Programming with Python - Basic

Python is a great programming language. It is a complete tutorial of using this programming language.

This slides is split into two parts, and it is the first part. Another part is at: https://speakerdeck.com/mosky/programming-with-python-adv.

Transcript

1. 1 Programming with Python Basic Topics Mosky
2. 2 Mosky: • The examples and the PDF version are available at: – j.mp/mosky-programming-with-python. • It is welcome to give me any advice of this slide or ask me the answers of the challenges. – mosky.tw
3. 3 Mosky • Projects – MoSQL mosql.mosky.tw – Clime clime.mosky.tw – Apt-Pool Apt-Add … • Pinkoi staff pinkoi.com • PyCon JP '12 Speaker pycon.jp • PyCon TW '12 Speaker pycon.tw
4. 4 Advertisement • COSCUP 2013 – coscup.org – 8/3-4 @ TICC – coscup-gereral@googlegroups.co m • PyCon Taiwan 2013 – pycon.tw – 5/25-26 @ Sinica – pythontw@googlegroups.com
5. 5 Topics • Basic Topics – Python 2 or 3? – Environment – hello.py – Common Types – Flow Control – File I/O – Documentation – Scope • Adv. Topics – Module and Package – Typing – Comprehension – Functional Technique – Object-oriented Prog. – Useful Libraries • Final Project – A Blog System
6. 6 An Investigation Do you know _________ ? – any other programming language – Object-oriented – Static Typing; Strong and Weak Typing – Dynamic Typing – Functor; Closure – Functional Programming – Web development
7. 7 Python 2 or 3? in short.
8. 8 Python 2 or 3? • Python 2.x – status quo – 2.7 is end-of-life release – harder for newcomers – more third-party lib. – 2to3.py – backported features: • What's News in Python 2.6 docs.python.org/release/2.6.4/whatsnew/2.6.html • What's News in Python 2.7 docs.python.org/dev/whatsnew/2.7.html • Python 3.x – present & future – under active development – easier for newcomers – less third-party lib. – 3to2.py – new features: • What's News in Python 3.0 docs.python.org/py3k/whatsnew/3.0.html
9. 9 Python 2 or 3? (cont.) • Use Python 3 if you can. • Decide Python 2 or 3 by the libraries you will use. • Today, we will go ahead with Python 2. And introduce you to the changes in Python3.
10. 10 Environment Is a python in your computer?
11. 11 On Linux or Mac • Python is built-in on Linux or Mac. • All you have to do is check the version. Type "python" in any terminal. Python 2.7.3 (default, Sep 26 2012, 21:51:14) [GCC 4.7.2] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>>
12. 12 On Windows • Download the installer from: "http://python.org/download" • Install it. • Add the Python's PATH. – Computer → System Properties → Advanced system settings → Advanced tab → Environment Variables → System Variables → find PATH. – "...;C:\Python27"
13. 13 Editor / IDE • The Editors – Sublime Text 2 www.sublimetext.com – VIM wiki.python.org/moin/Vim – Gnome Text Editor (gedit) – Notepad++ notepad-plus-plus.org – ... • The IDE – IDLE • Debian-base: sudo apt-get install idle • Windows: Use the Start Menu to search "IDLE" • The others: – wiki.python.org/moin/PythonEditors
14. 14 The Python Shell • Type "python" in terminal. – >>> – ... • Leaving a shell: – exit() – Linux or Mac: Ctrl+D – Windows: Ctrl+Z<Enter>
15. 15 The python Command • Enter Python shell without arguments. • python hello.py • python -c 'print "Hello, World!"' • python -m SimpleHTTPServer
16. 16 hello.py Say hello to Python.
17. 17 hello.py #!/usr/bin/env python # -*- coding: utf-8 -*- # file: hello.py def hello(name=None): if name:\n return 'Hello, %s!' % name else: return 'Hello, Python!' • #! the shebang. • # -*- defines the encoding of this file. • # means the comments. • : starts a block. • A block uses 4-space indent. • A statement ends with \n.
18. 18 hello.py (cont.) if __name__ == '__main__': import sys if len(sys.argv) >= 2: print hello(sys.argv[1]) else: print hello() • __name__, the name of module. • import is important. The usage: • import sys • from sys import argv • … as alias
20. 20 The print Statement print 'End with a new line char.' print 'Print', 'multiple', 'strings.' print 'End with a space.', print # print a new line char
21. 21 The print function in Python 3 print('End with a new line char.') print('Print', 'multiple', 'strings.') print('End with a space.', end=' ') print() # print a new line char print('End with a space.', end='') print('a', 'b', 'c', seq=',')
22. 22 Common Types Without it we can do noting
23. 23 Common Types • Numeric – Integer 100 – Float 10.0 – Long 100L – Complex 1+1j – Boolean True, False • Sequence – String "" – Unicode u"" – List [,] – Tuple (,)
24. 24 Common Types (cont.) • Mapping – Dictionary {:} • Set – Set {,} – Frozen Set forzenset(...)
25. 25 Integer, Float and Long • 3+3 • 3-3 • 3*3 • 3/3 • 6/2*(1+2) → int (as long in C) • divmod(5, 2) → tuple (not numeric) • 5/2 → int (truncated) • 5.0/2 → float (as double in C) • 5.0//2 → float (floored) • 2**1000 → long (∞ precision)
26. 26 Integer and Float in Python 3 • 3+3 • 3-3 • 3*3 • 3/3 • 6/2*(1+2) → int (∞ precision) • divmod(5, 2) → tuple (not numeric) • 5/2 → float • 5.0/2 → float (as double in C) • 5.0//2 → float (floored) • 2**1000 → int (∞ precision)
27. 27 Note: The Variables • x = 1 • x + 1 → 2 • y = 2 • x + y → 3 • y += 3 → 5 • x < y True → • bin(y) → '0b101' • bin(y | 0b011) → '0b111'
28. 28 A Trap of the Integer • weight = 49 • height = 163 • bmi = weight / (height / 100) ** 2 • bmi → 49 • (height / 100) → 1
29. 29 A Trap of the Integer (cont.) • weight = 49.0 • height = 163.0 • bmi = weight / (height / 100) ** 2 • bmi → 18.442545824080696
30. 30 Complex • 1j * 1J • 1j * complex(0,1) • 3 + 1j*3 • (3+1j)*3 • (1+2j)/(1+1j) → complex • a = 3.0+4.0j • float(a) → TypeError • a.real → 3.0 • a.imag → 4.0 • abs(a) # = sqrt(a.real**2 + a.imag**2) 5 →
31. 31 Boolean • not False • True and True • False or True • False +1 1 → • True +1 2 → Comparison: – 10 < 100 – 10 < 10.0 – 10 <= 10.0 – 10 == 10.0 – 10 != 10.0 – x is y
32. 32 String and Unicode '...' is equal to "..." String (immutable seq.) – ' 中文 ' – ' 嗨， \nPython ！ ' – r' 嗨， \nPython ！ ' – ''' ... ''' Unicode (immutable seq.) – u' 嗨， \nPython ！ ' – ur' 嗨， \nPython ！ ' – u''' ... ''' Functions – ord( 'A') – chr(65) – ord(u'中 ') – unichr(20013); chr(20013) Decoding (String → Unicode) – '中文 '.decode('utf-8') – unicode( '中文 ', 'utf-8') Encoding (Unicode → String) – u'中文 '.encode('utf-8') – str(u'中文 ') – str(u'中文 ', 'utf-8')
33. 33 Bytes and String in Python 3 '...' is equal to "..." Bytes (immutable seq.) – b' 中文 ' – b' 嗨， \nPython ！ ' – br' 嗨， \nPython ！ ' – b''' ... ''' String (immutable seq.) – ' 嗨， \nPython ！ ' – r' 嗨， \nPython ！ ' – ''' ... ''' Functions – ord(b'A') – chr(65) – ord( '中 ') – unichr(20013); chr(20013) Decoding (Bytes → String) – b'中文 '.decode('utf-8') – str(b'中文 ', 'utf-8') Encoding (String → Bytes) – '中文 '.encode('utf-8') – bytes( '中文 ') – bytes( '中文 ', 'utf-8')
34. 34 Unicode Does Matter! • b = ' 中文 ' • len(b) → 6 • len(b.decode('utf-8')) → 2
35. 35 String and Unicode (cont.) • They have a lot of methods: capitalize center count decode encode endswith expandtabs find rfind format index rindex isalnum isalpha isdigit islower isspace istitle isupper join ljust rjust lower partition rpartition replace split rsplit splitlines startswith rstrip strip lstrip swapcase title translate upper zfill • ref: docs.python.org/2/library/stdtypes.html#string-methods
36. 36 String and Unicode (cont.) String formatting: – % (modulo) • ref: docs.python.org/2/library/stdtypes.html#string-formatting-operations – str.format • ref: docs.python.org/2/library/string.html#formatstrings
37. 37 List and Tuple List (mutable seq.) – [] – ['item'] – ['s', 100, u'unicode'] – list('abc') – 'a b c'.split(' ') – '\n'.join(['spam', 'eggs']) – x, y = [1, 2] – x, y = [y, x] Tuple (seq.) – tuple() – ('item', ) – ('s', 100, u'unicode') – tuple('abc') – '\n'.join(('spam', 'eggs')) – x, y = (1, 2) – x, y = (y, x)
38. 38 Sequence Sequence – x in s # performance? – x not in s – s + t – s * n, n * s – s[i] – s[i:j] – s[i:j:k] – len(s) – s.index(x) – s.count(x) Mutable Seq. – s[i] = x – s[i:j] = t – del s[i:j] – s[i:j:k] = t – s.append(x) – s.insert(i, x) – s.pop([i]) – s.remove(x) # performance? – s.extend(t) in-place – s.sort([cmp[, key[, reverse]]]) – s.sort([key[, reverse]]) # Py 3 – s.reverse()
39. 39 Sequence Comparison • (0, 0, 0) < (0, 0, 1) • [0, 0, 0] < [0, 0, 1] • (0, ) < (0, 0) • 'ABC' < 'C' < 'Pascal' < 'Python' • (1, 2, 3) == (1.0, 2.0, 3.0) • 'A' == 'A' • 'A' > 65 • 'A' > 66 • ('A', ) > (66, )
40. 40 Sequence Comparison in Python 3 • (0, 0, 0) < (0, 0, 1) • [0, 0, 0] < [0, 0, 1] • (0, ) < (0, 0) • 'ABC' < 'C' < 'Pascal' < 'Python' • (1, 2, 3) == (1.0, 2.0, 3.0) • 'A' == 'A' • 'A' > 65 TypeError → • 'A' > 66 TypeError → • ('A', ) > (66, ) TypeError →
41. 41 Sequence (cont.) Slicing and Slice object: – s = range(10) – t = s – t[0] = 'A' – print s – t is s – t = s[:] – t is s – s = 'I am a str.' – s[:-3] – s.reverse() → TypeError – s[::-1] – ''.join(reversed(s)) – slice(None, None, -1)
42. 42 Mapping Dict. (mutable map.) – {} – {'A ': 1, 'B': 2, 'C': 3} – dict({...}) – dict(A=1, B=2, C=3) – k = 'ABC' – v = [1, 2, 3] – pairs = zip(k, v) – dict(pairs) – len(d) – d[k] – d[k] = v – del d[k] – k in d, k not in d – d.copy() – d.get(key[, default]) – d.setdefault(key[, default]) – d.items(), d.keys(), d.values() – d.pop(key[, default) – d.update([other]) ...
43. 43 Set Set (mutable set) – set() – {'A', 'B', 'C'} # Py3 – set('ABC') – set(['A','B','C']) – len(s) – x in s, x not in s – s.copy() – s.add(elem) – s.discard(elem) – s.pop() – s |= other – s &= other – s | other | ... – s & other & ... – s < | <= | == | > = | > other ...
44. 44 Flow Control in Python is grace and easy to learn.
45. 45 The if Statement if [condition 1]: … elif [condition 2]: … elif [condition 3]: … else: … [exp. if true] if [condition] else [exp. if false]
46. 46 Truth Value Testing They are same as False in a boolean context: – None – False – Zeros (ex. 0, 0.0, 0L, 0j) – Empty containers (ex. '', [], {}) – __nonzero__() or __len__() returns 0 or False
47. 47 Truth Value Testing (cont.) • if not None: ... • if not []: ... • if [0]: ... • if [[]]: ... • if "": ... • if {}: ... • if not {0: False}: … …
48. 48 The for Statement for [item] in [iterable]: … for i in [0, 1, 2]: print i for i in xrange(3): print i for i in range(3): print i
49. 49 The for Statement in Python 3 for [item] in [iterable]: … for i in [0, 1, 2]: print i for i in xrange(3): print i for i in range(3): print i
50. 50 The for Statement (cont.) for i in range(1, 3): print i for i in range(3, -1, -1): print i s = [...] for i, item in enumerate(s): print i, item s = [1, 2, 3] t = 'xyz' for i, j in zip(s, t): print i, j
51. 51 The for Statement (cont.) • It is like for … each in other language. – Note: Python hasn't other for loop. • It can iterate all of iterable object. – In other words, the object which defined __iter__. – ex. sequence, mapping, set, ...
52. 52 Challenge 1: A Pyramid • Use for loop to build a pyramid on right. – without limit. – limit: in two lines • hint: string formatting * *** ***** *******
53. 53 Challenge 2-1: Count the Chars • Use for loop to count the sentence on right. – without limit. – limit: without if • hint: use get "Please count the characters here." {'P': 1, ...}
54. 54 Challenge 2-2: Collect the Chars • Use for loop to collect the chars. – limit: use setdefault "Here are UPPERCASE and lowercase chars." {'c': ['C', 'c', 'c'], ...}
55. 55 The while Statement tasks = [...] while tasks: … while 1: … • A infinite loop. • It is better to use block mechanism in a loop. – ex. I/O block • It leaves the loop once the tasks is empty.
56. 56 The break, continue Statement loop …: if …: break loop …: if …: continue • It continues with the next iteration. • It terminates a loop.
57. 57 The break, continue Statement (cont.) • They do the same thing in both C and Python. • Using break or continue is encouraged. – take the place of the complicated condition in a while. – faster, because Python is interpreted. • Just use them.
58. 58 The pass Statement • Do nothing.
59. 59 The else Clause on Loops loop …: … else: … • No a clause on the if statement! • If the loop isn't broken by any break statement, the else block is executed. • It replaces the flags we usually used.
60. 60 Challenge 3-1: The Primes • Try to filter the primes from [2, 100). – without limit. – limit: use loop's else [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]
61. 61 The try Statement try: … except LookupError, e: … except (IndexError, KeyError), e: … else: … finally: …
62. 62 The try Statement in Python 3 try: … except LookupError as e: … except (IndexError, KeyError) as e: … else: … finally: …
63. 63 The try Statement (cont.) • For avoiding to catch the exception we don't expect, you should: – reduce your code in try block. • move them to else block. – make the exception precise in except statement. • Avoid using Exception. • ref: docs.python.org/2/library/exceptions.html#exception-hierarchy • Release the resource in finally block. – or use context manager – ex. file, socket, … • raise SomeError
64. 64 The def Statement def f(x, y): return (x, y) def f(x, y=2): return (x, y) f(1) f(x=1) f(*(1, )) f(**{'x': 1}) f(1, 2) f(y=2, x=1) f(*(1, 2)) f(**{'y': 2, 'x': 1})
65. 65 The def Statement (cont.) def f(*args): return args def f(**kargs): return kargs # f(1, 2) # → TypeError f(x=1, y=2, z=3) # f(*(1, 2)) # → TypeError f(**{'x': 1, 'y': 2, 'z': 3}) f(1, 2, 3) # f(y=2, x=1) # → TypeError f(*(1, 2, 3, 4)) # f(**{'y': 2 ,'x': 1}) # → TypeError
66. 66 The def Statement (cont.) def f(x, *args): return x, args def f(x, **kargs): return kargs # f(1, 2) # → TypeError f(x=1, y=2, z=3) # f(*(1, 2)) # → TypeError f(**{'x': 1, 'y': 2, 'z': 3}) f(1, 2, 3) # f(y=2, x=1) # → TypeError f(*(1, 2, 3, 4)) # f(**{'y': 2, 'x': 1}) # → TypeError
67. 67 The def Statement (cont.) def f(*args, y): return kargs def f(*args, **kargs): return args, kargs f(1, 2, 3) f(y=2, x=1) f(*(1, 2, 3, 4)) f(**{'y': 2, 'x': 1}) → SyntaxError
68. 68 The def Statement in Python 3 def f(*args, k): return kargs def f(*args, k, **kargs): return args, kargs f(1, 2, 3) f(x=1, k=2) f(*(1, 2, 3, 4)) f(**{'x': 1, 'k': 2}) F(1, 2, 3) # f(x=1, k=2) # → TypeError f(*(1, 2, 3, 4)) # f(**{'x': 1, 'k': 2}) # → TypeError
69. 69 The def Statement (cont.) def f(): pass def g(): pass d = {'x': f, 'y': g} d['x']() • Python functions are first-class functions. – It means you can pass functions as arguments, and assign functions to variables. – It is like the function pointers in C.
70. 70 An Example of Using while, try and def. # file: ex_try.py def take_int(prompt='Give me a int: '): while 1: try: user_input = int(raw_input(prompt)) except ValueError, e: print 'It is not a int!' else: return user_input if __name__ == '__main__': x = take_int() print 'I got a int from user: %d' % x $ python ex_try.py Give me a int: str It is not a int! Give me a int: abc It is not a int! Give me a int: 100 I got a int from user: 100 $
71. 71 A Trap of the Default Value # file: ex_defval_trap.py def f(items=[]): items.append(1) return items if __name__ == '__main__': print f() # -> [1] print f() # -> [1, 1] print f() # -> [1, 1, 1] • Because the list is created when the function is defined. • Avoid to use the mutable types as the default value.
72. 72 Challenge 4: A BMI Calculator • BMI: Body Mass Index – BMI = weight (KG) ÷ height (M)2 – < 18.5 → Underweight – [18.5, 25) → Normal weight – [25, 30) → Overweight – >= 30 → Obesity • Write a BMI calculator. – without limit. – limit: only one if • hint: use loop Enter your height (M): 1.63 Enter your weight (KG): 49 --- Your BMI is: 18.44 (Underweight) Ideal weight is between: 49.15 ~ 66.42
73. 73 File I/O Open anything with the open.
74. 74 The file Object f = open('input.txt') print f.read() f.seek(0) for line in f: print line, f.close() f = open('output.txt', 'w') f.write('a line.\n') f.close()
75. 75 The Context Manager with open('input.txt') as f: for line in f: print line, f.close() • Python 2.5↑ – Python 2.5.x: from __future__ import with_statement – Python 2.6↑: It is mandatory.
76. 76 Challenge 2: Count the Chars (cont.) – limit 3: with the files The path of input: input.txt The path of output: output.txt --- The result was written.
77. 77 The csv Moudle #!/usr/bin/env python # -*- coding: utf-8 -*- # file: ex_csv.py import csv with open('ex_csv.csv') as f: for row in csv.reader(f): print row 1, apple 2, orange 3, watermelon ['1', ' apple'] ['2', ' orange'] ['3', ' watermelon']
78. 78 The os.path Moudle # file: ex_os_path.py from os import walk from os.path import join def list_files(path): paths = [] for root, dir_names, file_names in walk(path): for file_name in file_names: paths.append(join(root, file_name)) return paths if __name__ == '__main__': import sys from os.path import abspath, dirname if len(sys.argv) == 2: path = abspath(dirname(sys.argv[1])) for path in list_files(path): print path else: print 'It requires a path as argument.' $ python ex_os_path.py It requires a path as argument. $ python ex_os_path.py . …/1 …/b/4 …/a/2 …/a/3
79. 79 Documentation The documentation is everywhere.
80. 80 The help Function • In Python shell: – help(open) – dir(open) – '\n'.join(dir(open)) • In terminal: – $ pydoc SimpleHTTPServer – $ pydoc csv – $ pydoc os.path
81. 81 Your Documentation # file: ex_doc.py '''module-level doc.''' def f(x): '''A short sentence describes this function. About the parameters, return value or any other detail ... ''' pass $ pydoc ex_doc Help on module ex_doc: NAME ex_doc - module-level doc. FILE /home/mosky/programming-with-python/ex_doc.py FUNCTIONS f(x) A short sentence describes this function. About the parameters, return value or any other detail ...
82. 82 Scope Where is the x?
83. 83 Scope # file: ex_scope.py x = 'global' def f(): if 1: x = 'local' return x if __name__ == '__main__': print x print f() $ python ex_scope.py global local $ • Scopes are decided by functions.
84. 84 The LEGB Rule # file: ex_LEGB.py global_var = 100 def f(): enclosed_var = 10 def g(): local_var = 1 return sum([local_var, enclosed_var, global_var]) return g() if __name__ == '__main__': print f() # -> 111 • return … – Local (in function) – Enclosed – Global – Built-in
85. 85 Challenge 3-2: The Primes (cont.) – limit 1: Sieve of Eratosthenes. – limit 2: use set. [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]
86. 86 Challenge 5: Mix All • You have many functions now. Try to write a CLI program to trigger your functions. – without limit – limit: without if. $ python mix.py pyramid 10 … $ python mix.py primes 100 … $ python mix.py bmi 1.63 49 … $ python mix.py blah blah Please check your args.
87. 87 Adv. Topics There is another slide.