Intro to Python

An introduction to the programming language Python. This talk was written for a hackday centred around Django, and given to a group of software developers who were experienced in PHP but had minimal knowledge of Python.

Transcript

1. Intro to Python Rae Knowler Django Hackday, 10.02.2015 1
2. What is Python? • Open source general-purpose language. • Object Oriented, Procedural, Functional • Easy to interface with C/ObjC/Java/Fortran • Easy-ish to interface with C++ (via SWIG) • Great interactive environment (http://tdc-www.harvard.edu/Python.pdf) 2
3. What can you use Python for? • Web development -- e.g with Django or Pylons • Scraping • Scripting • Scientific programming • Even games 3
4. Python interpreter 4
5. Running programs $ python my-program.py 5
6. Basic syntax 6
7. Whitespace 7
8. Whitespace def counter(x): if x > 10: print "x is already > 10!" else: print x 8
9. No variable declarations >>> x = 3 >>> type(x) <type 'int'> >>> y = "fish" >>> type(y) <type 'str'> 9
10. Control flow 10
11. If if name == "Freda": greeting = "Hi Freda!" elif name == "Bobby": greeting = "Go away, Bobby!" else: greeting = "Welcome, " + name 11
12. While n = 1 t = 1 while n < 5: print 'The %s. triangle number is %s' % (n, t) n += 1 t += n 12
13. While Output: The 1. triangle number is 1 The 2. triangle number is 3 The 3. triangle number is 6 The 4. triangle number is 10 13
14. For shopping_list = ["bread", "apples", "jam"] for item in shopping_list: print "Don't forget to buy %s" % item 14
15. For Output: Don't forget to buy bread Don't forget to buy apples Don't forget to buy jam 15
16. Try try: self.add_friend(user) log.debug('Added friend!') except Error as e: log.debug('Failed to add friend: %s' % e.value) finally: log.debug('We tried to add a friend.') 16
17. Basic data types 17
18. Bool False: False, 0, None, empty sequences or maps True: True, 1, non-empty sequences ... Cast to bool: bool([x]) 18
19. Int >>> x = 3 >>> x = int(3) >>> x = int(3.3) >>> x 3 >>> 5 / 3 1 >>> 5.0 / 3 1.6666666666666667 19
20. Float >>> float(3) 3.0 >>> 1.1 + 2.2 3.3000000000000003 20
21. Strongly typed... >>> 1 + '2 fish' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for +: 'int' and 'str' 21
22. ...but still dynamic >>> x = 1 >>> x = 'flower' >>> x 'flower' 22
23. Sequence types 23
24. List >>> numbers = [1, 2, 3] >>> numbers + [4] [1, 2, 3, 4] >>> numbers * 2 [1, 2, 3, 1, 2, 3] >>> numbers[0] = 5 >>> numbers [5, 2, 3] 24
25. Tuple >>> animals = ('bee', 'bear') >>> animals * 2 ('bee', 'bear', 'bee', 'bear') >>> animals[0] 'bee' >>> animals[0] = 'cat' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment 25
26. String >>> flower1 = "tulep" >>> flower2 = "daffodil" >>>flowers = ", ".join([flower1, flower2]) >>>flowers 'tulep, daffodil' 26
27. String >>>flowers 'tulep, daffodil' >>> flowers[3] = "i" Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'str' object does not support item assignment 27
28. Looping over sequences >>> flower = "tulip" >>> for letter in flower: ... print letter ... t u l i p 28
29. Slicing >>> flower = "tulip" >>> flower[1:4] 'uli' >>> flower1[1:4:2] 'ui' >>> flower1[::2] 'tlp' >>> flower1[::-1] 'pilut' 29
30. Dictionaries 30
31. Dictionaries >>> books = {} >>> books["War and Peace"] = 2 >>> books["Jane Eyre"] = 3 >>> books {'Jane Eyre': 3, 'War and Peace': 2} 31
32. Del >>> del books["Jane Eyre"] >>> books {'War and Peace': 2} 32
33. Keys >>> books.keys() ['Jane Eyre', 'War and Peace'] >>> for key in books: ... print key ... Jane Eyre War and Peace >>> "Love and Rockets" in books False 33
34. iteritems() >>> for title, count in books.iteritems(): ... print title, count ... Jane Eyre 3 War and Peace 1 iteritems() creates a generator of key, value pairs 34
35. Comprehensions 35
36. List comprehensions >>> numbers = list(range(10)) >>> odds = [] >>> for n in numbers: ... if n % 2 == 1: ... odds.append(n) ... >>> odds [1, 3, 5, 7, 9] 36
37. List comprehensions [expression for item in list if conditional] >>> odds = [n for n in numbers if n % 2 == 1] >>> odds [1, 3, 5, 7, 9] 37
38. List comprehensions [expression for item in list if conditional] >>> squares = [n * n for n in numbers] >>> squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] 38
39. Dict comprehensions {key:value for item in list if conditional} >>>{n: n * n for n in numbers if n % 2 == 1} {1: 1, 3: 9, 5: 25, 7: 49, 9: 81} Python 3.x only 39
40. Functions 40
41. Functions def is_prime(n): m = 2 while m * m < n: if n % m == 0: return False m += 1 return True 41
42. Optional arguments >>> def greet(name, time="day"): ... print "Good " + time + ", " + name ... >>> greet("Freda") Good day, Freda >>> greet("Bobby", "morning") Good morning, Bobby >>> greet(name="Jenny", time="evening") Good evening, Jenny 42
43. *args and **kwargs >>> def what_are_flowers(f1, f2, f3): ... print "The flowers are %s, %s and %s." % (f1, f2, f3) ... >>> what_are_flowers("lily", "rose", "tulip") The flowers are lily, rose and tulip. 43
44. *args and **kwargs The flowers are lily, rose and tulip >>> def what_are_flowers(*args): ... print "The flowers are " + ", ".join(args) ... >>> what_are_flowers("rose", "iris") The flowers are rose, iris >>> what_are_flowers("rose", "iris", "pansy") The flowers are rose, iris, pansy 44
45. *args and **kwargs >>> flowers = {"poppy": 3, "rose": 2} >>> def what_are_flowers(**kwargs): ... for k, v in kwargs: ... print v + " flowers are " + k ... >>> what_are_flowers(**flowers) 3 flowers are poppy 2 flowers are rose 45
46. *args and **kwargs >>> def inventory(bag, *args, **kwargs): ... print "bag: " + bag ... if args: ... print [a for a in args] ... if kwargs: ... print [k, v for k, v in kwargs. iteritems()] 46
47. *args and **kwargs >>> inventory("sack", "apple", "cat", pencils=2, candles=3) bag: sack ['apple', 'cat'] {'pencils': 2, 'candles': 3} 47
48. Splatting >>> a, b, c = (1, 2, 3) >>> a, b, c (1, 2, 3) >>> d, e, f = [1, 2, 3] >>> d, e, f (1, 2, 3) >>> g, h, i = '123' >>> g, h, i ('1', '2', '3') 48
49. Classes 49
50. Syntax https://github.com/opendatazurich/ckanext-stadtzh- harvest/blob/master/ckanext/stadtzhharvest/harvester.py#L22 50
51. __init__() 51
52. Modules 52
53. Import syntax 53
54. __init__.py 54
55. Module search path 1. Built-in modules 2. The current directory 3. PYTHONPATH 4. Installation-dependent default 55
56. PYTHONPATH $ export PYTHONPATH=$PYTHONPATH: /path/to/some/package 56
57. Libraries 57
58. Built-in libraries datetime difflib random re os math … 58
59. Other useful libraries Requests BeautifulSoup lxml NumPy and SciPy nose Hypothesis (made by my housemate) … 59
60. Pip "The PyPA recommended tool for installing Python packages." (https://pip.pypa.io/en/latest/) 60
61. Pip $ wget https://bootstrap.pypa.io/get-pip.py $ python get-pip.py $ pip install <whatever> 61
62. Virtualenv 62
63. What does virtualenv do? "A tool to create isolated Python environments" (https://virtualenv.pypa.io/en/latest/) 63
64. How to use virtualenv? $ sudo pip install virtualenv $ virtualenv venv $ cd venv $ ls bin include lib local $ source bin/activate (venv) $ 64
65. Examples 65
66. File I/O https://gist.github. com/bellisk/577a41e20522dd28af07 Updates a metadata file and creates a diff. 66
67. Scraping https://github. com/bellisk/TestScraper/blob/master/scraper.py Uses Requests and BeautifulSoup to extract data from an online catalogue 67
68. Useful links 68
69. Useful links The Python docs: https://docs.python.org/3/ Django: https://www.djangoproject.com/ IPython interactive shells: http://ipython.org/ Introduction to Python (Harvard): http://tdc- www.harvard.edu/Python.pdf 69
70. Thanks! 70