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Python WATs: Uncovering Odd Behavior
- 2. Hacker School Alum Software Engineer at Venmo ! @amygdalama mathamy.com Who am I?
- 3. Allison Kaptur Tom Ballinger Gary Bernhardt Thanks!
- 5. a. Trivia b. Why? c. Tips for theme in themes:
- 6. Identity
- 7. > "" is "" ??? Trivia
- 8. > "" is "" True Trivia
- 9. > 0 is 0 ??? Trivia
- 10. > 0 is 0 True Trivia
- 11. > [] is [] ??? Trivia
- 12. > [] is [] False Trivia
- 13. > {} is {} ??? Trivia
- 14. > {} is {} False Trivia
- 15. > a = 256 > b = 256 > a is b ??? Trivia
- 16. > a = 256 > b = 256 > a is b True Trivia
- 17. > a = 257 > b = 257 > a is b ??? Trivia
- 18. > a = 257 > b = 257 > a is b False Trivia
- 19. > a = 257; b = 257 > a is b ??? Trivia
- 20. > a = 257; b = 257 > a is b True Trivia
- 21. a Why?-‐5 … 255 256 … > a = 256 ! !
- 22. a Why?-‐5 … 255 256 … b > a = 256 > b = 256 !
- 23. a Why?-‐5 … 255 256 … b > a = 256 > b = 256 > a is b True
- 24. a Why?… 255 256 … 257 > a = 257 ! !
- 25. a Why?… 255 256 … 257 257 b > a = 257 > b = 257 !
- 26. a Why?… 255 256 … 257 257 b > a = 257 > b = 257 > a is b False
- 27. > a = 257 > b = 257 > id(a) == id(b) False Why?
- 28. > a = 257; b = 257 > id(a) == id(b) True Why?
- 29. • Use == instead of is to compare values • Exceptions: • x is None • tests for identity Tips
- 30. Mutability
- 31. > faves = ["cats", "dragons"] ! ! ! Trivia
- 32. > faves = ["cats", "dragons"] > temp = faves ! ! Trivia
- 33. > faves = ["cats", "dragons"] > temp = faves > temp.append("rainbows") ! Trivia
- 34. > faves = ["cats", "dragons"] > temp = faves > temp.append("rainbows") > temp ??? Trivia
- 35. > faves = ["cats", "dragons"] > temp = faves > temp.append("rainbows") > temp ["cats", "dragons", "rainbows"] Trivia
- 36. > faves = ["cats", "dragons"] > temp = faves > temp.append("rainbows") > faves ??? Trivia
- 37. > faves = ["cats", "dragons"] > temp = faves > temp.append("rainbows") > faves ["cats", "dragons", "rainbows"] Trivia
- 38. faves Why? "cats" "dragons" > faves = ["cats", "dragons"]
- 39. faves Why? "cats" "dragons" > temp = faves temp
- 41. • lists [] • dictionaries {} • sets () Tip: Mutable Objects
- 42. > temp = faves[:] > temp = list(faves) Tip: Make Real Copies
- 43. > temp = [] > for element in faves: temp.append(element) Tip: Shallow Copy
- 44. > faves = [["cats", "dragons"]] ! ! ! Tip: Deep Copy
- 45. > faves = [["cats", "dragons"]] > temp = faves[:] ! ! Tip: Deep Copy
- 46. > faves = [["cats", "dragons"]] > temp = faves[:] > temp[0].append("rainbows") ! Tip: Deep Copy
- 47. > faves = [["cats", "dragons"]] > temp = faves[:] > temp[0].append("rainbows") > faves ??? Tip: Deep Copy
- 48. > faves = [["cats", "dragons"]] > temp = faves[:] > temp[0].append("rainbows") > faves [["cats", "dragons", "rainbows"]] Tip: Deep Copy
- 49. • For arbitrarily-nested lists, make deep copies ! > import copy > temp = copy.deepcopy(faves) Tip: Deep Copy
- 52. def append_cat(l=[]): l.append(‘cat’) return l Trivia
- 53. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat() ??? ! Trivia
- 54. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat() [‘cat’] ! Trivia
- 55. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat() [‘cat’] > append_cat() ??? Trivia
- 56. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat() [‘cat’] > append_cat() [‘cat’, ‘cat’] Trivia
- 57. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat.func_defaults ??? Why?
- 58. def append_cat(l=[]): l.append(‘cat’) return l ! > append_cat.func_defaults ([],) Why?
- 59. def append_cat(l=[]): l.append('cat') return l ! > append_cat() > append_cat.func_defaults ??? Why?
- 60. def append_cat(l=[]): l.append('cat') return l ! > append_cat() > append_cat.func_defaults (['cat'],) Why?
- 61. > append_cat.func_defaults (['cat'],) ! ! Why?
- 62. > append_cat.func_defaults (['cat'],) > _[0].append('dragon') ! Why?
- 63. > append_cat.func_defaults (['cat'],) > _[0].append('dragon') > append_cat() ??? Why?
- 64. > append_cat.func_defaults (['cat'],) > _[0].append('dragon') > append_cat() ['cat', 'dragon', 'cat'] Why?
- 65. def append_cat(l=None): ! ! ! Tip: Use None
- 66. def append_cat(l=None): if l is None: l = [] ! Tip: Use None
- 67. def append_cat(l=None): if l is None: l = [] l.append(‘cat') return l Tip: Use None
- 68. def sorting_hat(student, cache={}): ! ! ! ! ! ! Tip: Take Advantage!
- 69. def sorting_hat(student, cache={}): if student in cache: print "Used cache!" return cache[student] else: ! ! Tip: Take Advantage!
- 70. def sorting_hat(student, cache={}): if student in cache: print "Used cache!" return cache[student] else: house = slow_alg(student) cache[student] = house return house Tip: Take Advantage!
- 71. > sorting_hat('Amy Hanlon') 'Ravenclaw' ! ! ! ! Tip: Take Advantage!
- 72. > sorting_hat('Amy Hanlon') 'Ravenclaw' > sorting_hat('Amy Hanlon') Used cache! ‘Ravenclaw' ! Tip: Take Advantage!
- 73. > sorting_hat('Amy Hanlon') 'Ravenclaw' > sorting_hat('Amy Hanlon') Used cache! ‘Ravenclaw' > sorting_hat.func_defaults ({'Amy Hanlon': 'Ravenclaw'},) Tip: Take Advantage!
- 74. Scope
- 75. > a = 1 ! ! ! ! Trivia
- 76. > a = 1 > def foo(): return a ! ! Trivia
- 77. > a = 1 > def foo(): return a ! > foo() ??? Trivia
- 78. > a = 1 > def foo(): return a ! > foo() 1 Trivia
- 80. Namespaces! ! • locals() • globals() # {'a' : 1} Why?
- 81. Namespaces! ! • locals() • globals() # {'a' : 1} • __builtins__ Why?
- 82. > a = 1 > def foo(): return a ! > foo() 1 Trivia
- 83. > a = 1 > def foo(): a += 1 return a ! > foo() ??? Trivia
- 84. > a = 1 > def foo(): a += 1 return a ! > foo() UnboundLocalError: local variable 'a' referenced before assignment Trivia
- 85. “When you make an assignment to a variable in a scope, that variable becomes local to that scope.” Why?
- 86. Why? > a = 1 > def foo(): # a += 1 a = a + 1 return a
- 87. > a = 1 > def foo(): global a a += 1 return a > foo() 2 > a 2 Tip: Use global
- 88. > def foo(a): a += 1 return a > a = 1 > a = foo(a) 2 Tip: Don’t Use global
- 89. Links • https://docs.python.org/3/ • http://eli.thegreenplace.net/ • http://akaptur.github.io/blog/ 2013/10/29/a-python-puzzle/ • http://mathamy.com/python-wats- mutable-default-arguments.html