This file is indexed.

/usr/share/doc/python/faq/library.html is in python 2.7.9-1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

   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
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.11: http://docutils.sourceforge.net/" />
<title>Python Library and Extension FAQ</title>
<meta name="date" content="2005-12-16" />
<style type="text/css">

/*
:Author: David Goodger (goodger@python.org)
:Id: $Id: html4css1.css 7614 2013-02-21 15:55:51Z milde $
:Copyright: This stylesheet has been placed in the public domain.

Default cascading style sheet for the HTML output of Docutils.

See http://docutils.sf.net/docs/howto/html-stylesheets.html for how to
customize this style sheet.
*/

/* used to remove borders from tables and images */
.borderless, table.borderless td, table.borderless th {
  border: 0 }

table.borderless td, table.borderless th {
  /* Override padding for "table.docutils td" with "! important".
     The right padding separates the table cells. */
  padding: 0 0.5em 0 0 ! important }

.first {
  /* Override more specific margin styles with "! important". */
  margin-top: 0 ! important }

.last, .with-subtitle {
  margin-bottom: 0 ! important }

.hidden {
  display: none }

a.toc-backref {
  text-decoration: none ;
  color: black }

blockquote.epigraph {
  margin: 2em 5em ; }

dl.docutils dd {
  margin-bottom: 0.5em }

object[type="image/svg+xml"], object[type="application/x-shockwave-flash"] {
  overflow: hidden;
}

/* Uncomment (and remove this text!) to get bold-faced definition list terms
dl.docutils dt {
  font-weight: bold }
*/

div.abstract {
  margin: 2em 5em }

div.abstract p.topic-title {
  font-weight: bold ;
  text-align: center }

div.admonition, div.attention, div.caution, div.danger, div.error,
div.hint, div.important, div.note, div.tip, div.warning {
  margin: 2em ;
  border: medium outset ;
  padding: 1em }

div.admonition p.admonition-title, div.hint p.admonition-title,
div.important p.admonition-title, div.note p.admonition-title,
div.tip p.admonition-title {
  font-weight: bold ;
  font-family: sans-serif }

div.attention p.admonition-title, div.caution p.admonition-title,
div.danger p.admonition-title, div.error p.admonition-title,
div.warning p.admonition-title, .code .error {
  color: red ;
  font-weight: bold ;
  font-family: sans-serif }

/* Uncomment (and remove this text!) to get reduced vertical space in
   compound paragraphs.
div.compound .compound-first, div.compound .compound-middle {
  margin-bottom: 0.5em }

div.compound .compound-last, div.compound .compound-middle {
  margin-top: 0.5em }
*/

div.dedication {
  margin: 2em 5em ;
  text-align: center ;
  font-style: italic }

div.dedication p.topic-title {
  font-weight: bold ;
  font-style: normal }

div.figure {
  margin-left: 2em ;
  margin-right: 2em }

div.footer, div.header {
  clear: both;
  font-size: smaller }

div.line-block {
  display: block ;
  margin-top: 1em ;
  margin-bottom: 1em }

div.line-block div.line-block {
  margin-top: 0 ;
  margin-bottom: 0 ;
  margin-left: 1.5em }

div.sidebar {
  margin: 0 0 0.5em 1em ;
  border: medium outset ;
  padding: 1em ;
  background-color: #ffffee ;
  width: 40% ;
  float: right ;
  clear: right }

div.sidebar p.rubric {
  font-family: sans-serif ;
  font-size: medium }

div.system-messages {
  margin: 5em }

div.system-messages h1 {
  color: red }

div.system-message {
  border: medium outset ;
  padding: 1em }

div.system-message p.system-message-title {
  color: red ;
  font-weight: bold }

div.topic {
  margin: 2em }

h1.section-subtitle, h2.section-subtitle, h3.section-subtitle,
h4.section-subtitle, h5.section-subtitle, h6.section-subtitle {
  margin-top: 0.4em }

h1.title {
  text-align: center }

h2.subtitle {
  text-align: center }

hr.docutils {
  width: 75% }

img.align-left, .figure.align-left, object.align-left {
  clear: left ;
  float: left ;
  margin-right: 1em }

img.align-right, .figure.align-right, object.align-right {
  clear: right ;
  float: right ;
  margin-left: 1em }

img.align-center, .figure.align-center, object.align-center {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

.align-left {
  text-align: left }

.align-center {
  clear: both ;
  text-align: center }

.align-right {
  text-align: right }

/* reset inner alignment in figures */
div.align-right {
  text-align: inherit }

/* div.align-center * { */
/*   text-align: left } */

ol.simple, ul.simple {
  margin-bottom: 1em }

ol.arabic {
  list-style: decimal }

ol.loweralpha {
  list-style: lower-alpha }

ol.upperalpha {
  list-style: upper-alpha }

ol.lowerroman {
  list-style: lower-roman }

ol.upperroman {
  list-style: upper-roman }

p.attribution {
  text-align: right ;
  margin-left: 50% }

p.caption {
  font-style: italic }

p.credits {
  font-style: italic ;
  font-size: smaller }

p.label {
  white-space: nowrap }

p.rubric {
  font-weight: bold ;
  font-size: larger ;
  color: maroon ;
  text-align: center }

p.sidebar-title {
  font-family: sans-serif ;
  font-weight: bold ;
  font-size: larger }

p.sidebar-subtitle {
  font-family: sans-serif ;
  font-weight: bold }

p.topic-title {
  font-weight: bold }

pre.address {
  margin-bottom: 0 ;
  margin-top: 0 ;
  font: inherit }

pre.literal-block, pre.doctest-block, pre.math, pre.code {
  margin-left: 2em ;
  margin-right: 2em }

pre.code .ln { color: grey; } /* line numbers */
pre.code, code { background-color: #eeeeee }
pre.code .comment, code .comment { color: #5C6576 }
pre.code .keyword, code .keyword { color: #3B0D06; font-weight: bold }
pre.code .literal.string, code .literal.string { color: #0C5404 }
pre.code .name.builtin, code .name.builtin { color: #352B84 }
pre.code .deleted, code .deleted { background-color: #DEB0A1}
pre.code .inserted, code .inserted { background-color: #A3D289}

span.classifier {
  font-family: sans-serif ;
  font-style: oblique }

span.classifier-delimiter {
  font-family: sans-serif ;
  font-weight: bold }

span.interpreted {
  font-family: sans-serif }

span.option {
  white-space: nowrap }

span.pre {
  white-space: pre }

span.problematic {
  color: red }

span.section-subtitle {
  /* font-size relative to parent (h1..h6 element) */
  font-size: 80% }

table.citation {
  border-left: solid 1px gray;
  margin-left: 1px }

table.docinfo {
  margin: 2em 4em }

table.docutils {
  margin-top: 0.5em ;
  margin-bottom: 0.5em }

table.footnote {
  border-left: solid 1px black;
  margin-left: 1px }

table.docutils td, table.docutils th,
table.docinfo td, table.docinfo th {
  padding-left: 0.5em ;
  padding-right: 0.5em ;
  vertical-align: top }

table.docutils th.field-name, table.docinfo th.docinfo-name {
  font-weight: bold ;
  text-align: left ;
  white-space: nowrap ;
  padding-left: 0 }

/* "booktabs" style (no vertical lines) */
table.docutils.booktabs {
  border: 0px;
  border-top: 2px solid;
  border-bottom: 2px solid;
  border-collapse: collapse;
}
table.docutils.booktabs * {
  border: 0px;
}
table.docutils.booktabs th {
  border-bottom: thin solid;
  text-align: left;
}

h1 tt.docutils, h2 tt.docutils, h3 tt.docutils,
h4 tt.docutils, h5 tt.docutils, h6 tt.docutils {
  font-size: 100% }

ul.auto-toc {
  list-style-type: none }

</style>
</head>
<body>
<div class="document" id="python-library-and-extension-faq">
<h1 class="title">Python Library and Extension FAQ</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Date:</th>
<td>2005-12-16</td></tr>
<tr><th class="docinfo-name">Version:</th>
<td>8684</td></tr>
<tr class="field"><th class="docinfo-name">Web site:</th><td class="field-body"><a class="reference external" href="http://www.python.org/">http://www.python.org/</a></td>
</tr>
</tbody>
</table>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="auto-toc simple">
<li><a class="reference internal" href="#general-library-questions" id="id3">1&nbsp;&nbsp;&nbsp;General Library Questions</a><ul class="auto-toc">
<li><a class="reference internal" href="#how-do-i-find-a-module-or-application-to-perform-task-x" id="id4">1.1&nbsp;&nbsp;&nbsp;How do I find a module or application to perform task X?</a></li>
<li><a class="reference internal" href="#where-is-the-math-py-socket-py-regex-py-etc-source-file" id="id5">1.2&nbsp;&nbsp;&nbsp;Where is the math.py (socket.py, regex.py, etc.) source file?</a></li>
<li><a class="reference internal" href="#how-do-i-make-a-python-script-executable-on-unix" id="id6">1.3&nbsp;&nbsp;&nbsp;How do I make a Python script executable on Unix?</a></li>
<li><a class="reference internal" href="#is-there-a-curses-termcap-package-for-python" id="id7">1.4&nbsp;&nbsp;&nbsp;Is there a curses/termcap package for Python?</a></li>
<li><a class="reference internal" href="#is-there-an-equivalent-to-c-s-onexit-in-python" id="id8">1.5&nbsp;&nbsp;&nbsp;Is there an equivalent to C's onexit() in Python?</a></li>
<li><a class="reference internal" href="#why-don-t-my-signal-handlers-work" id="id9">1.6&nbsp;&nbsp;&nbsp;Why don't my signal handlers work?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#common-tasks" id="id10">2&nbsp;&nbsp;&nbsp;Common tasks</a><ul class="auto-toc">
<li><a class="reference internal" href="#how-do-i-test-a-python-program-or-component" id="id11">2.1&nbsp;&nbsp;&nbsp;How do I test a Python program or component?</a></li>
<li><a class="reference internal" href="#how-do-i-create-documentation-from-doc-strings" id="id12">2.2&nbsp;&nbsp;&nbsp;How do I create documentation from doc strings?</a></li>
<li><a class="reference internal" href="#how-do-i-get-a-single-keypress-at-a-time" id="id13">2.3&nbsp;&nbsp;&nbsp;How do I get a single keypress at a time?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#threads" id="id14">3&nbsp;&nbsp;&nbsp;Threads</a><ul class="auto-toc">
<li><a class="reference internal" href="#how-do-i-program-using-threads" id="id15">3.1&nbsp;&nbsp;&nbsp;How do I program using threads?</a></li>
<li><a class="reference internal" href="#none-of-my-threads-seem-to-run-why" id="id16">3.2&nbsp;&nbsp;&nbsp;None of my threads seem to run: why?</a></li>
<li><a class="reference internal" href="#how-do-i-parcel-out-work-among-a-bunch-of-worker-threads" id="id17">3.3&nbsp;&nbsp;&nbsp;How do I parcel out work among a bunch of worker threads?</a></li>
<li><a class="reference internal" href="#what-kinds-of-global-value-mutation-are-thread-safe" id="id18">3.4&nbsp;&nbsp;&nbsp;What kinds of global value mutation are thread-safe?</a></li>
<li><a class="reference internal" href="#can-t-we-get-rid-of-the-global-interpreter-lock" id="id19">3.5&nbsp;&nbsp;&nbsp;Can't we get rid of the Global Interpreter Lock?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#input-and-output" id="id20">4&nbsp;&nbsp;&nbsp;Input and Output</a><ul class="auto-toc">
<li><a class="reference internal" href="#how-do-i-delete-a-file-and-other-file-questions" id="id21">4.1&nbsp;&nbsp;&nbsp;How do I delete a file? (And other file questions...)</a></li>
<li><a class="reference internal" href="#how-do-i-copy-a-file" id="id22">4.2&nbsp;&nbsp;&nbsp;How do I copy a file?</a></li>
<li><a class="reference internal" href="#how-do-i-read-or-write-binary-data" id="id23">4.3&nbsp;&nbsp;&nbsp;How do I read (or write) binary data?</a></li>
<li><a class="reference internal" href="#i-can-t-seem-to-use-os-read-on-a-pipe-created-with-os-popen-why" id="id24">4.4&nbsp;&nbsp;&nbsp;I can't seem to use os.read() on a pipe created with os.popen(); why?</a></li>
<li><a class="reference internal" href="#how-do-i-run-a-subprocess-with-pipes-connected-to-both-input-and-output" id="id25">4.5&nbsp;&nbsp;&nbsp;How do I run a subprocess with pipes connected to both input and output?</a></li>
<li><a class="reference internal" href="#how-do-i-access-the-serial-rs232-port" id="id26">4.6&nbsp;&nbsp;&nbsp;How do I access the serial (RS232) port?</a></li>
<li><a class="reference internal" href="#why-doesn-t-closing-sys-stdout-stdin-stderr-really-close-it" id="id27">4.7&nbsp;&nbsp;&nbsp;Why doesn't closing sys.stdout (stdin, stderr) really close it?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#network-internet-programming" id="id28">5&nbsp;&nbsp;&nbsp;Network/Internet Programming</a><ul class="auto-toc">
<li><a class="reference internal" href="#what-www-tools-are-there-for-python" id="id29">5.1&nbsp;&nbsp;&nbsp;What WWW tools are there for Python?</a></li>
<li><a class="reference internal" href="#how-can-i-mimic-cgi-form-submission-method-post" id="id30">5.2&nbsp;&nbsp;&nbsp;How can I mimic CGI form submission (METHOD=POST)?</a></li>
<li><a class="reference internal" href="#what-module-should-i-use-to-help-with-generating-html" id="id31">5.3&nbsp;&nbsp;&nbsp;What module should I use to help with generating HTML?</a></li>
<li><a class="reference internal" href="#how-do-i-send-mail-from-a-python-script" id="id32">5.4&nbsp;&nbsp;&nbsp;How do I send mail from a Python script?</a></li>
<li><a class="reference internal" href="#how-do-i-avoid-blocking-in-the-connect-method-of-a-socket" id="id33">5.5&nbsp;&nbsp;&nbsp;How do I avoid blocking in the connect() method of a socket?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#databases" id="id34">6&nbsp;&nbsp;&nbsp;Databases</a><ul class="auto-toc">
<li><a class="reference internal" href="#are-there-any-interfaces-to-database-packages-in-python" id="id35">6.1&nbsp;&nbsp;&nbsp;Are there any interfaces to database packages in Python?</a></li>
<li><a class="reference internal" href="#how-do-you-implement-persistent-objects-in-python" id="id36">6.2&nbsp;&nbsp;&nbsp;How do you implement persistent objects in Python?</a></li>
<li><a class="reference internal" href="#why-is-cpickle-so-slow" id="id37">6.3&nbsp;&nbsp;&nbsp;Why is cPickle so slow?</a></li>
<li><a class="reference internal" href="#if-my-program-crashes-with-a-bsddb-or-anydbm-database-open-it-gets-corrupted-how-come" id="id38">6.4&nbsp;&nbsp;&nbsp;If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</a></li>
<li><a class="reference internal" href="#i-tried-to-open-berkeley-db-file-but-bsddb-produces-bsddb-error-22-invalid-argument-help-how-can-i-restore-my-data" id="id39">6.5&nbsp;&nbsp;&nbsp;I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#mathematics-and-numerics" id="id40">7&nbsp;&nbsp;&nbsp;Mathematics and Numerics</a><ul class="auto-toc">
<li><a class="reference internal" href="#how-do-i-generate-random-numbers-in-python" id="id41">7.1&nbsp;&nbsp;&nbsp;How do I generate random numbers in Python?</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="general-library-questions">
<h1><a class="toc-backref" href="#id3">1&nbsp;&nbsp;&nbsp;General Library Questions</a></h1>
<div class="section" id="how-do-i-find-a-module-or-application-to-perform-task-x">
<h2><a class="toc-backref" href="#id4">1.1&nbsp;&nbsp;&nbsp;How do I find a module or application to perform task X?</a></h2>
<p>Check <a class="reference external" href="http://www.python.org/doc/lib/">the Library Reference</a> to see
if there's a relevant standard library module.  (Eventually you'll
learn what's in the standard library and will able to skip this step.)</p>
<p>Search the <a class="reference external" href="http://cheeseshop.python.org/pypi">Python Package Index</a>.</p>
<p>Next, check the <a class="reference external" href="http://www.vex.net/parnassus/">Vaults of Parnassus</a>,
an older index of packages.</p>
<p>Finally, try <a class="reference external" href="http://www.google.com">Google</a> or other Web search
engine.  Searching for &quot;Python&quot; plus a keyword or two for your topic
of interest will usually find something helpful.</p>
</div>
<div class="section" id="where-is-the-math-py-socket-py-regex-py-etc-source-file">
<h2><a class="toc-backref" href="#id5">1.2&nbsp;&nbsp;&nbsp;Where is the math.py (socket.py, regex.py, etc.) source file?</a></h2>
<p>If you can't find a source file for a module it may be a builtin
or dynamically loaded module implemented in C, C++ or other
compiled language.  In this case you may not have the source
file or it may be something like mathmodule.c, somewhere in
a C source directory (not on the Python Path).</p>
<p>There are (at least) three kinds of modules in Python:</p>
<ol class="arabic">
<li><p class="first">modules written in Python (.py);</p>
</li>
<li><p class="first">modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);</p>
</li>
<li><p class="first">modules written in C and linked with the interpreter; to get a list
of these, type:</p>
<pre class="literal-block">
import sys
print sys.builtin_module_names
</pre>
</li>
</ol>
</div>
<div class="section" id="how-do-i-make-a-python-script-executable-on-unix">
<h2><a class="toc-backref" href="#id6">1.3&nbsp;&nbsp;&nbsp;How do I make a Python script executable on Unix?</a></h2>
<p>You need to do two things: the script file's mode must be executable
and the first line must begin with <tt class="docutils literal">#!</tt>  followed by the path of
the Python interpreter.</p>
<p>The first is done by executing <tt class="docutils literal">chmod +x scriptfile</tt> or perhaps
<tt class="docutils literal">chmod 755 scriptfile</tt>.</p>
<p>The second can be done in a number of ways.  The most straightforward
way is to write</p>
<pre class="literal-block">
#!/usr/local/bin/python
</pre>
<p>as the very first line of your file, using the pathname for where the
Python interpreter is installed on your platform.</p>
<p>If you would like the script to be independent of where the Python
interpreter lives, you can use the &quot;env&quot; program.  Almost all
Unix variants support the following, assuming the python interpreter
is in a directory on the user's $PATH:</p>
<pre class="literal-block">
#! /usr/bin/env python
</pre>
<p><em>Don't</em> do this for CGI scripts.  The $PATH variable for
CGI scripts is often very minimal, so you need to use the actual
absolute pathname of the interpreter.</p>
<p>Occasionally, a user's environment is so full that the /usr/bin/env
program fails; or there's no env program at all.
In that case, you can try the following hack (due to Alex Rezinsky):</p>
<pre class="literal-block">
#! /bin/sh
&quot;&quot;&quot;:&quot;
exec python $0 ${1+&quot;$&#64;&quot;}
&quot;&quot;&quot;
</pre>
<p>The minor disadvantage is that this defines the script's __doc__ string.
However, you can fix that by adding</p>
<pre class="literal-block">
__doc__ = &quot;&quot;&quot;...Whatever...&quot;&quot;&quot;
</pre>
</div>
<div class="section" id="is-there-a-curses-termcap-package-for-python">
<h2><a class="toc-backref" href="#id7">1.4&nbsp;&nbsp;&nbsp;Is there a curses/termcap package for Python?</a></h2>
<p>For Unix variants: The standard Python source distribution comes with
a curses module in the Modules/ subdirectory, though it's not compiled
by default (note that this is not available in the Windows
distribution -- there is no curses module for Windows).</p>
<p>The curses module supports basic curses features as well as many
additional functions from ncurses and SYSV curses such as colour,
alternative character set support, pads, and mouse support. This means
the module isn't compatible with operating systems that only
have BSD curses, but there don't seem to be any currently maintained
OSes that fall into this category.</p>
<p>For Windows: use <a class="reference external" href="http://effbot.org/zone/console-index.htm">the consolelib module</a>.</p>
</div>
<div class="section" id="is-there-an-equivalent-to-c-s-onexit-in-python">
<h2><a class="toc-backref" href="#id8">1.5&nbsp;&nbsp;&nbsp;Is there an equivalent to C's onexit() in Python?</a></h2>
<p><a class="reference external" href="http://www.python.org/doc/lib/module-atexit.html">The atexit module</a> provides a
register function that is similar to C's onexit.</p>
</div>
<div class="section" id="why-don-t-my-signal-handlers-work">
<h2><a class="toc-backref" href="#id9">1.6&nbsp;&nbsp;&nbsp;Why don't my signal handlers work?</a></h2>
<p>The most common problem is that the signal handler is declared
with the wrong argument list.  It is called as</p>
<pre class="literal-block">
handler(signum, frame)
</pre>
<p>so it should be declared with two arguments:</p>
<pre class="literal-block">
def handler(signum, frame):
    ...
</pre>
</div>
</div>
<div class="section" id="common-tasks">
<h1><a class="toc-backref" href="#id10">2&nbsp;&nbsp;&nbsp;Common tasks</a></h1>
<div class="section" id="how-do-i-test-a-python-program-or-component">
<h2><a class="toc-backref" href="#id11">2.1&nbsp;&nbsp;&nbsp;How do I test a Python program or component?</a></h2>
<p>Python comes with two testing frameworks.  The <a class="reference external" href="http://www.python.org/doc/lib/module-doctest.html">doctest module</a> finds examples
in the docstrings for a module and runs them, comparing the output
with the expected output given in the docstring.</p>
<p>The <a class="reference external" href="http://www.python.org/doc/lib/module-unittest.html">unittest module</a> is a fancier
testing framework modelled on Java and Smalltalk testing frameworks.</p>
<p>For testing, it helps to write the program so that it may be easily
tested by using good modular design.  Your program should have almost
all functionality encapsulated in either functions or class methods --
and this sometimes has the surprising and delightful effect of making
the program run faster (because local variable accesses are faster
than global accesses).  Furthermore the program should avoid depending
on mutating global variables, since this makes testing much more
difficult to do.</p>
<p>The &quot;global main logic&quot; of your program may be as simple
as</p>
<pre class="literal-block">
if __name__==&quot;__main__&quot;:
     main_logic()
</pre>
<p>at the bottom of the main module of your program.</p>
<p>Once your program is organized as a tractable collection
of functions and class behaviours you should write test
functions that exercise the behaviours.  A test suite
can be associated with each module which automates
a sequence of tests.  This sounds like a lot of work, but
since Python is so terse and flexible it's surprisingly easy.
You can make coding much more pleasant and fun by
writing your test functions in parallel with the &quot;production
code&quot;, since this makes it easy to find bugs and even
design flaws earlier.</p>
<p>&quot;Support modules&quot; that are not intended to be the main module of a
program may include a self-test of the module.</p>
<pre class="literal-block">
if __name__ == &quot;__main__&quot;:
   self_test()
</pre>
<p>Even programs that interact with complex external interfaces may be
tested when the external interfaces are unavailable by using &quot;fake&quot;
interfaces implemented in Python.</p>
</div>
<div class="section" id="how-do-i-create-documentation-from-doc-strings">
<h2><a class="toc-backref" href="#id12">2.2&nbsp;&nbsp;&nbsp;How do I create documentation from doc strings?</a></h2>
<p>The <a class="reference external" href="http://www.python.org/doc/lib/module-pydoc.html">pydoc module</a>
can create HTML from the doc strings in your Python source code.  An
alternative is <a class="reference external" href="http://starship.python.net/crew/danilo/pythondoc/">pythondoc</a>.</p>
</div>
<div class="section" id="how-do-i-get-a-single-keypress-at-a-time">
<h2><a class="toc-backref" href="#id13">2.3&nbsp;&nbsp;&nbsp;How do I get a single keypress at a time?</a></h2>
<p>For Unix variants:There are several solutions.
It's straightforward to do this using curses, but curses is a
fairly large module to learn.  Here's a solution without curses:</p>
<pre class="literal-block">
import termios, fcntl, sys, os
fd = sys.stdin.fileno()

oldterm = termios.tcgetattr(fd)
newattr = termios.tcgetattr(fd)
newattr[3] = newattr[3] &amp; ~termios.ICANON &amp; ~termios.ECHO
termios.tcsetattr(fd, termios.TCSANOW, newattr)

oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)

try:
    while 1:
        try:
            c = sys.stdin.read(1)
            print &quot;Got character&quot;, `c`
        except IOError: pass
finally:
    termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
    fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)
</pre>
<p>You need the <tt class="docutils literal">termios</tt> and the <tt class="docutils literal">fcntl</tt> module for any of this to work,
and I've only tried it on Linux, though it should work elsewhere.
In this code, characters are read and printed one at a time.</p>
<p><tt class="docutils literal">termios.tcsetattr()</tt> turns off stdin's echoing and disables canonical
mode.  <tt class="docutils literal">fcntl.fnctl()</tt> is used to obtain stdin's file descriptor flags
and modify them for non-blocking mode.  Since reading stdin when it is
empty results in an <tt class="docutils literal">IOError</tt>, this error is caught and ignored.</p>
</div>
</div>
<div class="section" id="threads">
<h1><a class="toc-backref" href="#id14">3&nbsp;&nbsp;&nbsp;Threads</a></h1>
<div class="section" id="how-do-i-program-using-threads">
<h2><a class="toc-backref" href="#id15">3.1&nbsp;&nbsp;&nbsp;How do I program using threads?</a></h2>
<p>Be sure to use <a class="reference external" href="http://www.python.org/doc/lib/module-threading.html">the threading module</a> and not the
<tt class="docutils literal">thread</tt> module.  The <tt class="docutils literal">threading</tt> module builds convenient
abstractions on top of the low-level primitives provided by the
<tt class="docutils literal">thread</tt> module.</p>
<p>Aahz has a set of slides from his threading tutorial that are helpful;
see <a class="reference external" href="http://starship.python.net/crew/aahz/OSCON2001/">http://starship.python.net/crew/aahz/OSCON2001/</a>.</p>
</div>
<div class="section" id="none-of-my-threads-seem-to-run-why">
<h2><a class="toc-backref" href="#id16">3.2&nbsp;&nbsp;&nbsp;None of my threads seem to run: why?</a></h2>
<p>As soon as the main thread exits, all threads are killed.  Your main
thread is running too quickly, giving the threads no time to do any work.</p>
<p>A simple fix is to add a sleep to the end of the program
that's long enough for all the threads to finish:</p>
<pre class="literal-block">
import threading, time

def thread_task(name, n):
    for i in range(n): print name, i

for i in range(10):
    T = threading.Thread(target=thread_task, args=(str(i), i))
    T.start()

time.sleep(10) # &lt;----------------------------!
</pre>
<p>But now (on many platforms) the threads don't run in parallel,
but appear to run sequentially, one at a time!  The reason is
that the OS thread scheduler doesn't start a new thread until
the previous thread is blocked.</p>
<p>A simple fix is to add a tiny sleep to the start of the run
function:</p>
<pre class="literal-block">
def thread_task(name, n):
    time.sleep(0.001) # &lt;---------------------!
    for i in range(n): print name, i

for i in range(10):
    T = threading.Thread(target=thread_task, args=(str(i), i))
    T.start()

time.sleep(10)
</pre>
<p>Instead of trying to guess how long a <tt class="docutils literal">time.sleep()</tt> delay will be
enough, it's better to use some kind of semaphore mechanism.  One idea
is to use the <a class="reference external" href="http://www.python.org/doc/lib/module-Queue.html">Queue module</a> to create a queue
object, let each thread append a token to the queue when it finishes,
and let the main thread read as many tokens from the queue as there
are threads.</p>
</div>
<div class="section" id="how-do-i-parcel-out-work-among-a-bunch-of-worker-threads">
<h2><a class="toc-backref" href="#id17">3.3&nbsp;&nbsp;&nbsp;How do I parcel out work among a bunch of worker threads?</a></h2>
<p>Use the <a class="reference external" href="http://www.python.org/doc/lib/module-Queue.html">Queue module</a> to create a queue
containing a list of jobs.  The <tt class="docutils literal">Queue</tt> class maintains a list of
objects with <tt class="docutils literal">.put(obj)</tt> to add an item to the queue and <tt class="docutils literal">.get()</tt>
to return an item.  The class will take care of the locking necessary
to ensure that each job is handed out exactly once.</p>
<p>Here's a trivial example:</p>
<pre class="literal-block">
import threading, Queue, time

# The worker thread gets jobs off the queue.  When the queue is empty, it
# assumes there will be no more work and exits.
# (Realistically workers will run until terminated.)
def worker ():
    print 'Running worker'
    time.sleep(0.1)
    while True:
        try:
            arg = q.get(block=False)
        except Queue.Empty:
            print 'Worker', threading.currentThread(),
            print 'queue empty'
            break
        else:
            print 'Worker', threading.currentThread(),
            print 'running with argument', arg
            time.sleep(0.5)

# Create queue
q = Queue.Queue()

# Start a pool of 5 workers
for i in range(5):
    t = threading.Thread(target=worker, name='worker %i' % (i+1))
    t.start()

# Begin adding work to the queue
for i in range(50):
    q.put(i)

# Give threads time to run
print 'Main thread sleeping'
time.sleep(5)
</pre>
<p>When run, this will produce the following output:</p>
<blockquote>
Running worker
Running worker
Running worker
Running worker
Running worker
Main thread sleeping
Worker &lt;Thread(worker 1, started)&gt; running with argument 0
Worker &lt;Thread(worker 2, started)&gt; running with argument 1
Worker &lt;Thread(worker 3, started)&gt; running with argument 2
Worker &lt;Thread(worker 4, started)&gt; running with argument 3
Worker &lt;Thread(worker 5, started)&gt; running with argument 4
Worker &lt;Thread(worker 1, started)&gt; running with argument 5
...</blockquote>
<p>Consult the module's documentation for more details; the <tt class="docutils literal">Queue</tt>
class provides a featureful interface.</p>
</div>
<div class="section" id="what-kinds-of-global-value-mutation-are-thread-safe">
<h2><a class="toc-backref" href="#id18">3.4&nbsp;&nbsp;&nbsp;What kinds of global value mutation are thread-safe?</a></h2>
<p>A global interpreter lock (GIL) is used internally to ensure that only
one thread runs in the Python VM at a time.  In general, Python offers
to switch among threads only between bytecode instructions; how
frequently it switches can be set via <tt class="docutils literal">sys.setcheckinterval()</tt>.
Each bytecode instruction and therefore all the C implementation code
reached from each instruction is therefore atomic from the point of view of a Python program.</p>
<p>In theory, this means an exact accounting requires an exact
understanding of the PVM bytecode implementation.  In practice, it
means that operations on shared variables of builtin data types (ints,
lists, dicts, etc) that &quot;look atomic&quot; really are.</p>
<p>For example, the following operations are all atomic (L, L1, L2 are lists, D, D1, D2 are dicts, x, y
are objects, i, j are ints):</p>
<pre class="literal-block">
L.append(x)
L1.extend(L2)
x = L[i]
x = L.pop()
L1[i:j] = L2
L.sort()
x = y
x.field = y
D[x] = y
D1.update(D2)
D.keys()
</pre>
<p>These aren't:</p>
<pre class="literal-block">
i = i+1
L.append(L[-1])
L[i] = L[j]
D[x] = D[x] + 1
</pre>
<p>Operations that replace other objects may invoke those other
objects' <tt class="docutils literal">__del__</tt> method when their reference count reaches zero, and
that can affect things.  This is especially true for the mass updates
to dictionaries and lists.  When in doubt, use a mutex!</p>
</div>
<div class="section" id="can-t-we-get-rid-of-the-global-interpreter-lock">
<h2><a class="toc-backref" href="#id19">3.5&nbsp;&nbsp;&nbsp;Can't we get rid of the Global Interpreter Lock?</a></h2>
<p>The Global Interpreter Lock (GIL) is often seen as a hindrance to
Python's deployment on high-end multiprocessor server machines,
because a multi-threaded Python program effectively only uses one CPU,
due to the insistence that (almost) all Python code can only run while
the GIL is held.</p>
<p>Back in the days of Python 1.5, Greg Stein actually implemented a
comprehensive patch set (the &quot;free threading&quot; patches) that removed
the GIL and replaced it with fine-grained locking.  Unfortunately, even
on Windows (where locks are very efficient) this ran ordinary Python
code about twice as slow as the interpreter using the GIL.  On Linux
the performance loss was even worse because pthread locks aren't as
efficient.</p>
<p>Since then, the idea of getting rid of the GIL has occasionally come
up but nobody has found a way to deal with the expected slowdown, and
users who don't use threads would not be happy if their code ran at
half at the speed.  Greg's free threading patch set has not been kept
up-to-date for later Python versions.</p>
<p>This doesn't mean that you can't make good use of Python on multi-CPU
machines!  You just have to be creative with dividing the work up
between multiple <em>processes</em> rather than multiple <em>threads</em>.
Judicious use of C extensions will also help; if you use a C extension
to perform a time-consuming task, the extension can release the GIL
while the thread of execution is in the C code and allow other threads
to get some work done.</p>
<p>It has been suggested that the GIL should be a per-interpreter-state
lock rather than truly global; interpreters then wouldn't be able to
share objects.  Unfortunately, this isn't likely to happen either.  It
would be a tremendous amount of work, because many object
implementations currently have global state.  For example, small
integers and short strings are cached; these caches would have to be
moved to the interpreter state.  Other object types have their own
free list; these free lists would have to be moved to the interpreter
state.  And so on.</p>
<p>And I doubt that it can even be done in finite time, because the same
problem exists for 3rd party extensions.  It is likely that 3rd party
extensions are being written at a faster rate than you can convert
them to store all their global state in the interpreter state.</p>
<p>And finally, once you have multiple interpreters not sharing any
state, what have you gained over running each interpreter
in a separate process?</p>
</div>
</div>
<div class="section" id="input-and-output">
<h1><a class="toc-backref" href="#id20">4&nbsp;&nbsp;&nbsp;Input and Output</a></h1>
<div class="section" id="how-do-i-delete-a-file-and-other-file-questions">
<h2><a class="toc-backref" href="#id21">4.1&nbsp;&nbsp;&nbsp;How do I delete a file? (And other file questions...)</a></h2>
<p>Use <tt class="docutils literal">os.remove(filename)</tt> or <tt class="docutils literal">os.unlink(filename)</tt>; for
documentation, see <a class="reference external" href="http://www.python.org/doc/lib/module-posix.html">the POSIX module</a>.  The two
functions are identical; <tt class="docutils literal">unlink()</tt> is simply the name of the Unix
system call for this function.</p>
<p>To remove a directory, use <tt class="docutils literal">os.rmdir()</tt>; use <tt class="docutils literal">os.mkdir()</tt> to
create one.  <tt class="docutils literal">os.makedirs(path)</tt> will create any intermediate
directories in <tt class="docutils literal">path</tt> that don't exist. <tt class="docutils literal">os.removedirs(path)</tt> will
remove intermediate directories as long as they're empty; if you want
to delete an entire directory tree and its contents, use
<tt class="docutils literal">shutil.rmtree()</tt>.</p>
<p>To rename a file, use <tt class="docutils literal">os.rename(old_path, new_path)</tt>.</p>
<p>To truncate a file, open it using <tt class="docutils literal">f = open(filename, <span class="pre">&quot;r+&quot;)</span></tt>, and use
<tt class="docutils literal">f.truncate(offset)</tt>; offset defaults to the current seek position.
There's also <tt class="docutils literal">`os.ftruncate(fd, offset)</tt> for files opened with <tt class="docutils literal">os.open()</tt>,
where <tt class="docutils literal">fd</tt> is the file descriptor (a small integer).</p>
<p>The <tt class="docutils literal">shutil</tt> module also contains a number of functions to work on files
including <tt class="docutils literal">copyfile</tt>, <tt class="docutils literal">copytree</tt>, and <tt class="docutils literal">rmtree</tt>.</p>
</div>
<div class="section" id="how-do-i-copy-a-file">
<h2><a class="toc-backref" href="#id22">4.2&nbsp;&nbsp;&nbsp;How do I copy a file?</a></h2>
<p>The <tt class="docutils literal">shutil</tt> module contains a <tt class="docutils literal">copyfile()</tt> function.  Note that
on MacOS 9 it doesn't copy the resource fork and Finder info.</p>
</div>
<div class="section" id="how-do-i-read-or-write-binary-data">
<h2><a class="toc-backref" href="#id23">4.3&nbsp;&nbsp;&nbsp;How do I read (or write) binary data?</a></h2>
<p>or complex data formats, it's best to use <a class="reference external" href="http://www.python.org/doc/lib/module-struct.html">the struct module</a>.  It allows you
to take a string containing binary data (usually numbers) and convert
it to Python objects; and vice versa.</p>
<p>For example, the following code reads two 2-byte integers
and one 4-byte integer in big-endian format from a file:</p>
<pre class="literal-block">
import struct

f = open(filename, &quot;rb&quot;)  # Open in binary mode for portability
s = f.read(8)
x, y, z = struct.unpack(&quot;&gt;hhl&quot;, s)
</pre>
<p>The '&gt;' in the format string forces big-endian data; the letter
'h' reads one &quot;short integer&quot; (2 bytes), and 'l' reads one
&quot;long integer&quot; (4 bytes) from the string.</p>
<p>For data that is more regular (e.g. a homogeneous list of ints or
thefloats), you can also use <a class="reference external" href="http://www.python.org/doc/lib/module-array.html">the array module</a>.</p>
</div>
<div class="section" id="i-can-t-seem-to-use-os-read-on-a-pipe-created-with-os-popen-why">
<h2><a class="toc-backref" href="#id24">4.4&nbsp;&nbsp;&nbsp;I can't seem to use os.read() on a pipe created with os.popen(); why?</a></h2>
<p><tt class="docutils literal">os.read()</tt> is a low-level function which takes a file descriptor, a
small integer representing the opened file.  <tt class="docutils literal">os.popen()</tt> creates a
high-level file object, the same type returned by the builtin
<tt class="docutils literal">open()</tt> function.  Thus, to read n bytes from a pipe p created with
<tt class="docutils literal">os.popen()</tt>, you need to use <tt class="docutils literal">p.read(n)</tt>.</p>
</div>
<div class="section" id="how-do-i-run-a-subprocess-with-pipes-connected-to-both-input-and-output">
<h2><a class="toc-backref" href="#id25">4.5&nbsp;&nbsp;&nbsp;How do I run a subprocess with pipes connected to both input and output?</a></h2>
<p>Use <a class="reference external" href="http://www.python.org/doc/lib/module-popen2.html">the popen2 module</a>.  For example:</p>
<pre class="literal-block">
import popen2
fromchild, tochild = popen2.popen2(&quot;command&quot;)
tochild.write(&quot;input\n&quot;)
tochild.flush()
output = fromchild.readline()
</pre>
<p>Warning: in general it is unwise to do this because you can easily
cause a deadlock where your process is blocked waiting for output from
the child while the child is blocked waiting for input from you.  This
can be caused because the parent expects the child to output more text
than it does, or it can be caused by data being stuck in stdio buffers
due to lack of flushing.  The Python parent can of course explicitly
flush the data it sends to the child before it reads any output, but
if the child is a naive C program it may have been written to never
explicitly flush its output, even if it is interactive, since flushing
is normally automatic.</p>
<p>Note that a deadlock is also possible if you use <tt class="docutils literal">popen3</tt> to read
stdout and stderr. If one of the two is too large for the internal
buffer (increasing the buffer size does not help) and you <tt class="docutils literal">read()</tt>
the other one first, there is a deadlock, too.</p>
<p>Note on a bug in popen2: unless your program calls <tt class="docutils literal">wait()</tt>
or <tt class="docutils literal">waitpid()</tt>, finished child processes are never removed,
and eventually calls to popen2 will fail because of a limit on
the number of child processes.  Calling <tt class="docutils literal">os.waitpid</tt> with the
<tt class="docutils literal">os.WNOHANG</tt> option can prevent this; a good place to insert such
a call would be before calling <tt class="docutils literal">popen2</tt> again.</p>
<p>In many cases, all you really need is to run some data through a
command and get the result back.  Unless the amount of data is very
large, the easiest way to do this is to write it to a temporary file
and run the command with that temporary file as input.  The <a class="reference external" href="http://www.python.org/doc/lib/module-tempfile.html">standard
module tempfile</a>
exports a <tt class="docutils literal">mktemp()</tt> function  to generate unique temporary file names.</p>
<pre class="literal-block">
import tempfile
import os
class Popen3:
   &quot;&quot;&quot;
   This is a deadlock-safe version of popen that returns
   an object with errorlevel, out (a string) and err (a string).
   (capturestderr may not work under windows.)
   Example: print Popen3('grep spam','\n\nhere spam\n\n').out
   &quot;&quot;&quot;
   def __init__(self,command,input=None,capturestderr=None):
       outfile=tempfile.mktemp()
       command=&quot;( %s ) &gt; %s&quot; % (command,outfile)
       if input:
           infile=tempfile.mktemp()
           open(infile,&quot;w&quot;).write(input)
           command=command+&quot; &lt;&quot;+infile
       if capturestderr:
           errfile=tempfile.mktemp()
           command=command+&quot; 2&gt;&quot;+errfile
       self.errorlevel=os.system(command) &gt;&gt; 8
       self.out=open(outfile,&quot;r&quot;).read()
       os.remove(outfile)
       if input:
           os.remove(infile)
       if capturestderr:
           self.err=open(errfile,&quot;r&quot;).read()
           os.remove(errfile)
</pre>
<p>Note that many interactive programs (e.g. vi) don't work well with
pipes substituted for standard input and output.  You will have to use
pseudo ttys (&quot;ptys&quot;) instead of pipes. Or you can use a Python
interface to Don Libes' &quot;expect&quot; library.  A Python extension that
interfaces to expect is called &quot;expy&quot; and available from
<a class="reference external" href="http://expectpy.sourceforge.net">http://expectpy.sourceforge.net</a>.  A pure Python solution that works
like expect is ` pexpect &lt;<a class="reference external" href="http://pexpect.sourceforge.net">http://pexpect.sourceforge.net</a>&gt;`_.</p>
</div>
<div class="section" id="how-do-i-access-the-serial-rs232-port">
<h2><a class="toc-backref" href="#id26">4.6&nbsp;&nbsp;&nbsp;How do I access the serial (RS232) port?</a></h2>
<p>For Win32, POSIX (Linux, BSD, etc.), Jython:</p>
<blockquote>
<a class="reference external" href="http://pyserial.sourceforge.net">http://pyserial.sourceforge.net</a></blockquote>
<p>For Unix, see a Usenet post by Mitch Chapman:</p>
<blockquote>
<a class="reference external" href="http://groups.google.com/groups?selm=34A04430.CF9&#64;ohioee.com">http://groups.google.com/groups?selm=34A04430.CF9&#64;ohioee.com</a></blockquote>
</div>
<div class="section" id="why-doesn-t-closing-sys-stdout-stdin-stderr-really-close-it">
<h2><a class="toc-backref" href="#id27">4.7&nbsp;&nbsp;&nbsp;Why doesn't closing sys.stdout (stdin, stderr) really close it?</a></h2>
<p>Python file objects are a high-level layer of abstraction on top of C
streams, which in turn are a medium-level layer of abstraction on top
of (among other things) low-level C file descriptors.</p>
<p>For most file objects you create in Python via the builtin <tt class="docutils literal">file</tt>
constructor, <tt class="docutils literal">f.close()</tt> marks the Python file object as being closed
from Python's point of view, and also arranges to close the underlying
C stream.  This also happens automatically in f's destructor, when f
becomes garbage.</p>
<p>But stdin, stdout and stderr are treated specially by Python, because
of the special status also given to them by C.  Running
<tt class="docutils literal">sys.stdout.close()</tt> marks the Python-level file object as being
closed, but does <em>not</em> close the associated C stream.</p>
<p>To close the underlying C stream for one of these three, you should
first be sure that's what you really want to do (e.g., you may confuse
extension modules trying to do I/O).  If it is, use
os.close:</p>
<pre class="literal-block">
os.close(0)   # close C's stdin stream
os.close(1)   # close C's stdout stream
os.close(2)   # close C's stderr stream
</pre>
</div>
</div>
<div class="section" id="network-internet-programming">
<h1><a class="toc-backref" href="#id28">5&nbsp;&nbsp;&nbsp;Network/Internet Programming</a></h1>
<div class="section" id="what-www-tools-are-there-for-python">
<h2><a class="toc-backref" href="#id29">5.1&nbsp;&nbsp;&nbsp;What WWW tools are there for Python?</a></h2>
<p>See the chapters titled <a class="reference external" href="http://www.python.org/doc/lib/internet.html">&quot;Internet Protocols and Support&quot;</a> and <a class="reference external" href="http://www.python.org/doc/lib/netdata.html">&quot;Internet Data
Handling&quot;</a> in the
Library Reference Manual. Python has many modules that will help you
build server-side and client-side web systems.</p>
<p>A summary of available frameworks is maintained by Paul Boddie at
<a class="reference external" href="http://www.python.org/cgi-bin/moinmoin/WebProgramming">http://www.python.org/cgi-bin/moinmoin/WebProgramming</a> .</p>
<p>Cameron Laird maintains a useful set of pages about Python web technologies at
<a class="reference external" href="http://phaseit.net/claird/comp.lang.python/web_python.html">http://phaseit.net/claird/comp.lang.python/web_python.html</a>.</p>
<p>The <a class="reference external" href="http://www.python.org/topics/web/">Web Programming topic guide</a>
also points to many useful resources.</p>
</div>
<div class="section" id="how-can-i-mimic-cgi-form-submission-method-post">
<h2><a class="toc-backref" href="#id30">5.2&nbsp;&nbsp;&nbsp;How can I mimic CGI form submission (METHOD=POST)?</a></h2>
<p>I would like to retrieve web pages that are the result of POSTing a
form. Is there existing code that would let me do this easily?</p>
<p>Yes. Here's a simple example that uses httplib:</p>
<pre class="literal-block">
#!/usr/local/bin/python

import httplib, sys, time

### build the query string
qs = &quot;First=Josephine&amp;MI=Q&amp;Last=Public&quot;

### connect and send the server a path
httpobj = httplib.HTTP('www.some-server.out-there', 80)
httpobj.putrequest('POST', '/cgi-bin/some-cgi-script')
### now generate the rest of the HTTP headers...
httpobj.putheader('Accept', '*/*')
httpobj.putheader('Connection', 'Keep-Alive')
httpobj.putheader('Content-type', 'application/x-www-form-urlencoded')
httpobj.putheader('Content-length', '%d' % len(qs))
httpobj.endheaders()
httpobj.send(qs)
### find out what the server said in response...
reply, msg, hdrs = httpobj.getreply()
if reply != 200:
    sys.stdout.write(httpobj.getfile().read())
</pre>
<p>Note that in general for URL-encoded POST operations,  query
strings must be quoted by using <tt class="docutils literal">urllib.quote()</tt>.  For example to send name=&quot;Guy
Steele, Jr.&quot;:</p>
<pre class="literal-block">
&gt;&gt;&gt; from urllib import quote
&gt;&gt;&gt; x = quote(&quot;Guy Steele, Jr.&quot;)
&gt;&gt;&gt; x
'Guy%20Steele,%20Jr.'
&gt;&gt;&gt; query_string = &quot;name=&quot;+x
&gt;&gt;&gt; query_string
'name=Guy%20Steele,%20Jr.'
</pre>
</div>
<div class="section" id="what-module-should-i-use-to-help-with-generating-html">
<h2><a class="toc-backref" href="#id31">5.3&nbsp;&nbsp;&nbsp;What module should I use to help with generating HTML?</a></h2>
<p>There are many different modules available:</p>
<ul class="simple">
<li>HTMLgen is a class library of objects corresponding to all the HTML
3.2 markup tags. It's used when you are writing in Python and wish
to synthesize HTML pages for generating a web or for CGI forms, etc.</li>
<li>DocumentTemplate and Zope Page Templates are two different systems that are
part of Zope.</li>
<li>Quixote's PTL uses Python syntax to assemble strings of text.</li>
</ul>
<p>Consult the <a class="reference external" href="http://www.python.org/topics/web/">Web Programming topic guide</a> for more links.</p>
</div>
<div class="section" id="how-do-i-send-mail-from-a-python-script">
<h2><a class="toc-backref" href="#id32">5.4&nbsp;&nbsp;&nbsp;How do I send mail from a Python script?</a></h2>
<p>Use <a class="reference external" href="http://www.python.org/doc/lib/module-smtplib.html">the standard library module smtplib</a>.</p>
<p>Here's a very simple interactive mail sender that uses it.  This
method will work on any host that supports an SMTP listener.</p>
<pre class="literal-block">
import sys, smtplib

fromaddr = raw_input(&quot;From: &quot;)
toaddrs  = raw_input(&quot;To: &quot;).split(',')
print &quot;Enter message, end with ^D:&quot;
msg = ''
while 1:
    line = sys.stdin.readline()
    if not line:
        break
    msg = msg + line

# The actual mail send
server = smtplib.SMTP('localhost')
server.sendmail(fromaddr, toaddrs, msg)
server.quit()
</pre>
<p>A Unix-only alternative uses sendmail.  The location of the
sendmail program varies between systems; sometimes it is
<tt class="docutils literal">/usr/lib/sendmail</tt>, sometime <tt class="docutils literal">/usr/sbin/sendmail</tt>.  The sendmail
manual page will help you out.  Here's some sample code:</p>
<pre class="literal-block">
SENDMAIL = &quot;/usr/sbin/sendmail&quot; # sendmail location
import os
p = os.popen(&quot;%s -t -i&quot; % SENDMAIL, &quot;w&quot;)
p.write(&quot;To: receiver&#64;example.com\n&quot;)
p.write(&quot;Subject: test\n&quot;)
p.write(&quot;\n&quot;) # blank line separating headers from body
p.write(&quot;Some text\n&quot;)
p.write(&quot;some more text\n&quot;)
sts = p.close()
if sts != 0:
    print &quot;Sendmail exit status&quot;, sts
</pre>
</div>
<div class="section" id="how-do-i-avoid-blocking-in-the-connect-method-of-a-socket">
<h2><a class="toc-backref" href="#id33">5.5&nbsp;&nbsp;&nbsp;How do I avoid blocking in the connect() method of a socket?</a></h2>
<p>The select module is commonly used to help with asynchronous
I/O on sockets.</p>
<p>To prevent the TCP connect from blocking, you can set the socket to
non-blocking mode.  Then when you do the <tt class="docutils literal">connect()</tt>, you will
either connect immediately (unlikely) or get an exception that
contains the error number as <tt class="docutils literal">.errno</tt>.  <tt class="docutils literal">errno.EINPROGRESS</tt>
indicates that the connection is in progress, but hasn't finished yet.
Different OSes will return different values, so you're going to have
to check what's returned on your system.</p>
<p>You can use the <tt class="docutils literal">connect_ex()</tt> method to avoid creating an
exception.  It will just return the errno value.  To poll, you can
call <tt class="docutils literal">connect_ex()</tt> again later -- 0 or <tt class="docutils literal">errno.EISCONN</tt> indicate
that you're connected -- or you can pass this socket to select to
check if it's writeable.</p>
</div>
</div>
<div class="section" id="databases">
<h1><a class="toc-backref" href="#id34">6&nbsp;&nbsp;&nbsp;Databases</a></h1>
<div class="section" id="are-there-any-interfaces-to-database-packages-in-python">
<h2><a class="toc-backref" href="#id35">6.1&nbsp;&nbsp;&nbsp;Are there any interfaces to database packages in Python?</a></h2>
<p>Yes.</p>
<p>Python 2.3 includes the <tt class="docutils literal">bsddb</tt> package which provides an interface
to the <a class="reference external" href="http://www.python.org/doc/lib/module-bsddb.html">BerkeleyDB</a> library.
Interfaces to disk-based hashes such as <a class="reference external" href="http://www.python.org/doc/lib/module-dbm.html">DBM</a> and <a class="reference external" href="http://www.python.org/doc/lib/module-gdbm.html">GDBM</a> are also included
with standard Python.</p>
<p>Support for most relational databases is available.  See the <a class="reference external" href="http://www.python.org/topics/database">Database
Topic Guide</a> for details.</p>
</div>
<div class="section" id="how-do-you-implement-persistent-objects-in-python">
<h2><a class="toc-backref" href="#id36">6.2&nbsp;&nbsp;&nbsp;How do you implement persistent objects in Python?</a></h2>
<p>The <a class="reference external" href="http://www.python.org/doc/lib/module-pickle.html">pickle library module</a> solves this in a
very general way (though you still can't store things like open files,
sockets or windows), and the <a class="reference external" href="http://www.python.org/doc/lib/module-shelve.html">shelve library module</a> uses pickle and
(g)dbm to create persistent mappings containing arbitrary Python
objects.  For better performance, you can use
<a class="reference external" href="http://www.python.org/doc/lib/module-cPickle.html">the cPickle module</a>.</p>
<p>A more awkward way of doing things is to use pickle's little sister,
marshal.  <a class="reference external" href="http://www.python.org/doc/lib/module-marshal.html">The marshal module</a> provides very
fast ways to store noncircular basic Python types to files and
strings, and back again.  Although marshal does not do fancy things
like store instances or handle shared references properly, it does run
extremely fast.  For example loading a half megabyte of data may take
less than a third of a second.  This often beats doing something more
complex and general such as using gdbm with pickle/shelve.</p>
</div>
<div class="section" id="why-is-cpickle-so-slow">
<h2><a class="toc-backref" href="#id37">6.3&nbsp;&nbsp;&nbsp;Why is cPickle so slow?</a></h2>
<p>The default format used by the pickle module is a slow one that
results in readable pickles.  Making it the default, but it would
break backward compatibility:</p>
<pre class="literal-block">
largeString = 'z' * (100 * 1024)
myPickle = cPickle.dumps(largeString, protocol=1)
</pre>
</div>
<div class="section" id="if-my-program-crashes-with-a-bsddb-or-anydbm-database-open-it-gets-corrupted-how-come">
<h2><a class="toc-backref" href="#id38">6.4&nbsp;&nbsp;&nbsp;If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</a></h2>
<p>Databases opened for write access with the bsddb module (and often by
the anydbm module, since it will preferentially use bsddb) must
explicitly be closed using the <tt class="docutils literal">.close()</tt> method of the database.  The
underlying library caches database contents which need to be
converted to on-disk form and written.</p>
<p>If you have initialized a new bsddb database but not written anything to
it before the program crashes, you will often wind up with a zero-length
file and encounter an exception the next time the file is opened.</p>
</div>
<div class="section" id="i-tried-to-open-berkeley-db-file-but-bsddb-produces-bsddb-error-22-invalid-argument-help-how-can-i-restore-my-data">
<h2><a class="toc-backref" href="#id39">6.5&nbsp;&nbsp;&nbsp;I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data?</a></h2>
<p>Don't panic! Your data is probably intact. The most frequent cause
for the error is that you tried to open an earlier Berkeley DB file
with a later version of the Berkeley DB library.</p>
<p>Many Linux systems now have all three versions of Berkeley DB
available.  If you are migrating from version 1 to a newer version use
db_dump185 to dump a plain text version of the database.
If you are migrating from version 2 to version 3 use db2_dump to create
a plain text version of the database.  In either case, use db_load to
create a new native database for the latest version installed on your
computer.  If you have version 3 of Berkeley DB installed, you should
be able to use db2_load to create a native version 2 database.</p>
<p>You should move away from Berkeley DB version 1 files because
the hash file code contains known bugs that can corrupt your data.</p>
</div>
</div>
<div class="section" id="mathematics-and-numerics">
<h1><a class="toc-backref" href="#id40">7&nbsp;&nbsp;&nbsp;Mathematics and Numerics</a></h1>
<div class="section" id="how-do-i-generate-random-numbers-in-python">
<h2><a class="toc-backref" href="#id41">7.1&nbsp;&nbsp;&nbsp;How do I generate random numbers in Python?</a></h2>
<p>The <a class="reference external" href="http://www.python.org/doc/lib/module-random.html">standard module random</a> implements a random number
generator.  Usage is simple:</p>
<pre class="literal-block">
import random
random.random()
</pre>
<p>This returns a random floating point number in the range [0, 1).</p>
<p>There are also many other specialized generators in this module, such
as:</p>
<ul class="simple">
<li><tt class="docutils literal">randrange(a, b)</tt> chooses an integer in the range [a, b).</li>
<li><tt class="docutils literal">uniform(a, b)</tt> chooses a floating point number in the range [a, b).</li>
<li><tt class="docutils literal">normalvariate(mean, sdev)</tt> samples the normal (Gaussian) distribution.</li>
</ul>
<p>Some higher-level functions operate on sequences directly, such as:</p>
<ul class="simple">
<li><tt class="docutils literal">choice(S)</tt> chooses random element from a given sequence</li>
<li><tt class="docutils literal">shuffle(L)</tt> shuffles a list in-place, i.e. permutes it randomly</li>
</ul>
<p>There's also a <tt class="docutils literal">Random</tt> class you can instantiate
to create independent multiple random number generators.</p>
</div>
</div>
</div>
</body>
</html>