forked from UWPCE-PythonCert/IntroToPython-2014
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathsession04.html
More file actions
1184 lines (1073 loc) · 82.9 KB
/
session04.html
File metadata and controls
1184 lines (1073 loc) · 82.9 KB
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
<!DOCTYPE html>
<!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]-->
<!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]-->
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Session Four: Dictionaries, Sets, Exceptions, and Files — Introduction To Python 1.3 documentation</title>
<link href='https://fonts.googleapis.com/css?family=Lato:400,700|Roboto+Slab:400,700|Inconsolata:400,700' rel='stylesheet' type='text/css'>
<link rel="stylesheet" href="_static/css/theme.css" type="text/css" />
<link rel="top" title="Introduction To Python 1.3 documentation" href="index.html"/>
<link rel="next" title="Session Five: Advanced Argument passing, List and Dict Comprehensions, Lambda and Functional programming" href="session05.html"/>
<link rel="prev" title="Session Three: Sequences, Iteration and String Formatting" href="session03.html"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/modernizr/2.6.2/modernizr.min.js"></script>
</head>
<body class="wy-body-for-nav" role="document">
<div class="wy-grid-for-nav">
<nav data-toggle="wy-nav-shift" class="wy-nav-side">
<div class="wy-side-nav-search">
<a href="index.html" class="fa fa-home"> Introduction To Python</a>
<div role="search">
<form id ="rtd-search-form" class="wy-form" action="search.html" method="get">
<input type="text" name="q" placeholder="Search docs" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
</div>
</div>
<div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="session01.html">Session One: Introductions</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session01.html#introductions">Introductions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#introduction-to-this-class">Introduction to This Class</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#introduction-to-your-environment">Introduction to Your Environment</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#setting-up-your-environment">Setting Up Your Environment</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#introduction-to-ipython">Introduction to iPython</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#basic-python-syntax">Basic Python Syntax</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#id2">Homework</a></li>
<li class="toctree-l2"><a class="reference internal" href="session01.html#next-class">Next Class</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session02.html">Session Two: Functions, Booleans and Modules</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session02.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#git-work">Git Work</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#quick-intro-to-basics">Quick Intro to Basics</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#functions">Functions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#in-class-lab">In-Class Lab:</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#boolean-expressions">Boolean Expressions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#id1">In-Class Lab:</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#code-structure-modules-and-namespaces">Code Structure, Modules, and Namespaces</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#id4">In-Class Lab</a></li>
<li class="toctree-l2"><a class="reference internal" href="session02.html#homework">Homework</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session03.html">Session Three: Sequences, Iteration and String Formatting</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session03.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#sequences">Sequences</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#lists-tuples">Lists, Tuples...</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#mutability">Mutability</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#mutable-sequence-methods">Mutable Sequence Methods</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#id1">Iteration</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#string-features">String Features</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#one-last-trick">One Last Trick</a></li>
<li class="toctree-l2"><a class="reference internal" href="session03.html#homework">Homework</a></li>
</ul>
</li>
<li class="toctree-l1 current"><a class="current reference internal" href="">Session Four: Dictionaries, Sets, Exceptions, and Files</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="#a-little-warm-up">A little warm up</a></li>
<li class="toctree-l2"><a class="reference internal" href="#dictionaries-and-sets">Dictionaries and Sets</a></li>
<li class="toctree-l2"><a class="reference internal" href="#exceptions">Exceptions</a></li>
<li class="toctree-l2"><a class="reference internal" href="#file-reading-and-writing">File Reading and Writing</a></li>
<li class="toctree-l2"><a class="reference internal" href="#paths-and-directories">Paths and Directories</a></li>
<li class="toctree-l2"><a class="reference internal" href="#homework">Homework</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session05.html">Session Five: Advanced Argument passing, List and Dict Comprehensions, Lambda and Functional programming</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session05.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#advanced-argument-passing">Advanced Argument Passing</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#a-bit-more-on-mutability-and-copies">A bit more on mutability (and copies)</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#list-and-dict-comprehensions">List and Dict Comprehensions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#anonymous-functions">Anonymous functions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#functional-programming">Functional Programming</a></li>
<li class="toctree-l2"><a class="reference internal" href="session05.html#homework">Homework</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session06.html">Session Six: Object oriented programming: Classes, instances, attributes, and subclassing</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session06.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session06.html#object-oriented-programming">Object Oriented Programming</a></li>
<li class="toctree-l2"><a class="reference internal" href="session06.html#python-classes">Python Classes</a></li>
<li class="toctree-l2"><a class="reference internal" href="session06.html#subclassing-inheritance">Subclassing/Inheritance</a></li>
<li class="toctree-l2"><a class="reference internal" href="session06.html#more-on-subclassing">More on Subclassing</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session07.html">Session Seven: Testing, More OO</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session07.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#testing">Testing</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#more-on-subclassing">More on Subclassing</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#properties">Properties</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#static-and-class-methods">Static and Class Methods</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#special-methods">Special Methods</a></li>
<li class="toctree-l2"><a class="reference internal" href="session07.html#homework">Homework</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="session08.html">Session Eight: Generators, Iterators, Decorators, and Context Managers</a><ul>
<li class="toctree-l2"><a class="reference internal" href="session08.html#review-questions">Review/Questions</a></li>
<li class="toctree-l2"><a class="reference internal" href="session08.html#decorators">Decorators</a></li>
<li class="toctree-l2"><a class="reference internal" href="session08.html#iterators-and-generators">Iterators and Generators</a></li>
<li class="toctree-l2"><a class="reference internal" href="session08.html#context-managers">Context Managers</a></li>
<li class="toctree-l2"><a class="reference internal" href="session08.html#homework">Homework</a></li>
</ul>
</li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="homework/index.html">Homework Materials</a><ul>
<li class="toctree-l2"><a class="reference internal" href="homework/kata_fourteen.html">Kata Fourteen: Tom Swift Under Milk Wood</a></li>
<li class="toctree-l2"><a class="reference internal" href="homework/html_builder.html">HTML Renderer Homework Assignment</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="supplements/index.html">Supplemental Materials</a><ul>
<li class="toctree-l2"><a class="reference internal" href="supplements/python_learning_resources.html">Useful Python Learning Resources</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/python_for_mac.html">Setting up your Mac for Python and this class</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/python_for_windows.html">Setting up Windows for Python and this class</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/python_for_linux.html">Setting up Linux for Python and this class</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/virtualenv.html">Working with Virtualenv</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/sublime_as_ide.html">Turning Sublime Text Into a Lightweight Python IDE</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/shell.html">Shell Customizations for Python Development</a></li>
<li class="toctree-l2"><a class="reference internal" href="supplements/unicode.html">Unicode in Python 2</a></li>
</ul>
</li>
</ul>
</div>
</nav>
<section data-toggle="wy-nav-shift" class="wy-nav-content-wrap">
<nav class="wy-nav-top" role="navigation" aria-label="top navigation">
<i data-toggle="wy-nav-top" class="fa fa-bars"></i>
<a href="index.html">Introduction To Python</a>
</nav>
<div class="wy-nav-content">
<div class="rst-content">
<div role="navigation" aria-label="breadcrumbs navigation">
<ul class="wy-breadcrumbs">
<li><a href="index.html">Docs</a> »</li>
<li>Session Four: Dictionaries, Sets, Exceptions, and Files</li>
<li class="wy-breadcrumbs-aside">
<a href="_sources/session04.txt" rel="nofollow"> View page source</a>
</li>
</ul>
<hr/>
</div>
<div role="main">
<div class="section" id="session-four-dictionaries-sets-exceptions-and-files">
<h1>Session Four: Dictionaries, Sets, Exceptions, and Files<a class="headerlink" href="#session-four-dictionaries-sets-exceptions-and-files" title="Permalink to this headline">¶</a></h1>
<div class="section" id="review-questions">
<h2>Review/Questions<a class="headerlink" href="#review-questions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="review-of-previous-classes">
<h3>Review of Previous Classes<a class="headerlink" href="#review-of-previous-classes" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><ul class="simple">
<li>Sequences<ul>
<li>Slicing</li>
<li>Lists</li>
<li>Tuples</li>
<li>tuple vs lists - which to use?</li>
</ul>
</li>
<li>interating<ul>
<li>for</li>
<li>while<ul>
<li>break and continue</li>
</ul>
</li>
<li>else with loops</li>
</ul>
</li>
</ul>
</div></blockquote>
<p>Any questions?</p>
<p>A couple other nifty utilties with for loops:</p>
<p><strong>tuple unpacking:</strong></p>
<p>remember this?</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span>
</pre></div>
</div>
<p>You can do that in a for loop, also:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [4]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">),</span> <span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">)]</span>
<span class="gp">In [5]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="n">l</span><span class="p">:</span>
<span class="go"> print "i:%i, j:%i"%(i, j)</span>
<span class="go">i:1, j:2</span>
<span class="go">i:3, j:4</span>
<span class="go">i:5, j:6</span>
</pre></div>
</div>
</div>
<div class="section" id="looping-through-two-loops-at-once">
<h3>Looping through two loops at once:<a class="headerlink" href="#looping-through-two-loops-at-once" title="Permalink to this headline">¶</a></h3>
<p><strong>zip:</strong></p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [10]: </span><span class="n">l1</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="gp">In [11]: </span><span class="n">l2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="gp">In [12]: </span><span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">l1</span><span class="p">,</span> <span class="n">l2</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">print</span> <span class="s">"i:</span><span class="si">%i</span><span class="s">, j:</span><span class="si">%i</span><span class="s">"</span><span class="o">%</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">)</span>
<span class="gp"> ....:</span>
<span class="go">i:1, j:3</span>
<span class="go">i:2, j:4</span>
<span class="go">i:3, j:5</span>
</pre></div>
</div>
</div>
<div class="section" id="homework-comments">
<h3>Homework comments<a class="headerlink" href="#homework-comments" title="Permalink to this headline">¶</a></h3>
<p>Building up a long string.</p>
<p>The obvious thing to do is something like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">msg</span> <span class="o">=</span> <span class="s">u""</span>
<span class="k">for</span> <span class="n">piece</span> <span class="ow">in</span> <span class="n">list_of_stuff</span><span class="p">:</span>
<span class="n">msg</span> <span class="o">+=</span> <span class="n">piece</span>
</pre></div>
</div>
<p>But: strings are immutable – python needs to create a new string each time you add a piece – not efficient:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">msg</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">piece</span> <span class="ow">in</span> <span class="n">list_of_stuff</span><span class="p">:</span>
<span class="n">msg</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">piece</span><span class="p">)</span>
<span class="s">u" "</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span>
</pre></div>
</div>
<p>appending to lists is efficient – and so is the join() method of strings.</p>
<p>What is <tt class="docutils literal"><span class="pre">assert</span></tt> for?</p>
<p>Testing – NOT for issues expected to happen operationally:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">assert</span> <span class="n">m</span> <span class="o">>=</span> <span class="mi">0</span>
</pre></div>
</div>
<p>in operational code should be:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="n">m</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ValueError</span>
</pre></div>
</div>
<p>I’ll cover Exceptions later this class...</p>
<p>(Asserts get ignored if optimization is turned on!)</p>
</div>
</div>
<div class="section" id="a-little-warm-up">
<h2>A little warm up<a class="headerlink" href="#a-little-warm-up" title="Permalink to this headline">¶</a></h2>
<div class="section" id="fun-with-strings">
<h3>Fun with strings<a class="headerlink" href="#fun-with-strings" title="Permalink to this headline">¶</a></h3>
<ul>
<li><p class="first">Rewrite: <tt class="docutils literal"><span class="pre">the</span> <span class="pre">first</span> <span class="pre">3</span> <span class="pre">numbers</span> <span class="pre">are:</span> <span class="pre">%i,</span> <span class="pre">%i,</span> <span class="pre">%i"%(1,2,3)</span></tt></p>
<blockquote>
<div><ul class="simple">
<li>for an arbitrary number of numbers...</li>
</ul>
</div></blockquote>
</li>
<li><p class="first">Write a format string that will take:</p>
<blockquote>
<div><ul class="simple">
<li><tt class="docutils literal"><span class="pre">(</span> <span class="pre">2,</span> <span class="pre">123.4567,</span> <span class="pre">10000)</span></tt></li>
<li>and produce:</li>
<li>`` “file_002 : 123.46, 1e+04” ``</li>
</ul>
</div></blockquote>
</li>
</ul>
</div>
</div>
<div class="section" id="dictionaries-and-sets">
<h2>Dictionaries and Sets<a class="headerlink" href="#dictionaries-and-sets" title="Permalink to this headline">¶</a></h2>
<div class="section" id="dictionary">
<h3>Dictionary<a class="headerlink" href="#dictionary" title="Permalink to this headline">¶</a></h3>
<p>Python calls it a <tt class="docutils literal"><span class="pre">dict</span></tt></p>
<p>Other languages call it:</p>
<blockquote>
<div><ul class="simple">
<li>dictionary</li>
<li>associative array</li>
<li>map</li>
<li>hash table</li>
<li>hash</li>
<li>key-value pair</li>
</ul>
</div></blockquote>
</div>
<div class="section" id="dictionary-constructors">
<h3>Dictionary Constructors<a class="headerlink" href="#dictionary-constructors" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="p">{</span><span class="s">'key1'</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s">'key2'</span><span class="p">:</span> <span class="mi">5</span><span class="p">}</span>
<span class="go">{'key1': 3, 'key2': 5}</span>
<span class="gp">>>> </span><span class="nb">dict</span><span class="p">([(</span><span class="s">'key1'</span><span class="p">,</span> <span class="mi">3</span><span class="p">),(</span><span class="s">'key2'</span><span class="p">,</span> <span class="mi">5</span><span class="p">)])</span>
<span class="go">{'key1': 3, 'key2': 5}</span>
<span class="gp">>>> </span><span class="nb">dict</span><span class="p">(</span><span class="n">key1</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">key2</span><span class="o">=</span> <span class="mi">5</span><span class="p">)</span>
<span class="go">{'key1': 3, 'key2': 5}</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s">'key1'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">3</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s">'key2'</span><span class="p">]</span> <span class="o">=</span> <span class="mi">5</span>
<span class="gp">>>> </span><span class="n">d</span>
<span class="go">{'key1': 3, 'key2': 5}</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionary-indexing">
<h3>Dictionary Indexing<a class="headerlink" href="#dictionary-indexing" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s">'name'</span><span class="p">:</span> <span class="s">'Brian'</span><span class="p">,</span> <span class="s">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s">'score'</span><span class="p">]</span>
<span class="go">42</span>
<span class="gp">>>> </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="mi">1</span><span class="p">:</span> <span class="s">'one'</span><span class="p">,</span> <span class="mi">0</span><span class="p">:</span> <span class="s">'zero'</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="go">'zero'</span>
<span class="gp">>>> </span><span class="n">d</span><span class="p">[</span><span class="s">'non-existing key'</span><span class="p">]</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">KeyError</span>: <span class="n">'non-existing key'</span>
</pre></div>
</div>
<p>Keys can be any immutable:</p>
<blockquote>
<div><ul class="simple">
<li>number</li>
<li>string</li>
<li>tuple</li>
</ul>
</div></blockquote>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [325]: </span><span class="n">d</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="s">'string'</span>
<span class="gp">In [326]: </span><span class="n">d</span><span class="p">[</span><span class="mf">3.14</span><span class="p">]</span> <span class="o">=</span> <span class="s">'pi'</span>
<span class="gp">In [327]: </span><span class="n">d</span><span class="p">[</span><span class="s">'pi'</span><span class="p">]</span> <span class="o">=</span> <span class="mf">3.14</span>
<span class="gp">In [328]: </span><span class="n">d</span><span class="p">[</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span> <span class="p">]</span> <span class="o">=</span> <span class="s">'a tuple key'</span>
<span class="gp">In [329]: </span><span class="n">d</span><span class="p">[</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span> <span class="p">]</span> <span class="o">=</span> <span class="s">'a list key'</span>
<span class="go"> TypeError: unhashable type: 'list'</span>
</pre></div>
</div>
<p>Actually – any “hashable” type.</p>
<p>Hash functions convert arbitrarily large data to a small proxy (usually int)</p>
<p>Always return the same proxy for the same input</p>
<p>MD5, SHA, etc</p>
<p>Dictionaries hash the key to an integer proxy and use it to find the key and value.</p>
<p>Key lookup is efficient because the hash function leads directly to a bucket with very few keys (often just one)</p>
<p>What would happen if the proxy changed after storing a key?</p>
<p>Hashability requires immutability</p>
<p>Key lookup is very efficient</p>
<p>Same average time regardless of size</p>
<p>Note: Python name look-ups are implemented with dict – it’s highly optimized</p>
<p>Key to value:</p>
<blockquote>
<div><ul class="simple">
<li>lookup is one way</li>
</ul>
</div></blockquote>
<p>Value to key:</p>
<blockquote>
<div><ul class="simple">
<li>requires visiting the whole dict</li>
</ul>
</div></blockquote>
<p>If you need to check dict values often, create another dict or set</p>
<p>(up to you to keep them in sync)</p>
</div>
<div class="section" id="dictionary-ordering-not">
<h3>Dictionary Ordering (not)<a class="headerlink" href="#dictionary-ordering-not" title="Permalink to this headline">¶</a></h3>
<p>Dictionaries have no defined order</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [352]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s">'one'</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s">'two'</span><span class="p">:</span><span class="mi">2</span><span class="p">,</span> <span class="s">'three'</span><span class="p">:</span><span class="mi">3</span><span class="p">}</span>
<span class="gp">In [353]: </span><span class="n">d</span>
<span class="gh">Out[353]: </span><span class="go">{'one': 1, 'three': 3, 'two': 2}</span>
<span class="gp">In [354]: </span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="gh">Out[354]: </span><span class="go">['three', 'two', 'one']</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionary-iterating">
<h3>Dictionary Iterating<a class="headerlink" href="#dictionary-iterating" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">for</span></tt> iterates over the keys</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [15]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s">'name'</span><span class="p">:</span> <span class="s">'Brian'</span><span class="p">,</span> <span class="s">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [16]: </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">d</span><span class="p">:</span>
<span class="go"> print x</span>
<span class="gp"> ....:</span>
<span class="go">score</span>
<span class="go">name</span>
</pre></div>
</div>
<p>(note the different order...)</p>
</div>
<div class="section" id="dict-keys-and-values">
<h3>dict keys and values<a class="headerlink" href="#dict-keys-and-values" title="Permalink to this headline">¶</a></h3>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [20]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s">'name'</span><span class="p">:</span> <span class="s">'Brian'</span><span class="p">,</span> <span class="s">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [21]: </span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
<span class="gh">Out[21]: </span><span class="go">['score', 'name']</span>
<span class="gp">In [22]: </span><span class="n">d</span><span class="o">.</span><span class="n">values</span><span class="p">()</span>
<span class="gh">Out[22]: </span><span class="go">[42, 'Brian']</span>
<span class="gp">In [23]: </span><span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">()</span>
<span class="gh">Out[23]: </span><span class="go">[('score', 42), ('name', 'Brian')]</span>
</pre></div>
</div>
</div>
<div class="section" id="id1">
<h3>dict keys and values<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
<p>Iterating on everything</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [26]: </span><span class="n">d</span> <span class="o">=</span> <span class="p">{</span><span class="s">'name'</span><span class="p">:</span> <span class="s">'Brian'</span><span class="p">,</span> <span class="s">'score'</span><span class="p">:</span> <span class="mi">42</span><span class="p">}</span>
<span class="gp">In [27]: </span><span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="go"> print "%s: %s" % (k,v)</span>
<span class="gp"> ....:</span>
<span class="go">score: 42</span>
<span class="go">name: Brian</span>
</pre></div>
</div>
</div>
<div class="section" id="dictionary-performance">
<h3>Dictionary Performance<a class="headerlink" href="#dictionary-performance" title="Permalink to this headline">¶</a></h3>
<blockquote>
<div><blockquote>
<div><ul class="simple">
<li>indexing is fast and constant time: O(1)</li>
<li><tt class="docutils literal"><span class="pre">x</span> <span class="pre">in</span> <span class="pre">s</span></tt> constant time: O(1)</li>
<li>visiting all is proportional to n: O(n)</li>
<li>inserting is constant time: O(1)</li>
<li>deleting is constant time: O(1)</li>
</ul>
</div></blockquote>
<p><a class="reference external" href="http://wiki.python.org/moin/TimeComplexity">http://wiki.python.org/moin/TimeComplexity</a></p>
</div></blockquote>
</div>
<div class="section" id="other-dict-operations">
<h3>Other dict operations:<a class="headerlink" href="#other-dict-operations" title="Permalink to this headline">¶</a></h3>
<p>See them all here:</p>
<p><a class="reference external" href="https://docs.python.org/2/library/stdtypes.html#mapping-types-dict">https://docs.python.org/2/library/stdtypes.html#mapping-types-dict</a></p>
<p>Is it in there?</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [5]: </span><span class="n">d</span>
<span class="gh">Out[5]: </span><span class="go">{'that': 7, 'this': 5}</span>
<span class="gp">In [6]: </span><span class="s">'that'</span> <span class="ow">in</span> <span class="n">d</span>
<span class="gh">Out[6]: </span><span class="go">True</span>
<span class="gp">In [7]: </span><span class="s">'this'</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">d</span>
<span class="gh">Out[7]: </span><span class="go">False</span>
</pre></div>
</div>
<p>Containment is on the keys.</p>
<p>Getting something: (like indexing)</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [9]: </span><span class="n">d</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'this'</span><span class="p">)</span>
<span class="gh">Out[9]: </span><span class="go">5</span>
</pre></div>
</div>
<p>But you can specify a default</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [11]: </span><span class="n">d</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">u'something'</span><span class="p">,</span> <span class="s">u'a default'</span><span class="p">)</span>
<span class="gh">Out[11]: </span><span class="go">u'a default'</span>
</pre></div>
</div>
<p>Never raises an Exception (default default is None)</p>
<p>iterating</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [13]: </span><span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">iteritems</span><span class="p">():</span>
<span class="gp"> ....: </span> <span class="k">print</span> <span class="n">item</span>
<span class="gp"> ....:</span>
<span class="go">('this', 5)</span>
<span class="go">('that', 7)</span>
<span class="gp">In [15]: </span><span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">iterkeys</span><span class="p">():</span>
<span class="go"> print key</span>
<span class="gp"> ....:</span>
<span class="go">this</span>
<span class="go">that</span>
<span class="gp">In [16]: </span><span class="k">for</span> <span class="n">val</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">itervalues</span><span class="p">():</span>
<span class="go"> print val</span>
<span class="gp"> ....:</span>
<span class="go">5</span>
<span class="go">7</span>
</pre></div>
</div>
<p>the <tt class="docutils literal"><span class="pre">iter*</span></tt> methods don’t actually create the lists.</p>
<p>“Popping”: getting the value while removing it</p>
<p>pop out a particular key</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [19]: </span><span class="n">d</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s">'this'</span><span class="p">)</span>
<span class="gh">Out[19]: </span><span class="go">5</span>
<span class="gp">In [20]: </span><span class="n">d</span>
<span class="gh">Out[20]: </span><span class="go">{'that': 7}</span>
</pre></div>
</div>
<p>pop out an arbitrary key, value pair</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [23]: </span><span class="n">d</span><span class="o">.</span><span class="n">popitem</span><span class="p">()</span>
<span class="gh">Out[23]: </span><span class="go">('that', 7)</span>
<span class="gp">In [24]: </span><span class="n">d</span>
<span class="gh">Out[24]: </span><span class="go">{}</span>
</pre></div>
</div>
<p>This one is handy:</p>
<p><tt class="docutils literal"><span class="pre">setdefault(key[,</span> <span class="pre">default])</span></tt></p>
<p>gets the value if it’s there, sets it if it’s not</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [27]: </span><span class="n">d</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s">u'something'</span><span class="p">,</span> <span class="s">u'a value'</span><span class="p">)</span>
<span class="gh">Out[27]: </span><span class="go">u'a value'</span>
<span class="gp">In [28]: </span><span class="n">d</span>
<span class="gh">Out[28]: </span><span class="go">{u'something': u'a value'}</span>
<span class="gp">In [29]: </span><span class="n">d</span><span class="o">.</span><span class="n">setdefault</span><span class="p">(</span><span class="s">u'something'</span><span class="p">,</span> <span class="s">u'a value'</span><span class="p">)</span>
<span class="gh">Out[29]: </span><span class="go">u'a value'</span>
<span class="gp">In [30]: </span><span class="n">d</span>
<span class="gh">Out[30]: </span><span class="go">{u'something': u'a value'}</span>
</pre></div>
</div>
<p>dict View objects:</p>
<p>Like <tt class="docutils literal"><span class="pre">keys()</span></tt>, <tt class="docutils literal"><span class="pre">values()</span></tt>, <tt class="docutils literal"><span class="pre">items()</span></tt>, but maintain a link to the original dict</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [47]: </span><span class="n">d</span>
<span class="gh">Out[47]: </span><span class="go">{u'something': u'a value'}</span>
<span class="gp">In [48]: </span><span class="n">item_view</span> <span class="o">=</span> <span class="n">d</span><span class="o">.</span><span class="n">viewitems</span><span class="p">()</span>
<span class="gp">In [49]: </span><span class="n">d</span><span class="p">[</span><span class="s">'something else'</span><span class="p">]</span> <span class="o">=</span> <span class="s">u'another value'</span>
<span class="gp">In [50]: </span><span class="n">item_view</span>
<span class="gh">Out[50]: </span><span class="go">dict_items([('something else', u'another value'), (u'something', u'a value')])</span>
</pre></div>
</div>
</div>
<div class="section" id="sets">
<h3>Sets<a class="headerlink" href="#sets" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">set</span></tt> is an unordered collection of distinct values</p>
<p>Essentially a dict with only keys</p>
<p>Set Constructors</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="o">>>></span> <span class="nb">set</span><span class="p">()</span>
<span class="nb">set</span><span class="p">([])</span>
<span class="o">>>></span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">}</span>
<span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">()</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">update</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span>
<span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
</pre></div>
</div>
</div>
<div class="section" id="set-properties">
<h3>Set Properties<a class="headerlink" href="#set-properties" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">Set</span></tt> members must be hashable</p>
<p>Like dictionary keys – and for same reason (efficient lookup)</p>
<p>No indexing (unordered)</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="o">>>></span> <span class="n">s</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="s">'set'</span> <span class="nb">object</span> <span class="n">does</span> <span class="ow">not</span> <span class="n">support</span> <span class="n">indexing</span>
</pre></div>
</div>
</div>
<div class="section" id="set-methods">
<h3>Set Methods<a class="headerlink" href="#set-methods" title="Permalink to this headline">¶</a></h3>
<div class="highlight-ipython"><div class="highlight"><pre><span class="o">>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="c"># an arbitrary member</span>
<span class="mi">1</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">KeyError</span><span class="p">:</span> <span class="s">'pop from an empty set'</span>
<span class="o">>>></span> <span class="n">s</span> <span class="o">=</span> <span class="nb">set</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">s</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">KeyError</span><span class="p">:</span> <span class="mi">2</span>
</pre></div>
</div>
<p>All the “set” operations from math class...</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">s</span><span class="o">.</span><span class="n">isdisjoint</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">issubset</span><span class="p">(</span><span class="n">other</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">union</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">intersection</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">difference</span><span class="p">(</span><span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">symmetric_difference</span><span class="p">(</span> <span class="n">other</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="frozen-set">
<h3>Frozen Set<a class="headerlink" href="#frozen-set" title="Permalink to this headline">¶</a></h3>
<p>Another kind of set: <tt class="docutils literal"><span class="pre">frozenset</span></tt></p>
<p>immutable – for use as a key in a dict
(or another set...)</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">fs</span> <span class="o">=</span> <span class="nb">frozenset</span><span class="p">((</span><span class="mi">3</span><span class="p">,</span><span class="mi">8</span><span class="p">,</span><span class="mi">5</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">fs</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="mi">9</span><span class="p">)</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">AttributeError</span>: <span class="n">'frozenset' object has no attribute 'add'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="exceptions">
<h2>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="id2">
<h3>Exceptions<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h3>
<p>Another Branching structure:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">()</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="n">process</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="c"># never called if file missing</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"couldn't open missing.txt"</span>
</pre></div>
</div>
</div>
<div class="section" id="id3">
<h3>Exceptions<a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h3>
<p>Never Do this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">()</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="n">process</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="c"># never called if file missing</span>
<span class="k">except</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"couldn't open missing.txt"</span>
</pre></div>
</div>
</div>
<div class="section" id="id4">
<h3>Exceptions<a class="headerlink" href="#id4" title="Permalink to this headline">¶</a></h3>
<p>Use Exceptions, rather than your own tests:</p>
<p>Don’t do this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">do_something</span><span class="p">()</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">):</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="n">process</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="c"># never called if file missing</span>
</pre></div>
</div>
<p>It will almost always work – but the almost will drive you crazy</p>
<p>Example from homework</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="n">num_in</span><span class="o">.</span><span class="n">isdigit</span><span class="p">():</span>
<span class="n">num_in</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">num_in</span><span class="p">)</span>
</pre></div>
</div>
<p>but – <tt class="docutils literal"><span class="pre">int(num_in)</span></tt> will only work if the string can be converted to an integer.</p>
<p>So you can do</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">num_in</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">num_in</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">u"Input must be an integer, try again."</span>
</pre></div>
</div>
<p>Or let the Exception be raised....</p>
<p>“it’s Easier to Ask Forgiveness than Permission”</p>
<blockquote>
<div>– Grace Hopper</div></blockquote>
<p><a class="reference external" href="http://www.youtube.com/watch?v=AZDWveIdqjY">http://www.youtube.com/watch?v=AZDWveIdqjY</a></p>
<p>(Pycon talk by Alex Martelli)</p>
<p>For simple scripts, let exceptions happen.</p>
<p>Only handle the exception if the code can and will do something about it.</p>
<p>(much better debugging info when an error does occur)</p>
</div>
<div class="section" id="exceptions-finally">
<h3>Exceptions – finally<a class="headerlink" href="#exceptions-finally" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">()</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="n">process</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="c"># never called if file missing</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"couldn't open missing.txt"</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">do_some_clean</span><span class="o">-</span><span class="n">up</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">finally:</span></tt> clause will always run</p>
</div>
<div class="section" id="exceptions-else">
<h3>Exceptions – else<a class="headerlink" href="#exceptions-else" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">()</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"couldn't open missing.txt"</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">process</span><span class="p">(</span><span class="n">f</span><span class="p">)</span> <span class="c"># only called if there was no exception</span>
</pre></div>
</div>
<p>Advantage:</p>
<p>you know where the Exception came from</p>
</div>
<div class="section" id="exceptions-using-them">
<h3>Exceptions – using them<a class="headerlink" href="#exceptions-using-them" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="n">do_something</span><span class="p">()</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'missing.txt'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">IOError</span> <span class="k">as</span> <span class="n">the_error</span><span class="p">:</span>
<span class="k">print</span> <span class="n">the_error</span>
<span class="n">the_error</span><span class="o">.</span><span class="n">extra_info</span> <span class="o">=</span> <span class="s">"some more information"</span>
<span class="k">raise</span>
</pre></div>
</div>
<p>Particularly useful if you catch more than one exception:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">except</span> <span class="p">(</span><span class="ne">IOError</span><span class="p">,</span> <span class="n">BufferError</span><span class="p">,</span> <span class="ne">OSError</span><span class="p">)</span> <span class="k">as</span> <span class="n">the_error</span><span class="p">:</span>
<span class="n">do_something_with</span> <span class="p">(</span><span class="n">the_error</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="raising-exceptions">
<h3>Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">divide</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="n">b</span><span class="p">):</span>
<span class="k">if</span> <span class="n">b</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ZeroDivisionError</span><span class="p">(</span><span class="s">"b can not be zero"</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="n">a</span> <span class="o">/</span> <span class="n">b</span>
</pre></div>
</div>
<p>when you call it:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [515]: </span><span class="n">divide</span> <span class="p">(</span><span class="mi">12</span><span class="p">,</span><span class="mi">0</span><span class="p">)</span>
<span class="go">ZeroDivisionError: b can not be zero</span>
</pre></div>
</div>
</div>
<div class="section" id="built-in-exceptions">
<h3>Built in Exceptions<a class="headerlink" href="#built-in-exceptions" title="Permalink to this headline">¶</a></h3>
<p>You can create your own custom exceptions</p>
<p>But...</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">exp</span> <span class="o">=</span> \
<span class="p">[</span><span class="n">name</span> <span class="k">for</span> <span class="n">name</span> <span class="ow">in</span> <span class="nb">dir</span><span class="p">(</span><span class="n">__builtin__</span><span class="p">)</span> <span class="k">if</span> <span class="s">"Error"</span> <span class="ow">in</span> <span class="n">name</span><span class="p">]</span>
<span class="nb">len</span><span class="p">(</span><span class="n">exp</span><span class="p">)</span>
<span class="mi">32</span>
</pre></div>
</div>
<p>For the most part, you can/should use a built in one</p>
<p>Choose the best match you can for the built in Exception you raise.</p>
<p>Example (for last week’s ackerman homework):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="p">(</span><span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="nb">int</span><span class="p">))</span> <span class="ow">or</span> <span class="p">(</span><span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="nb">int</span><span class="p">)):</span>
<span class="k">raise</span> <span class="ne">ValueError</span>
</pre></div>
</div>
<p>Is it the <em>value</em> or the input the problem here?</p>
<p>Nope: the <em>type</em> is the problem:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="p">(</span><span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="nb">int</span><span class="p">))</span> <span class="ow">or</span> <span class="p">(</span><span class="ow">not</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="nb">int</span><span class="p">)):</span>
<span class="k">raise</span> <span class="ne">TypeError</span>
</pre></div>
</div>
<p>but should you be checking type anyway? (EAFP)</p>
</div>
</div>
<div class="section" id="file-reading-and-writing">
<h2>File Reading and Writing<a class="headerlink" href="#file-reading-and-writing" title="Permalink to this headline">¶</a></h2>
<div class="section" id="files">
<h3>Files<a class="headerlink" href="#files" title="Permalink to this headline">¶</a></h3>
<p>Text Files</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">io</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'secrets.txt'</span><span class="p">,</span> <span class="n">codec</span><span class="o">=</span><span class="s">'utf-8'</span><span class="p">)</span>
<span class="n">secret_data</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">secret_data</span></tt> is a (unicode) string</p>
<p><tt class="docutils literal"><span class="pre">codec</span></tt> defaults to <tt class="docutils literal"><span class="pre">sys.getdefaultencoding()</span></tt> – often NOT what you want.</p>
<p>(There is also the regular <tt class="docutils literal"><span class="pre">open()</span></tt> built in, but it won’t handle Unicode for you...)</p>
<p>Binary Files</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'secrets.bin'</span><span class="p">,</span> <span class="s">'rb'</span><span class="p">)</span>
<span class="n">secret_data</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p><tt class="docutils literal"><span class="pre">secret_data</span></tt> is a byte string</p>
<p>(with arbitrary bytes in it – well, not arbitrary – whatever is in the file.)</p>
<p>(See the <tt class="docutils literal"><span class="pre">struct</span></tt> module to unpack binary data )</p>
<p>File Opening Modes</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'secrets.txt'</span><span class="p">,</span> <span class="p">[</span><span class="n">mode</span><span class="p">])</span>
<span class="s">'r'</span><span class="p">,</span> <span class="s">'w'</span><span class="p">,</span> <span class="s">'a'</span>
<span class="s">'rb'</span><span class="p">,</span> <span class="s">'wb'</span><span class="p">,</span> <span class="s">'ab'</span>
<span class="n">r</span><span class="o">+</span><span class="p">,</span> <span class="n">w</span><span class="o">+</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span>
<span class="n">r</span><span class="o">+</span><span class="n">b</span><span class="p">,</span> <span class="n">w</span><span class="o">+</span><span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span>
<span class="n">U</span>
<span class="n">U</span><span class="o">+</span>
</pre></div>
</div>
<p>These follow the Unix conventions, and aren’t all that well documented on the Python docs. But these BSD docs make it pretty clear:</p>
<p><a class="reference external" href="http://www.manpagez.com/man/3/fopen/">http://www.manpagez.com/man/3/fopen/</a></p>
<p><strong>Gotcha</strong> – ‘w’ modes always clear the file</p>
<p>Text is default</p>
<blockquote>
<div><ul class="simple">
<li>Newlines are translated: <tt class="docutils literal"><span class="pre">\r\n</span> <span class="pre">-></span> <span class="pre">\n</span></tt></li>
<li>– reading and writing!</li>
<li>Use *nix-style in your code: <tt class="docutils literal"><span class="pre">\n</span></tt></li>
<li><tt class="docutils literal"><span class="pre">io.open()</span></tt> returns various “stream” objects – but they act like file objects.</li>
<li>In text mode, io.open() defaults to “Universal” newline mode.</li>
</ul>
</div></blockquote>
<p>Gotcha:</p>
<blockquote>
<div><ul class="simple">
<li>no difference between text and binary on *nix</li>
<li>breaks on Windows</li>
</ul>
</div></blockquote>
<p><tt class="docutils literal"><span class="pre">io.open(file,</span> <span class="pre">mode='r',</span> <span class="pre">buffering=-1,</span> <span class="pre">encoding=None,</span> <span class="pre">errors=None,</span> <span class="pre">newline=None,</span> <span class="pre">closefd=True)</span></tt></p>
<blockquote>
<div><ul class="simple">
<li><tt class="docutils literal"><span class="pre">file</span></tt> is generally a file name or full path</li>
<li><tt class="docutils literal"><span class="pre">mode</span></tt> is the mode for opening: ‘r’, ‘w’, etc.</li>
<li><tt class="docutils literal"><span class="pre">buffering</span></tt> controls the buffering mode (0 for no buffering)</li>
<li><tt class="docutils literal"><span class="pre">encoding</span></tt> sets the unicode encoding – only for text files – when set, you can ONLY write unicode object to the file.</li>
<li><tt class="docutils literal"><span class="pre">errors</span></tt> sets the encoding error mode: ‘strict’, ‘ignore’, ‘replace’,...</li>
<li><tt class="docutils literal"><span class="pre">newline</span></tt> controls Universal Newline mode: lets you write DOS-type files on *nix, for instance (text mode only).</li>
<li><tt class="docutils literal"><span class="pre">closedfd</span></tt> controls close() behavior if a file descriptor, rather than a name is passed in (advanced usage!)</li>
</ul>
</div></blockquote>
<p>(<a class="reference external" href="https://docs.python.org/2/library/io.html?highlight=io.open#io.open">https://docs.python.org/2/library/io.html?highlight=io.open#io.open</a>)</p>
</div>
<div class="section" id="file-reading">
<h3>File Reading<a class="headerlink" href="#file-reading" title="Permalink to this headline">¶</a></h3>
<p>Reading part of a file</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">header_size</span> <span class="o">=</span> <span class="mi">4096</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'secrets.txt'</span><span class="p">)</span>
<span class="n">secret_header</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="n">header_size</span><span class="p">)</span>
<span class="n">secret_rest</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>Common Idioms</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'secrets.txt'</span><span class="p">):</span>
<span class="k">print</span> <span class="n">line</span>
</pre></div>
</div>
<p>(the file object is an iterator!)</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">f</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'secrets.txt'</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="n">line</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">line</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">do_something_with_line</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="file-writing">
<h3>File Writing<a class="headerlink" href="#file-writing" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="n">outfile</span> <span class="o">=</span> <span class="n">io</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'output.txt'</span><span class="p">,</span> <span class="s">'w'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">outfile</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"this is line: </span><span class="si">%i</span><span class="se">\n</span><span class="s">"</span><span class="o">%</span><span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="file-methods">
<h3>File Methods<a class="headerlink" href="#file-methods" title="Permalink to this headline">¶</a></h3>
<p>Commonly Used Methods</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span> <span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> <span class="n">f</span><span class="o">.</span><span class="n">readlines</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="nb">str</span><span class="p">)</span> <span class="n">f</span><span class="o">.</span><span class="n">writelines</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span>
<span class="n">f</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="n">offset</span><span class="p">)</span> <span class="n">f</span><span class="o">.</span><span class="n">tell</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>
<span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="file-like-objects">
<h3>File Like Objects<a class="headerlink" href="#file-like-objects" title="Permalink to this headline">¶</a></h3>
<p>Many classes implement the file interface:</p>
<blockquote>
<div><ul class="simple">
<li>loggers</li>
<li><tt class="docutils literal"><span class="pre">sys.stdout</span></tt></li>
<li><tt class="docutils literal"><span class="pre">urllib.open()</span></tt></li>
<li>pipes, subprocesses</li>
<li>StringIO</li>
</ul>
</div></blockquote>
<p><a class="reference external" href="https://docs.python.org/2/library/stdtypes.html#file-objects">https://docs.python.org/2/library/stdtypes.html#file-objects</a></p>
</div>
<div class="section" id="stringio">
<h3>StringIO<a class="headerlink" href="#stringio" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre><span class="n">In</span> <span class="p">[</span><span class="mi">417</span><span class="p">]:</span> <span class="kn">import</span> <span class="nn">StringIO</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">420</span><span class="p">]:</span> <span class="n">f</span> <span class="o">=</span> <span class="n">StringIO</span><span class="o">.</span><span class="n">StringIO</span><span class="p">()</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">421</span><span class="p">]:</span> <span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">u"somestuff"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">422</span><span class="p">]:</span> <span class="n">f</span><span class="o">.</span><span class="n">seek</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">423</span><span class="p">]:</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">423</span><span class="p">]:</span> <span class="s">'somestuff'</span>
</pre></div>
</div>
<p>(handy for testing file handling code...)</p>
</div>
</div>
<div class="section" id="paths-and-directories">
<h2>Paths and Directories<a class="headerlink" href="#paths-and-directories" title="Permalink to this headline">¶</a></h2>
<div class="section" id="paths">
<h3>Paths<a class="headerlink" href="#paths" title="Permalink to this headline">¶</a></h3>
<p>Paths are generally handled with simple strings (or Unicode strings)</p>
<p>Relative paths:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="s">u'secret.txt'</span>
<span class="s">u'./secret.txt'</span>
</pre></div>
</div>
<p>Absolute paths:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="s">u'/home/chris/secret.txt'</span>
</pre></div>
</div>
<p>Either work with <tt class="docutils literal"><span class="pre">open()</span></tt> , etc.</p>
<p>(working directory only makes sense with command-line programs...)</p>
</div>
<div class="section" id="os-module">
<h3>os module<a class="headerlink" href="#os-module" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python"><div class="highlight"><pre>os.getcwd() -- os.getcwdu() (u for Unicode)
chdir(path)
os.path.abspath()
os.path.relpath()
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">split</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">splitext</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">basename</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">dirname</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>(all platform independent)</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">os</span><span class="o">.</span><span class="n">listdir</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">mkdir</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">walk</span><span class="p">()</span>
</pre></div>
</div>
<p>(higher level stuff in <tt class="docutils literal"><span class="pre">shutil</span></tt> module)</p>
</div>
<div class="section" id="pathlib">
<h3>pathlib<a class="headerlink" href="#pathlib" title="Permalink to this headline">¶</a></h3>
<p><tt class="docutils literal"><span class="pre">pathlib</span></tt> is a new package for handling paths in an OO way:</p>
<p><a class="reference external" href="http://pathlib.readthedocs.org/en/pep428/">http://pathlib.readthedocs.org/en/pep428/</a></p>
<p>It is now part of the Python3 standard library, and has been back-ported for use with Python2:</p>
<div class="highlight-bash"><div class="highlight"><pre><span class="nv">$ </span>pip install pathlib
</pre></div>
</div>
<p>All the stuff in os.path and more:</p>
<div class="highlight-ipython"><div class="highlight"><pre><span class="gp">In [64]: </span><span class="kn">import</span> <span class="nn">pathlib</span>
<span class="gp">In [65]: </span><span class="n">pth</span> <span class="o">=</span> <span class="n">pathlib</span><span class="o">.</span><span class="n">Path</span><span class="p">(</span><span class="s">'./'</span><span class="p">)</span>
<span class="gp">In [66]: </span><span class="n">pth</span><span class="o">.</span><span class="n">is_dir</span><span class="p">()</span>
<span class="gh">Out[66]: </span><span class="go">True</span>
<span class="gp">In [67]: </span><span class="n">pth</span><span class="o">.</span><span class="n">absolute</span><span class="p">()</span>
<span class="gh">Out[67]: </span><span class="go">PosixPath('/Users/Chris/PythonStuff/CodeFellowsClass/sea-f2-python-sept14/Examples/Session04')</span>
<span class="gp">In [68]: </span><span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="n">pth</span><span class="o">.</span><span class="n">iterdir</span><span class="p">():</span>
<span class="go"> print f</span>
<span class="go">junk2.txt</span>
<span class="go">junkfile.txt</span>
<span class="go">...</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="homework">