensions from shared libraries.
  Call the ``enable_load_extension(True)`` method to enable extensions,
  and then call :meth:`~sqlite3.Connection.load_extension` to load a particular shared library.
  (Updated by Gerhard Häring.)

* The :mod:`struct` module will no longer silently ignore overflow
  errors when a value is too large for a particular integer format
  code (one of ``bBhHiIlLqQ``); it now always raises a
  :exc:`struct.error` exception.  (Changed by Mark Dickinson;
  :issue:`1523`.)  The :func:`~struct.pack` function will also
  attempt to use :meth:`__index__` to convert and pack non-integers
  before trying the :meth:`__int__` method or reporting an error.
  (Changed by Mark Dickinson; :issue:`8300`.)

* New function: the :mod:`subprocess` module's
  :func:`~subprocess.check_output` runs a command with a specified set of arguments
  and returns the command's output as a string when the command runs without
  error, or raises a :exc:`~subprocess.CalledProcessError` exception otherwise.

  ::

    >>> subprocess.check_output(['df', '-h', '.'])
    'Filesystem     Size   Used  Avail Capacity  Mounted on\n
    /dev/disk0s2    52G    49G   3.0G    94%    /\n'

    >>> subprocess.check_output(['df', '-h', '/bogus'])
      ...
    subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1

  (Contributed by Gregory P. Smith.)

  The :mod:`subprocess` module will now retry its internal system calls
  on receiving an :const:`EINTR` signal.  (Reported by several people; final
  patch by Gregory P. Smith in :issue:`1068268`.)

* New function: :func:`~symtable.is_declared_global` in the :mod:`symtable` module
  returns true for variables that are explicitly declared to be global,
  false for ones that are implicitly global.
  (Contributed by Jeremy Hylton.)

* The ``sys.version_info`` value is now a named tuple, with attributes
  named :attr:`major`, :attr:`minor`, :attr:`micro`,
  :attr:`releaselevel`, and :attr:`serial`.  (Contributed by Ross
  Light; :issue:`4285`.)

  :func:`sys.getwindowsversion` also returns a named tuple,
  with attributes named :attr:`major`, :attr:`minor`, :attr:`build`,
  :attr:`platform`, :attr:`service_pack`, :attr:`service_pack_major`,
  :attr:`service_pack_minor`, :attr:`suite_mask`, and
  :attr:`product_type`.  (Contributed by Brian Curtin; :issue:`7766`.)

* The :mod:`tarfile` module's default error handling has changed, to
  no longer suppress fatal errors.  The default error level was previously 0,
  which meant that errors would only result in a message being written to the
  debug log, but because the debug log is not activated by default,
  these errors go unnoticed.  The default error level is now 1,
  which raises an exception if there's an error.
  (Changed by Lars Gustäbel; :issue:`7357`.)

  :mod:`tarfile` now supports filtering the :class:`~tarfile.TarInfo`
  objects being added to a tar file.  When you call :meth:`~tarfile.TarFile.add`,
  instance, you may supply an optional *filter* argument
  that's a callable.  The *filter* callable will be passed the
  :class:`~tarfile.TarInfo` for every file being added, and can modify and return it.
  If the callable returns ``None``, the file will be excluded from the
  resulting archive.  This is more powerful than the existing
  *exclude* argument, which has therefore been deprecated.
  (Added by Lars Gustäbel; :issue:`6856`.)
  The :class:`~tarfile.TarFile` class also now supports the context manager protocol.
  (Added by Lars Gustäbel; :issue:`7232`.)

* The :meth:`~threading.Event.wait` method of the :class:`threading.Event` class
  now returns the internal flag on exit.  This means the method will usually
  return true because :meth:`~threading.Event.wait` is supposed to block until the
  internal flag becomes true.  The return value will only be false if
  a timeout was provided and the operation timed out.
  (Contributed by Tim Lesher; :issue:`1674032`.)

* The Unicode database provided by the :mod:`unicodedata` module is
  now used internally to determine which characters are numeric,
  whitespace, or represent line breaks.  The database also
  includes information from the :file:`Unihan.txt` data file (patch
  by Anders Chrigström and Amaury Forgeot d'Arc; :issue:`1571184`)
  and has been updated to version 5.2.0 (updated by
  Florent Xicluna; :issue:`8024`).

* The :class:`~UserDict.UserDict` class is now a new-style class.  (Changed by
  Benjamin Peterson.)

* The ElementTree library, :mod:`xml.etree`, no longer escapes
  ampersands and angle brackets when outputting an XML processing
  instruction (which looks like ``<?xml-stylesheet href="#style1"?>``)
  or comment (which looks like ``<!-- comment -->``).
  (Patch by Neil Muller; :issue:`2746`.)

* The :mod:`zipfile` module's :class:`~zipfile.ZipFile` now supports the context
  management protocol, so you can write ``with zipfile.ZipFile(...) as f: ...``.
  (Contributed by Brian Curtin; :issue:`5511`.)

  :mod:`zipfile` now supports archiving empty directories and
  extracts them correctly.  (Fixed by Kuba Wieczorek; :issue:`4710`.)
  Reading files out of an archive is now faster, and interleaving
  :meth:`~zipfile.ZipFile.read` and :meth:`~zipfile.ZipFile.readline` now works correctly.
  (Contributed by Nir Aides; :issue:`7610`.)

  The :func:`~zipfile.is_zipfile` function now
  accepts a file object, in addition to the path names accepted in earlier
  versions.  (Contributed by Gabriel Genellina; :issue:`4756`.)

  The :meth:`~zipfile.ZipFile.writestr` method now has an optional *compress_type* parameter
  that lets you override the default compression method specified in the
  :class:`~zipfile.ZipFile` constructor.  (Contributed by Ronald Oussoren;
  :issue:`6003`.)


New module: sysconfig
---------------------------------

XXX A new :mod:`sysconfig` module has been extracted from
:mod:`distutils` and put in the standard library.

The :mod:`sysconfig` module provides access to Python's configuration
information like the list of installation paths and the configuration
variables relevant for the current platform. (contributed by Tarek)

Updated module: ElementTree 1.3
---------------------------------

XXX write this.

.. ======================================================================
.. whole new modules get described in subsections here


Unit Testing Enhancements
---------------------------------

The :mod:`unittest` module was enhanced in several ways.
The progress messages now shows 'x' for expected failures
and 'u' for unexpected successes when run in verbose mode.
(Contributed by Benjamin Peterson.)
Test cases can raise the :exc:`~unittest.SkipTest` exception to skip a test.
(:issue:`1034053`.)

.. XXX describe test discovery (Contributed by Michael Foord; :issue:`6001`.)

The error messages for :meth:`~unittest.TestCase.assertEqual`,
:meth:`~unittest.TestCase.assertTrue`, and :meth:`~unittest.TestCase.assertFalse`
failures now provide more information.  If you set the
:attr:`~unittest.TestCase.longMessage` attribute of your :class:`~unittest.TestCase` classes to
True, both the standard error message and any additional message you
provide will be printed for failures.  (Added by Michael Foord; :issue:`5663`.)

The :meth:`~unittest.TestCase.assertRaises` method now
return a context handler when called without providing a callable
object to run.  For example, you can write this::

  with self.assertRaises(KeyError):
      {}['foo']

(Implemented by Antoine Pitrou; :issue:`4444`.)

.. rev 78774

Module- and class-level setup and teardown fixtures are now supported.
Modules can contain :func:`~unittest.setUpModule` and :func:`~unittest.tearDownModule`
functions.  Classes can have :meth:`~unittest.TestCase.setUpClass` and
:meth:`~unittest.TestCase.tearDownClass` methods that must be defined as class methods
(using ``@classmethod`` or equivalent).  These functions and
methods are invoked when the test runner switches to a test case in a
different module or class.

The methods :meth:`~unittest.TestCase.addCleanup` and
:meth:`~unittest.TestCase.doCleanups` were added.
:meth:`~unittest.TestCase.addCleanup` allows you to add cleanup functions that
will be called unconditionally (after :meth:`~unittest.TestCase.setUp` if
:meth:`~unittest.TestCase.setUp` fails, otherwise after :meth:`~unittest.TestCase.tearDown`). This allows
for much simpler resource allocation and deallocation during tests
(:issue:`5679`).

A number of new methods were added that provide more specialized
tests.  Many of these methods were written by Google engineers
for use in their test suites; Gregory P. Smith, Michael Foord, and
GvR worked on merging them into Python's version of :mod:`unittest`.

* :meth:`~unittest.TestCase.assertIsNone` and :meth:`~unittest.TestCase.assertIsNotNone` take one
  expression and verify that the result is or is not ``None``.

* :meth:`~unittest.TestCase.assertIs` and :meth:`~unittest.TestCase.assertIsNot`
  take two values and check whether the two values evaluate to the same object or not.
  (Added by Michael Foord; :issue:`2578`.)

* :meth:`~unittest.TestCase.assertIsInstance` and
  :meth:`~unittest.TestCase.assertNotIsInstance` check whether
  the resulting object is an instance of a particular class, or of
  one of a tuple of classes.  (Added by Georg Brandl; :issue:`7031`.)

* :meth:`~unittest.TestCase.assertGreater`, :meth:`~unittest.TestCase.assertGreaterEqual`,
  :meth:`~unittest.TestCase.assertLess`, and :meth:`~unittest.TestCase.assertLessEqual` compare
  two quantities.

* :meth:`~unittest.TestCase.assertMultiLineEqual` compares two strings, and if they're
  not equal, displays a helpful comparison that highlights the
  differences in the two strings.  This comparison is now used by
  default when Unicode strings are compared with :meth:`~unittest.TestCase.assertEqual`.

* :meth:`~unittest.TestCase.assertRegexpMatches` checks whether its first argument is a
  string matching a regular expression provided as its second argument.

  .. XXX add assertNotRegexpMatches see issue 8038

* :meth:`~unittest.TestCase.assertRaisesRegexp` checks whether a particular exception
  is raised, and then also checks that the string representation of
  the exception matches the provided regular expression.

* :meth:`~unittest.TestCase.assertIn` and :meth:`~unittest.TestCase.assertNotIn`
  tests whether *first* is or is not in  *second*.

* :meth:`~unittest.TestCase.assertItemsEqual` tests whether two provided sequences
  contain the same elements.

* :meth:`~unittest.TestCase.assertSetEqual` compares whether two sets are equal, and
  only reports the differences between the sets in case of error.

* Similarly, :meth:`~unittest.TestCase.assertListEqual` and :meth:`~unittest.TestCase.assertTupleEqual`
  compare the specified types and explain any differences without necessarily
  printing their full values; these methods are now used by default
  when comparing lists and tuples using :meth:`~unittest.TestCase.assertEqual`.
  More generally, :meth:`~unittest.TestCase.assertSequenceEqual` compares two sequences
  and can optionally check whether both sequences are of a
  particular type.

* :meth:`~unittest.TestCase.assertDictEqual` compares two dictionaries and reports the
  differences; it's now used by default when you compare two dictionaries
  using :meth:`~unittest.TestCase.assertEqual`.  :meth:`~unittest.TestCase.assertDictContainsSubset` checks whether
  all of the key/value pairs in *first* are found in *second*.

* :meth:`~unittest.TestCase.assertAlmostEqual` and :meth:`~unittest.TestCase.assertNotAlmostEqual` test
  whether *first* and *second* are approximately equal by computing
  their difference, rounding the result to an optionally-specified number
  of *places* (the default is 7), and comparing to zero.

* :meth:`~unittest.TestLoader.loadTestsFromName` properly honors the
  :attr:`~unittest.TestLoader.suiteClass` attribute of
  the :class:`~unittest.TestLoader`. (Fixed by Mark Roddy; :issue:`6866`.)

* A new hook lets you extend the :meth:`~unittest.TestCase.assertEqual` method to handle
  new data types.  The :meth:`~unittest.TestCase.addTypeEqualityFunc` method takes a type
  object and a function. The function will be used when both of the
  objects being compared are of the specified type.  This function
  should compare the two objects and raise an exception if they don't
  match; it's a good idea for the function to provide additional
  information about why the two objects are matching, much as the new
  sequence comparison methods do.

:func:`unittest.main` now takes an optional ``exit`` argument.  If
False, :func:`~unittest.main` doesn't call :func:`sys.exit`, allowing it to be
used from the interactive interpreter. (Contributed by J. Pablo
Fernández; :issue:`3379`.)

A new command-line switch, :option:`-f` or :option:`--failfast`, makes
test execution stop immediately when a test fails instead of
continuing to execute further tests.  (Suggested by Cliff Dyer and
implemented by Michael Foord; :issue:`8074`.)

.. XXX document the other new switches

:class:`~unittest.TestResult` has new :meth:`~unittest.TestResult.startTestRun` and
:meth:`~unittest.TestResult.stopTestRun` methods that are called immediately before
and after a test run.  (Contributed by Robert Collins; :issue:`5728`.)

With all these changes, the :file:`unittest.py` was becoming awkwardly
large, so the module was turned into a package and the code split into
several files (by Benjamin Peterson).  This doesn't affect how the
module is imported.


.. _importlib-section:

importlib: Importing Modules
------------------------------

Python 3.1 includes the :mod:`importlib` package, a re-implementation
of the logic underlying Python's :keyword:`import` statement.
:mod:`importlib` is useful for implementors of Python interpreters and
to users who wish to write new importers that can participate in the
import process.  Python 2.7 doesn't contain the complete
:mod:`importlib` package, but instead has a tiny subset that contains
a single function, :func:`~importlib.import_module`.

``import_module(name, package=None)`` imports a module.  *name* is
a string containing the module or package's name.  It's possible to do
relative imports by providing a string that begins with a ``.``
character, such as ``..utils.errors``.  For relative imports, the
*package* argument must be provided and is the name of the package that
will be used as the anchor for
the relative import.  :func:`~importlib.import_module` both inserts the imported
module into ``sys.modules`` and returns the module object.

Here are some examples::

    >>> from importlib import import_module
    >>> anydbm = import_module('anydbm')  # Standard absolute import
    >>> anydbm
    <module 'anydbm' from '/p/python/Lib/anydbm.py'>
    >>> # Relative import
    >>> sysconfig = import_module('..sysconfig', 'distutils.command')
    >>> sysconfig
    <module 'distutils.sysconfig' from '/p/python/Lib/distutils/sysconfig.pyc'>

:mod:`importlib` was implemented by Brett Cannon and introduced in
Python 3.1.


ttk: Themed Widgets for Tk
--------------------------

Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform's widgets.  This widget
set was originally called Tile, but was renamed to Ttk (for "themed Tk")
on being added to Tcl/Tck release 8.5.

XXX write a brief discussion and an example here.

The :mod:`ttk` module was written by Guilherme Polo and added in
:issue:`2983`.  An alternate version called ``Tile.py``, written by
Martin Franklin and maintained by Kevin Walzer, was proposed for
inclusion in :issue:`2618`, but the authors argued that Guilherme
Polo's work was more comprehensive.


Deprecations and Removals
=========================

* :func:`contextlib.nested`, which allows handling more than one context manager
  with one :keyword:`with` statement, has been deprecated; :keyword:`with`
  supports multiple context managers syntactically now.

.. ======================================================================


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

Changes to Python's build process and to the C API include:

* The latest release of the GNU Debugger, GDB 7, can be `scripted
  using Python
  <http://sourceware.org/gdb/current/onlinedocs/gdb/Python.html>`__.
  When you begin debugging an executable program P, GDB will look for
  a file named ``P-gdb.py`` and automatically read it.  Dave Malcolm
  contributed a :file:`python-gdb.py` that adds a number of useful
  commands when debugging Python itself.  For example, there are
  ``py-up`` and ``py-down`` that go up or down one Python stack frame,
  which usually corresponds to several C stack frames.  ``py-print``
  prints the value of a Python variable, and ``py-bt`` prints the
  Python stack trace.  (Added as a result of :issue:`8032`.)

* If you use the :file:`.gdbinit` file provided with Python,
  the "pyo" macro in the 2.7 version now works correctly when the thread being
  debugged doesn't hold the GIL; the macro now acquires it before printing.
  (Contributed by Victor Stinner; :issue:`3632`.)

* :cfunc:`Py_AddPendingCall` is now thread-safe, letting any
  worker thread submit notifications to the main Python thread.  This
  is particularly useful for asynchronous IO operations.
  (Contributed by Kristjan Valur Jonsson; :issue:`4293`.)

* New function: :cfunc:`PyCode_NewEmpty` creates an empty code object;
  only the filename, function name, and first line number are required.
  This is useful to extension modules that are attempting to
  construct a more useful traceback stack.  Previously such
  extensions needed to call :cfunc:`PyCode_New`, which had many
  more arguments.  (Added by Jeffrey Yasskin.)

* New function: :cfunc:`PyErr_NewExceptionWithDoc` creates a new
  exception class, just as the existing :cfunc:`PyErr_NewException` does,
  but takes an extra ``char *`` argument containing the docstring for the
  new exception class.  (Added by the 'lekma' user on the Python bug tracker;
  :issue:`7033`.)

* New function: :cfunc:`PyFrame_GetLineNumber` takes a frame object
  and returns the line number that the frame is currently executing.
  Previously code would need to get the index of the bytecode
  instruction currently executing, and then look up the line number
  corresponding to that address.  (Added by Jeffrey Yasskin.)

* New functions: :cfunc:`PyLong_AsLongAndOverflow` and
  :cfunc:`PyLong_AsLongLongAndOverflow`  approximates a Python long
  integer as a C :ctype:`long` or :ctype:`long long`.
  If the number is too large to fit into
  the output type, an *overflow* flag is set and returned to the caller.
  (Contributed by Case Van Horsen; :issue:`7528` and :issue:`7767`.)

* New function: stemming from the rewrite of string-to-float conversion,
  a new :cfunc:`PyOS_string_to_double` function was added.  The old
  :cfunc:`PyOS_ascii_strtod` and :cfunc:`PyOS_ascii_atof` functions
  are now deprecated.

* New macros: the Python header files now define the following macros:
  :cmacro:`Py_ISALNUM`,
  :cmacro:`Py_ISALPHA`,
  :cmacro:`Py_ISDIGIT`,
  :cmacro:`Py_ISLOWER`,
  :cmacro:`Py_ISSPACE`,
  :cmacro:`Py_ISUPPER`,
  :cmacro:`Py_ISXDIGIT`,
  and :cmacro:`Py_TOLOWER`, :cmacro:`Py_TOUPPER`.
  All of these functions are analogous to the C
  standard macros for classifying characters, but ignore the current
  locale setting, because in
  several places Python needs to analyze characters in a
  locale-independent way.  (Added by Eric Smith;
  :issue:`5793`.)

  .. XXX these macros don't seem to be described in the c-api docs.

* New format codes: the :cfunc:`PyFormat_FromString`,
  :cfunc:`PyFormat_FromStringV`, and :cfunc:`PyErr_Format` now
  accepts ``%lld`` and ``%llu`` format codes for displaying values of
  C's :ctype:`long long` types.
  (Contributed by Mark Dickinson; :issue:`7228`.)

* The complicated interaction between threads and process forking has
  been changed.  Previously, the child process created by
  :func:`os.fork` might fail because the child is created with only a
  single thread running, the thread performing the :func:`os.fork`.
  If other threads were holding a lock, such as Python's import lock,
  when the fork was performed, the lock would still be marked as
  "held" in the new process.  But in the child process nothing would
  ever release the lock, since the other threads weren't replicated,
  and the child process would no longer be able to perform imports.

  Python 2.7 now acquires the import lock before performing an
  :func:`os.fork`, and will also clean up any locks created using the
  :mod:`threading` module.  C extension modules that have internal
  locks, or that call :cfunc:`fork()` themselves, will not benefit
  from this clean-up.

  (Fixed by Thomas Wouters; :issue:`1590864`.)

* The :cfunc:`Py_Finalize` function now calls the internal
  :func:`threading._shutdown` function; this prevents some exceptions from
  being raised when an interpreter shuts down.
  (Patch by Adam Olsen; :issue:`1722344`.)

* Global symbols defined by the :mod:`ctypes` module are now prefixed
  with ``Py``, or with ``_ctypes``.  (Implemented by Thomas
  Heller; :issue:`3102`.)

* New configure option: the :option:`--with-system-expat` switch allows
  building the :mod:`pyexpat` module to use the system Expat library.
  (Contributed by Arfrever Frehtes Taifersar Arahesis; :issue:`7609`.)

* New configure option: compiling Python with the
  :option:`--with-valgrind` option will now disable the pymalloc
  allocator, which is difficult for the Valgrind memory-error detector
  to analyze correctly.
  Valgrind will therefore be better at detecting memory leaks and
  overruns. (Contributed by James Henstridge; :issue:`2422`.)

* New configure option: you can now supply no arguments to
  :option:`--with-dbmliborder=` in order to build none of the various
  DBM modules.  (Added by Arfrever Frehtes Taifersar Arahesis;
  :issue:`6491`.)

* The :program:`configure` script now checks for floating-point rounding bugs
  on certain 32-bit Intel chips and defines a :cmacro:`X87_DOUBLE_ROUNDING`
  preprocessor definition.  No code currently uses this definition,
  but it's available if anyone wishes to use it.
  (Added by Mark Dickinson; :issue:`2937`.)

  :program:`configure` also now sets a :envvar:`LDCXXSHARED` Makefile
  variable for supporting C++ linking.  (Contributed by Arfrever
  Frehtes Taifersar Arahesis; :issue:`1222585`.)

* The build process now creates the necessary files for pkg-config
  support.  (Contributed by Clinton Roy; :issue:`3585`.)

* The build process now supports Subversion 1.7.  (Contributed by
  Arfrever Frehtes Taifersar Arahesis; :issue:`6094`.)


.. ======================================================================

Port-Specific Changes: Windows
-----------------------------------

* The :mod:`msvcrt` module now contains some constants from
  the :file:`crtassem.h` header file:
  :data:`CRT_ASSEMBLY_VERSION`,
  :data:`VC_ASSEMBLY_PUBLICKEYTOKEN`,
  and :data:`LIBRARIES_ASSEMBLY_NAME_PREFIX`.
  (Contributed by David Cournapeau; :issue:`4365`.)

* The new :cfunc:`_beginthreadex` API is used to start threads, and
  the native thread-local storage functions are now used.
  (Contributed by Kristjan Valur Jonsson; :issue:`3582`.)

* The :func:`os.listdir` function now correctly fails
  for an empty path.  (Fixed by Hirokazu Yamamoto; :issue:`5913`.)

* The :mod:`mimelib` module will now read the MIME database from
  the Windows registry when initializing.
  (Patch by Gabriel Genellina; :issue:`4969`.)

.. ======================================================================

Port-Specific Changes: Mac OS X
-----------------------------------

* The path ``/Library/Python/2.7/site-packages`` is now appended to
  ``sys.path``, in order to share added packages between the system
  installation and a user-installed copy of the same version.
  (Changed by Ronald Oussoren; :issue:`4865`.)


Other Changes and Fixes
=======================

* Two benchmark scripts, :file:`iobench` and :file:`ccbench`, were
  added to the :file:`Tools` directory.  :file:`iobench` measures the
  speed of built-in file I/O objects (as returned by :func:`open`)
  while performing various operations, and :file:`ccbench` is a
  concurrency benchmark that tries to measure computing throughput,
  thread switching latency, and IO processing bandwidth when
  performing several tasks using a varying number of threads.

* When importing a module from a :file:`.pyc` or :file:`.pyo` file
  with an existing :file:`.py` counterpart, the :attr:`co_filename`
  attributes of the resulting code objects are overwritten when the
  original filename is obsolete.  This can happen if the file has been
  renamed, moved, or is accessed through different paths.  (Patch by
  Ziga Seilnacht and Jean-Paul Calderone; :issue:`1180193`.)

* The :file:`regrtest.py` script now takes a :option:`--randseed=`
  switch that takes an integer that will be used as the random seed
  for the :option:`-r` option that executes tests in random order.
  The :option:`-r` option also reports the seed that was used
  (Added by Collin Winter.)

* Another :file:`regrtest.py` switch is :option:`-j`, which
  takes an integer specifying how many tests run in parallel. This
  allows reducing the total runtime on multi-core machines.
  This option is compatible with several other options, including the
  :option:`-R` switch which is known to produce long runtimes.
  (Added by Antoine Pitrou, :issue:`6152`.)  This can also be used
  with a new :option:`-F` switch that runs selected tests in a loop
  until they fail.  (Added by Antoine Pitrou; :issue:`7312`.)

.. ======================================================================

Porting to Python 2.7
=====================

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

* When using :class:`Decimal` instances with a string's
  :meth:`format` method, the default alignment was previously
  left-alignment.  This has been changed to right-alignment, which might
  change the output of your programs.
  (Changed by Mark Dickinson; :issue:`6857`.)

  Another :meth:`format`-related change: the default precision used
  for floating-point and complex numbers was changed from 6 decimal
  places to 12, which matches the precision used by :func:`str`.
  (Changed by Eric Smith; :issue:`5920`.)

* Because of an optimization for the :keyword:`with` statement, the special
  methods :meth:`__enter__` and :meth:`__exit__` must belong to the object's
  type, and cannot be directly attached to the object's instance.  This
  affects new-style classes (derived from :class:`object`) and C extension
  types.  (:issue:`6101`.)

* The :meth:`readline` method of :class:`StringIO` objects now does
  nothing when a negative length is requested, as other file-like
  objects do.  (:issue:`7348`).

In the standard library:

* The ElementTree library, :mod:`xml.etree`, no longer escapes
  ampersands and angle brackets when outputting an XML processing
  instruction (which looks like `<?xml-stylesheet href="#style1"?>`)
  or comment (which looks like `<!-- comment -->`).
  (Patch by Neil Muller; :issue:`2746`.)

For C extensions:

* C extensions that use integer format codes with the ``PyArg_Parse*``
  family of functions will now raise a :exc:`TypeError` exception
  instead of triggering a :exc:`DeprecationWarning` (:issue:`5080`).

* Use the new :cfunc:`PyOS_string_to_double` function instead of the old
  :cfunc:`PyOS_ascii_strtod` and :cfunc:`PyOS_ascii_atof` functions,
  which are now deprecated.


.. ======================================================================


.. _acks27:

Acknowledgements
================

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Ryan Lovett, Hugh Secker-Walker.