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Investigating Python Wats

Amy Hanlon, profile picture
Amy Hanlon

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
Venmo Hacker School Recurse Center ! @amygdalama mathamy.com
WAT
Identity Mutability Scope
Identity
>>>  a  =  256   >>>  b  =  256   >>>  a  is  b   ???
>>>  a  =  256   >>>  b  =  256   >>>  a  is  b   True
>>>  a  =  257   >>>  b  =  257   >>>  a  is  b   ???
>>>  a  =  257   >>>  b  =  257   >>>  a  is  b   False
$  python   ! ! -­‐5 … 255 256
-­‐5 … 255 256 a >>>  a  =  256   ! !
b a >>>  a  =  256   >>>  b  =  256   ! -­‐5 … 255 256
b a >>>  a  =  256   >>>  b  =  256   >>>  a  is  b   True -­‐5 … 255 256
a … 255 256 … 257 >>>  a  =  257   ! !
a … 255 256 … 257 257 b >>>  a  =  257   >>>  b  =  257   !
a … 255 256 … 257 257 b >>>  a  =  257   >>>  b  =  257   >>>  a  is  b   False
>>>  a  =  257;  b  =  257   >>>  a  is  b   ???
>>>  a  =  257;  b  =  257   >>>  a  is  b   True
>>>  a  =  257   >>>  b  =  257   >>>  a  is  b   False   ! >>>  a  =  257;  b  =  257   >>>  a  is  b   True
>>>  a  =  257   >>>   ! ! ! ! !
>>>  a  =  257   >>>  source  =  "a  =  257"   >>>  code_obj_a  =  compile(   ...          source=source,   ...          filename="",   ...          mode="exec")   !
>>>  a  =  257   >>>  source  =  "a  =  257"   >>>  code_obj_a  =  compile(   ...          source=source,   ...          filename="",   ...          mode="exec")   >>>  code_obj_a.co_consts   (257,  None)
>>>  source  =  "b  =  257"   >>>  code_obj_b  =  compile(   ...          source=source,   ...          filename="",   ...          mode="exec")   !
>>>  source  =  "b  =  257"   >>>  code_obj_b  =  compile(   ...          source=source,   ...          filename="",   ...          mode="exec")   >>>  code_obj_b.co_consts   (257,  None)
>>>  source  =  "a  =  257;  b  =  257"   ! ! ! ! !
>>>  source  =  "a  =  257;  b  =  257"   >>>  code_obj  =  compile(     ...          source=source,   ...          filename="",   ...          mode="exec")   !
>>>  source  =  "a  =  257;  b  =  257"   >>>  code_obj  =  compile(     ...          source=source,   ...          filename="",   ...          mode="exec")   >>>  code_obj.co_consts   (257,  None)
Mutability
>>>  row  =  [""]  *  3   >>>  row   ['',  '',  '']   ! ! ! !
>>>  row  =  [""]  *  3   >>>  row   ['',  '',  '']   >>>  board  =  [row]  *  3   ! ! !
>>>  row  =  [""]  *  3   >>>  row   ['',  '',  '']   >>>  board  =  [row]  *  3   >>>  board   [['',  '',  ''],    ['',  '',  ''],    ['',  '',  '']]
>>>  board[0]   ['',  '',  '']   !
>>>  board[0]   ['',  '',  '']   >>>  board[0][0]   ''
>>>  board[0][0]  =  "X"   ! ! !
>>>  board[0][0]  =  "X"   >>>  board   ???   !
>>>  board[0][0]  =  "X"   >>>  board   [['X',  '',  ''],    ['X',  '',  ''],    ['X',  '',  '']]
row "" "" "" >>>  row  =  [""]  *  3
>>>  board  =  [row]  *  3 row board[0] board[1] board[2] "" "" ""
>>>  board[0][0]  =  "X" row board[0] board[1] board[2] "X" "" ""
Mutable Default Arguments
def  append_cat(l=[]):   ! ! ! ! ! !
def  append_cat(l=[]):          l.append('cat')          return  l   ! ! ! !
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat()   ???   !
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat()   ['cat']   !
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat()   ['cat']   >>>  append_cat()   ???
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat()   ['cat']   >>>  append_cat()   ['cat',  'cat']
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat.__defaults__   ???   !
def  append_cat(l=[]):          l.append(‘cat’)          return  l   ! >>>  append_cat.__defaults__   ([],)   !
def  append_cat(l=[]):          l.append('cat')          return  l   ! >>>  append_cat()   >>>  append_cat.__defaults__   ???  
def  append_cat(l=[]):          l.append('cat')          return  l   ! >>>  append_cat()   >>>  append_cat.__defaults__   (['cat'],)  
>>>  append_cat.__defaults__   (['cat'],)   ! !
>>>  append_cat.__defaults__   (['cat'],)   >>>  _[0].append('dragon')   !
>>>  append_cat.__defaults__   (['cat'],)   >>>  _[0].append('dragon')   >>>  append_cat()   ???
>>>  append_cat.__defaults__   (['cat'],)   >>>  _[0].append('dragon')   >>>  append_cat()   ['cat',  'dragon',  'cat']
Scope
>>>  a  =  1   ! ! ! !
>>>  a  =  1   >>>  def  foo():   ...          return  a   ! !
>>>  a  =  1   >>>  def  foo():   ...          return  a   ! >>>  foo()   ???
>>>  a  =  1   >>>  def  foo():   ...          return  a   ! >>>  foo()   1
>>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   ! ! >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   ! ! >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   • enclosing        #  {}   ! >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   • enclosing        #  {}   ! >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   • enclosing        #  {}   • globals()        #  {'a':  1}   >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   ???
• locals()          #  {}   • enclosing        #  {}   ✓ globals()        #  {'a':  1}   >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   1
• locals()          #  {}   • enclosing        #  {}   ✓ globals()        #  {'a':  1}   • builtins          #  {'True':  True,…} >>>  a  =  1   >>>  def  foo():   ...          return  a   >>>  foo()   1
>>>  a  =  1   >>>  def  foo():   ...          return  a   ! >>>  from  dis  import  dis   ! !
>>>  a  =  1   >>>  def  foo():   ...          return  a   ! >>>  from  dis  import  dis   >>>  dis(foo)      2              0  LOAD_GLOBAL                    0  (a)                        3  RETURN_VALUE      
>>>  a  =  1   >>>  def  foo():   ...          a  +=  1   ...          return  a   ! >>>  foo()   ???
>>>  a  =  1   >>>  def  foo():   ...          a  +=  1   ...          return  a   ! >>>  foo()   UnboundLocalError:  local   variable  'a'  referenced   before  assignment
“When you make an assignment to a variable in a scope, that variable becomes local to that scope.”
>>>  a  =  1   >>>  def  foo():   ...          #  a  +=  1   ...          a  =  a  +  1   ...          return  a
>>>  a  =  1   >>>  def  foo():   ...          #  a  +=  1   ...          a  =  a  +  1   ...          return  a
>>>  a  =  1   >>>  def  foo():   ...          #  a  +=  1   ...          a  =  a  +  1   ...          return  a
>>>  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            
>>>  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            
“Knowledge is power — it’s measured in wats.” ! - Rose Ames
Thank you! @amygdalama mathamy.com
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/

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Investigating Python Wats

  • 3. WAT
  • 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
  • 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)
  • 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/