The document discusses odd behaviors in Python related to identity, mutability, and scope. It provides examples testing the identity and mutability of various Python objects. It also discusses issues that can arise from using mutable default arguments and provides tips on how to avoid these issues, such as using None as a default instead of a mutable object.

1. 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!

4. Identity
Mutability
Scope
Themes

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

40. faves
Why?
"cats"
"dragons"
"rainbows"
>
temp.append("rainbows")
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

50. Mutable Default
Arguments

51. def
append_cat(l=[]):
!
Trivia

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

79. Namespaces!
!
• locals()
!
Why?

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

90. Thank you!
Questions?