Investigating Python Wats
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The document discusses Python concepts related to identity, mutability, and scope. It provides examples to illustrate that integers with values below 256 have shared identity due to interning, while higher values do not. It also demonstrates how lists are mutable, so modifying nested list elements modifies the original lists. The document shows how default arguments are evaluated only once at definition time, leading to unexpected behavior. Finally, it gives an example of how variables defined in the global scope are accessible from within a function.
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Investigating Python Wats
- 3. WAT
- 5. Identity
- 6. >>> a = 256 >>> b = 256 >>> a is b ???
- 7. >>> a = 256 >>> b = 256 >>> a is b True
- 8. >>> a = 257 >>> b = 257 >>> a is b ???
- 9. >>> a = 257 >>> b = 257 >>> a is b False
- 11. -‐5 … 255 256 a >>> a = 256 ! !
- 12. b a >>> a = 256 >>> b = 256 ! -‐5 … 255 256
- 13. b a >>> a = 256 >>> b = 256 >>> a is b True -‐5 … 255 256
- 14. a … 255 256 … 257 >>> a = 257 ! !
- 15. a … 255 256 … 257 257 b >>> a = 257 >>> b = 257 !
- 16. a … 255 256 … 257 257 b >>> a = 257 >>> b = 257 >>> a is b False
- 17. >>> a = 257; b = 257 >>> a is b ???
- 18. >>> a = 257; b = 257 >>> a is b True
- 19. >>> a = 257 >>> b = 257 >>> a is b False ! >>> a = 257; b = 257 >>> a is b True
- 20. >>> a = 257 >>> ! ! ! ! !
- 21. >>> a = 257 >>> source = "a = 257" >>> code_obj_a = compile( ... source=source, ... filename="", ... mode="exec") !
- 22. >>> a = 257 >>> source = "a = 257" >>> code_obj_a = compile( ... source=source, ... filename="", ... mode="exec") >>> code_obj_a.co_consts (257, None)
- 23. >>> source = "b = 257" >>> code_obj_b = compile( ... source=source, ... filename="", ... mode="exec") !
- 24. >>> source = "b = 257" >>> code_obj_b = compile( ... source=source, ... filename="", ... mode="exec") >>> code_obj_b.co_consts (257, None)
- 25. >>> source = "a = 257; b = 257" ! ! ! ! !
- 26. >>> source = "a = 257; b = 257" >>> code_obj = compile( ... source=source, ... filename="", ... mode="exec") !
- 27. >>> source = "a = 257; b = 257" >>> code_obj = compile( ... source=source, ... filename="", ... mode="exec") >>> code_obj.co_consts (257, None)
- 28. Mutability
- 29. >>> row = [""] * 3 >>> row ['', '', ''] ! ! ! !
- 30. >>> row = [""] * 3 >>> row ['', '', ''] >>> board = [row] * 3 ! ! !
- 31. >>> row = [""] * 3 >>> row ['', '', ''] >>> board = [row] * 3 >>> board [['', '', ''], ['', '', ''], ['', '', '']]
- 32. >>> board[0] ['', '', ''] !
- 33. >>> board[0] ['', '', ''] >>> board[0][0] ''
- 34. >>> board[0][0] = "X" ! ! !
- 35. >>> board[0][0] = "X" >>> board ??? !
- 36. >>> board[0][0] = "X" >>> board [['X', '', ''], ['X', '', ''], ['X', '', '']]
- 37. row "" "" "" >>> row = [""] * 3
- 38. >>> board = [row] * 3 row board[0] board[1] board[2] "" "" ""
- 39. >>> board[0][0] = "X" row board[0] board[1] board[2] "X" "" ""
- 42. def append_cat(l=[]): l.append('cat') return l ! ! ! !
- 43. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat() ??? !
- 44. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat() ['cat'] !
- 45. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat() ['cat'] >>> append_cat() ???
- 46. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat() ['cat'] >>> append_cat() ['cat', 'cat']
- 47. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat.__defaults__ ??? !
- 48. def append_cat(l=[]): l.append(‘cat’) return l ! >>> append_cat.__defaults__ ([],) !
- 49. def append_cat(l=[]): l.append('cat') return l ! >>> append_cat() >>> append_cat.__defaults__ ???
- 50. def append_cat(l=[]): l.append('cat') return l ! >>> append_cat() >>> append_cat.__defaults__ (['cat'],)
- 52. >>> append_cat.__defaults__ (['cat'],) >>> _[0].append('dragon') !
- 53. >>> append_cat.__defaults__ (['cat'],) >>> _[0].append('dragon') >>> append_cat() ???
- 54. >>> append_cat.__defaults__ (['cat'],) >>> _[0].append('dragon') >>> append_cat() ['cat', 'dragon', 'cat']
- 55. Scope
- 56. >>> a = 1 ! ! ! !
- 57. >>> a = 1 >>> def foo(): ... return a ! !
- 58. >>> a = 1 >>> def foo(): ... return a ! >>> foo() ???
- 59. >>> a = 1 >>> def foo(): ... return a ! >>> foo() 1
- 60. >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 61. • locals() # {} ! ! >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 62. • locals() # {} ! ! >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 63. • locals() # {} • enclosing # {} ! >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 64. • locals() # {} • enclosing # {} ! >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 65. • locals() # {} • enclosing # {} • globals() # {'a': 1} >>> a = 1 >>> def foo(): ... return a >>> foo() ???
- 66. • locals() # {} • enclosing # {} ✓ globals() # {'a': 1} >>> a = 1 >>> def foo(): ... return a >>> foo() 1
- 67. • locals() # {} • enclosing # {} ✓ globals() # {'a': 1} • builtins # {'True': True,…} >>> a = 1 >>> def foo(): ... return a >>> foo() 1
- 68. >>> a = 1 >>> def foo(): ... return a ! >>> from dis import dis ! !
- 69. >>> a = 1 >>> def foo(): ... return a ! >>> from dis import dis >>> dis(foo) 2 0 LOAD_GLOBAL 0 (a) 3 RETURN_VALUE
- 70. >>> a = 1 >>> def foo(): ... a += 1 ... return a ! >>> foo() ???
- 71. >>> a = 1 >>> def foo(): ... a += 1 ... return a ! >>> foo() UnboundLocalError: local variable 'a' referenced before assignment
- 72. “When you make an assignment to a variable in a scope, that variable becomes local to that scope.”
- 73. >>> a = 1 >>> def foo(): ... # a += 1 ... a = a + 1 ... return a
- 74. >>> a = 1 >>> def foo(): ... # a += 1 ... a = a + 1 ... return a
- 75. >>> a = 1 >>> def foo(): ... # a += 1 ... a = a + 1 ... return a
- 76. >>> a = 1 >>> def foo(): ... a += 1 ... return a ... >>> dis(foo) 2 0 LOAD_FAST 0 (a) 3 LOAD_CONST 1 (1) 6 INPLACE_ADD 7 STORE_FAST 0 (a) ! 3 10 LOAD_FAST 0 (a) 13 RETURN_VALUE
- 77. >>> a = 1 >>> def foo(): ... a += 1 ... return a ... >>> dis(foo) 2 0 LOAD_FAST 0 (a) 3 LOAD_CONST 1 (1) 6 INPLACE_ADD 7 STORE_FAST 0 (a) ! 3 10 LOAD_FAST 0 (a) 13 RETURN_VALUE
- 78. “Knowledge is power — it’s measured in wats.” ! - Rose Ames
- 80. Links • https://www.destroyallsoftware.com/talks/wat • http://akaptur.github.io/blog/2013/10/29/a-python-puzzle/ • https://docs.python.org/3.4/c-api/long.html • https://docs.python.org/3/reference/ compound_stmts.html#function-definitions • http://effbot.org/zone/default-values.htm • https://docs.python.org/3/reference/ executionmodel.html#naming-and-binding • http://eli.thegreenplace.net/2011/05/15/understanding- unboundlocalerror-in-python/ • http://rose.github.io/posts/measured-in-wats/