The document provides an overview of Python lists, including how to create and access list elements using indexes, loop through lists, modify lists by adding, removing, or changing elements, and several common list methods. Key points include: lists allow storing multiple values in a single variable; values can be accessed by index with list[index]; common operations include append() to add elements, sort() and reverse() modify the list in-place, and in tests for membership in a list.

1. Python
Lists
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.

2. Loops let us do things many times
Python Lists

3. Loops let us do things many times
Collections let us store many values together
Python Lists

4. Loops let us do things many times
Collections let us store many values together
Most popular collection is a list
Python Lists

5. Create using [value, value, ...]
Python Lists

6. Create using [value, value, ...]
Get/set values using var[index]
Python Lists

7. Create using [value, value, ...]
Get/set values using var[index]
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases
['He', 'Ne', 'Ar', 'Kr']
Python Lists

8. Create using [value, value, ...]
Get/set values using var[index]
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases
['He', 'Ne', 'Ar', 'Kr']
print gases[1]
Ne
Python Lists

9. Index from 0, not 1
Python Lists

10. Index from 0, not 1
Reasons made sense for C in 1970...
Python Lists

11. Index from 0, not 1
Reasons made sense for C in 1970...
It's an error to try to access out of range
Python Lists

12. Index from 0, not 1
Reasons made sense for C in 1970...
It's an error to try to access out of range
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases[4]
IndexError: list index out of range
Python Lists

13. Use len(list) to get length of list
Python Lists

14. Use len(list) to get length of list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4
Python Lists

15. Use len(list) to get length of list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4
Returns 0 for the empty list
etheric = []
print len(etheric)
0
Python Lists

16. Some negative indices work
Python Lists

17. Some negative indices work
values[-1] is last element, values[-2] next-to-last, ...
Python Lists

18. Some negative indices work
values[-1] is last element, values[-2] next-to-last, ...
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

19. Some negative indices work
values[-1] is last element, values[-2] next-to-last, ...
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases[-1], gases[-4]
Kr He
Python Lists

20. Some negative indices work
values[-1] is last element, values[-2] next-to-last, ...
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases[-1], gases[-4]
Kr He
values[-1] is much nicer than values[len(values)-Python Lists

21. Some negative indices work
values[-1] is last element, values[-2] next-to-last, ...
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases[-1], gases[-4]
Kr He
values[-1] is much nicer than values[len(values)-less error prone
Python Lists

22. Mutable : can change it after it is created
Python Lists

23. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
Python Lists

24. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
gases[3] = 'Kr'
Python Lists

25. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
gases[3] = 'Kr'
print gases
['He', 'Ne', 'Ar', 'Kr']
Python Lists

26. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
gases[3] = 'Kr'
print gases
['He', 'Ne', 'Ar', 'Kr']
Location must exist before assignment
Python Lists

27. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
gases[3] = 'Kr'
print gases
['He', 'Ne', 'Ar', 'Kr']
Location must exist before assignment
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

28. Mutable : can change it after it is created
gases = ['He', 'Ne', 'Ar', 'K'] # last entry misspelled
gases[3] = 'Kr'
print gases
['He', 'Ne', 'Ar', 'Kr']
Location must exist before assignment
gases = ['He', 'Ne', 'Ar', 'Kr']
gases[4] = 'Xe'
IndexError: list assignment index out of range
Python Lists

29. Heterogeneous : can store values of many kinds
Python Lists

30. Heterogeneous : can store values of many kinds
helium = ['He', 2]
neon = ['Ne', 8]
Python Lists

31. Heterogeneous : can store values of many kinds
helium = ['He', 2]
[string, int]
neon = ['Ne', 8]
Python Lists

32. Heterogeneous : can store values of many kinds
helium = ['He', 2]
neon = ['Ne', 8]
'He
'
2
'Ne'
8
helium
neon
Python Lists

33. Heterogeneous : can store values of many kinds
helium = ['He', 2]
neon = ['Ne', 8]
gases = [helium, neon]
Python Lists

34. Heterogeneous : can store values of many kinds
helium = ['He', 2]
neon = ['Ne', 8]
gases = [helium, neon]
'He
'
2
'Ne'
8
helium
gases
neon
Python Lists

35. Heterogeneous : can store values of many kinds
helium = ['He', 2]
neon = ['Ne', 8]
gases = [helium, neon]
'He
'
2
'Ne'
8
helium
gases
neon
Devote a whole
episode to this
Python Lists

36. Loop over elements to "do all"
Python Lists

37. Loop over elements to "do all"
Use while to step through all possible indices
Python Lists

38. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
Python Lists

39. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
First legal index
Python Lists

40. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1 Next index
Python Lists

41. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
Defines set of legal indices
Python Lists

42. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
He
Ne
Ar
Kr
Python Lists

43. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
He
Ne
Ar
Kr
Tedious to type in over and over again
Python Lists

44. Loop over elements to "do all"
Use while to step through all possible indices
gases = ['He', 'Ne', 'Ar', 'Kr']
i = 0
while i < len(gases):
print gases[i]
i += 1
He
Ne
Ar
Kr
Tedious to type in over and over again
And it's easy to forget the "+= 1" at the end
Python Lists

45. Use a for loop to access each value in turn
Python Lists

46. Use a for loop to access each value in turn
gases = ['He', 'Ne', 'Ar', 'Kr']
for gas in gases:
print gas
He
Ne
Ar
Kr
Python Lists

47. Use a for loop to access each value in turn
gases = ['He', 'Ne', 'Ar', 'Kr']
for gas in gases:
print gas
He
Ne
Ar
Kr
Loop variable assigned each value in turn
Python Lists

48. Use a for loop to access each value in turn
gases = ['He', 'Ne', 'Ar', 'Kr']
for gas in gases:
print gas
He
Ne
Ar
Kr
Loop variable assigned each value in turn
Not each index
Python Lists

49. Use a for loop to access each value in turn
gases = ['He', 'Ne', 'Ar', 'Kr']
for gas in gases:
print gas
He
Ne
Ar
Kr
Loop variable assigned each value in turn
Not each index
Because that's the most common case
Python Lists

50. Can delete entries entirely (shortens the list)
Python Lists

51. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

52. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
del gases[0]
Python Lists

53. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
del gases[0]
print gases
['Ne', 'Ar', 'Kr']
Python Lists

54. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
del gases[0]
print gases
['Ne', 'Ar', 'Kr']
del gases[2]
Python Lists

55. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
del gases[0]
print gases
['Ne', 'Ar', 'Kr']
del gases[2]
print gases
['Ne', 'Ar']
Python Lists

56. Can delete entries entirely (shortens the list)
gases = ['He', 'Ne', 'Ar', 'Kr']
del gases[0]
print gases
['Ne', 'Ar', 'Kr']
del gases[2]
print gases
['Ne', 'Ar']
Yes, deleting an index that doesn't exist is an error
Python Lists

57. Appending values to a list lengthens it
Python Lists

58. Appending values to a list lengthens it
gases = []
Python Lists

59. Appending values to a list lengthens it
gases = []
gases.append('He')
Python Lists

60. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
Python Lists

61. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
gases.append('Ar')
Python Lists

62. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
gases.append('Ar')
print gases
['He', 'Ne', 'Ar']
Python Lists

63. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
gases.append('Ar')
print gases
['He', 'Ne', 'Ar']
Most operations on lists are methods
Python Lists

64. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
gases.append('Ar')
print gases
['He', 'Ne', 'Ar']
Most operations on lists are methods
A function that belongs to (and usually operates on)
specific data
Python Lists

65. Appending values to a list lengthens it
gases = []
gases.append('He')
gases.append('Ne')
gases.append('Ar')
print gases
['He', 'Ne', 'Ar']
Most operations on lists are methods
A function that belongs to (and usually operates on)
specific data
thing . method (args)
Python Lists

66. Some useful list methods
Python Lists

67. Some useful list methods
gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated
Python Lists

68. Some useful list methods
gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated
print gases.count('He')
2
Python Lists

69. Some useful list methods
gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated
print gases.count('He')
2 print gases.index('Ar')
2
Python Lists

70. Some useful list methods
gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated
print gases.count('He')
2 print gases.index('Ar')
2 gases.insert(1, 'Ne')
Python Lists

71. Some useful list methods
gases = ['He', 'He', 'Ar', 'Kr'] # 'He' is duplicated
print gases.count('He')
2 print gases.index('Ar')
2 gases.insert(1, 'Ne')
print gases
['He', 'Ne', 'He', 'Ar', 'Kr']
Python Lists

72. Two that are often used incorrectly
Python Lists

73. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

74. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
Python Lists

75. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
print gases
['Ar', 'He', 'Kr', 'Ne']
Python Lists

76. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
print gases
['Ar', 'He', 'Kr', 'Ne']
print gases.reverse()
None
Python Lists

77. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
print gases
['Ar', 'He', 'Kr', 'Ne']
print gases.reverse()
None
print gases
['Ne', 'Kr', 'He', 'Ar']
Python Lists

78. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
print gases
['Ar', 'He', 'Kr', 'Ne']
print gases.reverse()
None
print gases
['Ne', 'Kr', 'He', 'Ar']
A common bug
Python Lists

79. Two that are often used incorrectly
gases = ['He', 'Ne', 'Ar', 'Kr']
print gases.sort()
None
print gases
['Ar', 'He', 'Kr', 'Ne']
print gases.reverse()
None
print gases
['Ne', 'Kr', 'He', 'Ar']
A common bug
gases = gases.sort() assigns None to gases
Python Lists

80. Use in to test for membership
Python Lists

81. Use in to test for membership
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

82. Use in to test for membership
gases = ['He', 'Ne', 'Ar', 'Kr']
print 'He' in gases
True
Python Lists

83. Use in to test for membership
gases = ['He', 'Ne', 'Ar', 'Kr']
print 'He' in gases
True
if 'Pu' in gases:
print 'But plutonium is not a gas!'
else:
print 'The universe is well ordered.'
Python Lists

84. Use in to test for membership
gases = ['He', 'Ne', 'Ar', 'Kr']
print 'He' in gases
True
if 'Pu' in gases:
print 'But plutonium is not a gas!'
else:
print 'The universe is well ordered.'
The universe is well ordered.
Python Lists

85. Use range to construct lists of numbers
Python Lists

86. Use range to construct lists of numbers
print range(5)
[0, 1, 2, 3, 4]
Python Lists

87. Use range to construct lists of numbers
print range(5)
[0, 1, 2, 3, 4]
print range(2, 6)
[2, 3, 4, 5]
Python Lists

88. Use range to construct lists of numbers
print range(5)
[0, 1, 2, 3, 4]
print range(2, 6)
[2, 3, 4, 5]
print range(0, 10, 3)
[0, 3, 6, 9]
Python Lists

89. Use range to construct lists of numbers
print range(5)
[0, 1, 2, 3, 4]
print range(2, 6)
[2, 3, 4, 5]
print range(0, 10, 3)
[0, 3, 6, 9]
print range(10, 0)
[]
Python Lists

90. So range(len(list)) is all indices for the list
Python Lists

91. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
Python Lists

92. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4
Python Lists

93. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4 print range(len(gases))
[0, 1, 2, 3]
Python Lists

94. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4 print range(len(gases))
[0, 1, 2, 3]
for i in range(len(gases)):
print i, gases[i]
Python Lists

95. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4 print range(len(gases))
[0, 1, 2, 3]
for i in range(len(gases)):
print i, gases[i]
0 He
1 Ne
2 Ar
3 Kr
Python Lists

96. So range(len(list)) is all indices for the list
gases = ['He', 'Ne', 'Ar', 'Kr']
print len(gases)
4 print range(len(gases))
[0, 1, 2, 3]
for i in range(len(gases)):
print i, gases[i]
0 He
1 Ne
2 Ar
3 Kr
A very common idiom in Python
Python Lists

97. narrated by
Dominique Vuvan
October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.