Pytfalls

Conhecendo e explorando comportamentos não intuitivos do Python

Transcript

1. PYTFALLS
2. WHOAMI ▸ desenvolvedor na Jusbrasil ▸ contribui com projetos open source como Django, Docker, OpenStack, pypy, splinter, tsuru ▸ github.com/andrewsmedina
3. PYTFALLS??!!
4. A HIDDEN OR UNSUSPECTED DANGER OR DIFFICULTY PITFALL
5. AN UNEXPECTED PROBLEM OR USUALLY UNPLEASANT SURPRISE GOTCHA
9. Há muito tempo, numa linguagem muito distante…
10. > 1 + 1
11. > 1 + 1 2
12. > 1 + “1”
13. > 1 + “1” “11”
14. > 1 + “1” “11”
15. > “11” - 1
16. > “11” - 1
17. > “11” - 1 10
18. > “11” - 1 10
19. > “JS” - 1
20. > “JS” - 1 NaN
21. > “JS” - 1 NaN
22. > Array(16).join("JS" - 2) + " Batman!"
23. > Array(16).join("JS" - 2) + " Batman!" "NaNNaNNaNNaNNaNNaNNaNNaNNa NNaNNaNNaNNaNNaNNaN Batman!"
25. PYTHON
26. É UMA LINGUAGEM INTUITIVA PYTHON
27. ZEN OF PYTHON PYTHON
28. É LEGÍVEL PYTHON
29. CRIADA EM 1991 PYTHON
30. “There should be one-- and preferably only one --obvious way to do it.”
31. urllib httplib urllib3
32. urllib httplib urllib3
33. “Explicit is better than implicit.”
34. >>> "Oi" + u" mundo" u'Oi mundo'
35. >>> "Oi" + u" mundo" u'Oi mundo'
36. #1
37. Python 2
38. USE PYTHON3 TL;DR
39. #2
40. def append(element, to=[]): to.append(element) return to
41. >>> my_list = append(12) >>> print(my_list) ??? >>> my_other_list = append(42) print(my_other_list) ???
42. >>> my_list = append(12) >>> print(my_list) [12] >>> my_other_list = append(42) print(my_other_list) ???
43. >>> my_list = append(12) >>> print(my_list) [12] >>> my_other_list = append(42) print(my_other_list) [12, 42]
44. >>> my_list = append(12) >>> print(my_list) [12] >>> my_other_list = append(42) print(my_other_list) [12, 42]
45. def append(element, to=[]): to.append(element) return to
46. def append(element, to=None): if to is None: to = [] to.append(element) return to
47. def fib(n): if n <= 2: data = 1 else: data = fib(n - 2) + fib(n - 1) return data
48. def fib(n, cache={}): if n not in cache: if n <= 2: data = 1 else: data = fib(n - 2) + fib(n - 1) cache[n] = data return cache[n]
49. #3
50. def create_multipliers(): return [lambda x : i * x for i in range(5)]
51. for multiplier in create_multipliers(): print multiplier(2)
52. for multiplier in create_multipliers(): print multiplier(2) ? ? ? ?
53. for multiplier in create_multipliers(): print multiplier(2) ? (0) ? (2) ? (4) ? (6)
54. for multiplier in create_multipliers(): print multiplier(2) 8 8 8 8
55. for multiplier in create_multipliers(): print multiplier(2) 8 8 8 8
56. def create_multipliers(): return [lambda x : i * x for i in range(5)]
57. def create_multipliers(): multipliers = [] for i in range(5): def multiplier(x): return i * x multipliers.append(multiplier) return multipliers
58. def soma(x, y): return x+y
59. soma = lambda x, y: x+y
60. def soma(x, y): return x+y
61. def create_multipliers(): return [lambda x : i * x for i in range(5)]
62. def create_multipliers(): multipliers = [] for i in range(5): def multiplier(x): return i * x multipliers.append(multiplier) return multipliers
63. def create_multipliers(): return [lambda x, i=i: i * x for i in range(5)]
64. #4
65. >>> a = 256 >>> b = 256 >>> a is b True
66. >>> a = 257 >>> b = 257 >>> a is b ???
67. >>> a = 257 >>> b = 257 >>> a is b False
68. >>> a = 257 >>> b = 257 >>> a is b False
69. >>> a = 256 >>> b = 256 >>> a is b True
70. >>> help(id)
71. >>> help(id) id(obj, /) Return the identity of an object.
72. >>> a = 256 >>> id(a) 4307458704 >>> b = 256 >>> id(b) 4307458704
73. >>> a = 257 >>> id(257) 4312868592 >>> b = 257 >>> id(b) 4311130416
74. #5
75. # random.py import random random.randrange(100)
76. $ python random.py Traceback (most recent call last): File "random.py", line 1, in <module> import random File "random.py", line 3, in <module> print(random.randrange(100)) AttributeError: 'module' object has no attribute 'randrange'
77. #6
78. *.pyc
79. $ find . -name "*.pyc" -delete
80. export PYTHONDONTWRITEBYTECODE=1
81. >>> import sys >>> sys.dont_write_bytecode True
82. $ python -B main.py
83. #7
84. >>> linguagens = {} >>> linguagens[1] = “Python” >>> linguagens[1.0] = “Go” >>> linguagens[1] “Go”
85. >>> hash(1) == hash(1.0) True
86. #8
87. >>> x = [[]] * 5 >>> x [[], [], [], [], []] >>> x[0].append(1) ????
88. >>> x[0].append(1) ????
89. >>> x[0].append(1) # [[0], [], [], [], []] ok?
90. >>> x[0].append(1) [[1], [1], [1], [1], [1]]
91. >>> x[0].append(1) [[1], [1], [1], [1], [1]]
92. >>> id(x[0]) 4390176008 >>> id(x[1]) 4390176008
93. #9
94. class Hero: life = 100 class Dragon(Hero): pass class Giant(Hero): pass
95. >>> Hero.life, Giant.life, Dragon.life 100, 100, 100
96. >>> Hero.life = 10 >>> Hero.life, Giant.life, Dragon.life
97. >>> Hero.life = 10 >>> Hero.life, Giant.life, Dragon.life ?, ?, ?
98. >>> Hero.life = 10 >>> Hero.life, Giant.life, Dragon.life 10, 10, 10
99. >>> Giant.life = 300 >>> Hero.life, Giant.life, Dragon.life ?, ?, ?
100. >>> Giant.life = 300 >>> Hero.life, Giant.life, Dragon.life 10, 300, 10
101. >>> Hero.life = 50 >>> Hero.life, Giant.life, Dragon.life ?, ?, ?
102. >>> Hero.life = 50 >>> Hero.life, Giant.life, Dragon.life 50, 300, 50
103. >>> Hero.life = 50 >>> Hero.life, Giant.life, Dragon.life 50, 300, 50
104. Hero (life=50) Giant (life=300) Dragon
105. #10
106. class Hero: spells = []
107. >>> dragon = Hero() >>> dragon.spells.append(“fireball”) >>> dragon.spells [“fireball”]
108. >>> priest = Hero() >>> priest.spells.append(“heal”) >>> priest.spells ???
109. >>> priest = Hero() >>> priest.spells.append(“heal”) >>> priest.spells [“fireball”, “heal”]
110. >>> priest = Hero() >>> priest.spells.append(“heal”) >>> priest.spells [“fireball”, “heal”]
111. class Hero: spells = []
112. class Hero: def __init__(self): self.spells = []
113. >>> dragon = Hero() >>> dragon.spells.append(“fireball”) >>> dragon.spells [“fireball”] >>> priest = Hero() >>> priest.spells.append(“heal”) >>> priest.spells [“heal”]
114. #DICAS
118. OBRIGADO!