1. Python Aliasing Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.

2. An alias is a second name for a piece of data2

3. An alias is a second name for a piece of data
Often easier (and more useful) than making a
second copy 3

4. An alias is a second name for a piece of data
Often easier (and more useful) than making a
second copy
If the data is immutable, aliases don't matter 4

5. An alias is a second name for a piece of data
Often easier (and more useful) than making a
second copy
If the data is immutable, aliases don't matter
Because the data can't change 5

6. An alias is a second name for a piece of data
Often easier (and more useful) than making a
second copy
If the data is immutable, aliases don't matter
Because the data can't change
But if data can change, aliases can result in a lot
of hard-to-find bugs 6

7. Aliasing happens whenever one variable's value
is assigned to another variable 7

8. Aliasing happens whenever one variable's value
is assigned to another variable
first = 'isaac'
variable
value
first
'isaac' 8

9. Aliasing happens whenever one variable's value
is assigned to another variable
first = 'isaac'
second = first
variable
value
first
second
'isaac' 9

10. Aliasing happens whenever one variable's value
is assigned to another variable
first = 'isaac'
second = first
But as we've already seen…
variable
value
first
second
'isaac' 10

11. Aliasing happens whenever one variable's value
is assigned to another variable
first = 'isaac'
second = first
But as we've already seen…
first = first + ' newton'
variable
value
first
second
'isaac'
'isaac newton' 11

12. But lists are mutable 12

13. But lists are mutable first = ['isaac'] variable value first 'isaac'13

14. But lists are mutable first = ['isaac'] second = first variable value14

15. But lists are mutable first = ['isaac'] second = first
first = first.append('newton')
print first
['isaac', 'newton']
variable
value
first
second
'isaac' 'newton' 15

16. But lists are mutable first = ['isaac'] second = first
first = first.append('newton')
print first
['isaac', 'newton']
print second
variable
value
first
second
'isaac' 'newton' 16

17. But lists are mutable Didn't explicitly modify second
first = ['isaac']
second = first
first = first.append('newton')
print first
['isaac', 'newton']
print second
variable
value
first
second
Didn't explicitly
modify second
'isaac' 'newton' 17

18. But lists are mutable Didn't explicitly modify second A side effect
first = ['isaac']
second = first
first = first.append('newton')
print first
['isaac', 'newton']
print second
variable
value
first
second
Didn't explicitly
modify second
A side effect
'isaac' 'newton' 18

19. Example: use lists of lists to implement a 2D grid19

20. Example: use lists of lists to implement a 2D grid3 5 7 7 5 8 6 3 2 4 2 6 5 4 3 8 20

21. Example: use lists of lists to implement a 2D grid3 5
grid 7 7 5 8 6 3 2 4 2 6 5 4 3 8 21

22. Example: use lists of lists to implement a 2D grid3 5
grid[0] 7 7 5 8 6 3 2 4 2 6 5 4 3 8 22

23. Example: use lists of lists to implement a 2D grid3 5
grid[0][1] 7 7 5 8 6 3 2 4 2 6 5 4 3 8 23

24. # Correct code grid = [] for x in range(N): temp = []
for y in range(N):
temp.append(1)
grid.append(temp) 24

25. Outer "spine" of structure
# Correct code
grid = []
for x in range(N):
temp = []
for y in range(N):
temp.append(1)
grid.append(temp)
Outer "spine" of structure 25

26. Add N sub-lists to outer list
# Correct code
grid = []
for x in range(N):
temp = []
for y in range(N):
temp.append(1)
grid.append(temp)
Add N sub-lists to outer list 26

27. Create a sublist of N 1's # Correct code grid = [] for x in range(N):
temp = []
for y in range(N):
temp.append(1)
grid.append(temp)
Create a sublist of N 1's 27

28. # Equivalent code grid = [] for x in range(N): grid.append([])
for y in range(N):
grid[-1].append(1) 28

29. Last element of outer list is the sublist currently being filled in
# Equivalent code
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1)
Last element of outer list is the sublist currently
being filled in 29

30. # Incorrect code grid = [] EMPTY = [] for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) 30

31. # Incorrect code grid = [] EMPTY = [] # Equivalent code
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1)
# Equivalent code
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) 31

32. Aren't meaningful variable names supposed to be a good thing?
# Incorrect code
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1)
Aren't meaningful variable
names supposed to be
a good thing? 32

33. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x
grid
EMPTY 33

34. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x
grid
EMPTY 34

35. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x y
grid
EMPTY 35

36. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x y
grid
EMPTY 1 36

37. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x y
grid
EMPTY 2 1 37

38. grid = [] EMPTY = [] for x in range(N): grid.append(EMPTY)
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x y
grid
EMPTY 1 2 1 38

39. You see the problem... grid = [] EMPTY = [] for x in range(N):
variable
value
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) x y
grid
EMPTY 1 2 1
You see the problem... 39

40. No Aliasing
first = []
second = [] 40

41. second = [] second = first
No Aliasing Aliasing
first = [] first = []
second = [] second = first 41

42. grid = [] for x in range(N): grid.append([]) for y in range(N):
variable
value
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) x
grid 42

43. grid = [] for x in range(N): grid.append([]) for y in range(N):
variable
value
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) x
grid 43

44. grid = [] for x in range(N): grid.append([]) for y in range(N):
variable
value
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) x y
grid 2 1 44

45. grid = [] for x in range(N): grid.append([]) for y in range(N):
variable
value
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) x y
grid 1 2 1 45

46. grid = [] for x in range(N): grid.append([]) for y in range(N):
variable
value
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) x y
grid 1 1 1 46

47. If aliasing can cause bugs, why allow it?47

48. If aliasing can cause bugs, why allow it? Some languages don't48

49. If aliasing can cause bugs, why allow it? Some languages don't
Or at least appear not to 49

50. If aliasing can cause bugs, why allow it? Some languages don't
Or at least appear not to
Aliasing a million-element list is more efficient
than copying it 50

51. If aliasing can cause bugs, why allow it? Some languages don't
Or at least appear not to
Aliasing a million-element list is more efficient
than copying it
Sometimes really do want to update a structure
in place 51

52. Greg Wilson created by October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.