The document discusses aliasing in Python programming. It explains that an alias is a second name for a piece of data. It provides examples of how aliasing can cause bugs when working with mutable data like lists, but not with immutable data. It also discusses why Python allows aliasing despite the potential for bugs - for efficiency when working with large data structures and because sometimes in-place updates are desired.

1. Python
Aliasing
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. An alias is a second name for a piece of data
Python Aliasing

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

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

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

6. An alias is a second name for a piece of data
Often easier (and more useful) than making a
sseeccoonndd ccooppyy
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
Python Aliasing

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

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

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

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

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

12. But lists are mutable
Python Aliasing

13. But lists are mutable
first = ['isaac']
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
'isaac'
Python Aliasing

14. But lists are mutable
first = ['isaac']
second = first
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
second
'isaac'
Python Aliasing

15. But lists are mutable
first = ['isaac']
second = first
first = first.append('newton')
pppprrrriiiinnnntttt first
['isaac', 'newton']
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
second
'isaac' 'newton'
Python Aliasing

16. But lists are mutable
first = ['isaac']
second = first
first = first.append('newton')
pppprrrriiiinnnntttt first
['isaac', 'newton']
pppprrrriiiinnnntttt second
['isaac', 'newton']
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
second
'isaac' 'newton'
Python Aliasing

17. But lists are mutable
first = ['isaac']
second = first
first = first.append('newton')
pppprrrriiiinnnntttt first
['isaac', 'newton']
pppprrrriiiinnnntttt second
['isaac', 'newton']
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
Didn't explicitly
second
'isaac' 'newton'
modify second
Python Aliasing

18. But lists are mutable
first = ['isaac']
second = first
first = first.append('newton')
pppprrrriiiinnnntttt first
['isaac', 'newton']
pppprrrriiiinnnntttt second
['isaac', 'newton']
vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
first
Didn't explicitly
second
'isaac' 'newton'
modify second
A side effect
Python Aliasing

19. Example: use lists of lists
to implement a 2D grid
Python Aliasing

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

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

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

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

24. # Correct code
grid = []
ffffoooorrrr x iiiinnnn range(N):
temp = []
ffffoooorrrr y iiiinnnn range(N):
temp.append(1)
grid.append(temp)
Python Aliasing

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

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

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

28. # Equivalent code
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
Python Aliasing

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

30. # Incorrect code
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
Python Aliasing

31. # Incorrect code
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
# Equivalent code
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid.append([])
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
Python Aliasing

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

33. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x 0
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
Python Aliasing

34. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x 0
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
Python Aliasing

35. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x
y
0
0
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
Python Aliasing

36. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x
y
0
0
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
1
Python Aliasing

37. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x
y
0
2
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
1 1 1
Python Aliasing

38. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x
y
1
2
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
1 1 1
Python Aliasing

39. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
EMPTY = []
ffffoooorrrr x iiiinnnn range(N):
x
y
1
2
grid.append(EMPTY)
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
EMPTY
1 1 1
You see the problem...
Python Aliasing

40. No Aliasing
ffiirrsstt == [[]]
second = []
Python Aliasing

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

42. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
x 0
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
Python Aliasing

43. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
x 0
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
Python Aliasing

44. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
x
y
0
2
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
1 1 1
Python Aliasing

45. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
x
y
1
2
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
1 1 1
Python Aliasing

46. vvvvaaaarrrriiiiaaaabbbblllleeee vvvvaaaalllluuuueeee
grid = []
ffffoooorrrr x iiiinnnn range(N):
grid.append([])
x
y
1
0
ffffoooorrrr y iiiinnnn range(N):
grid[-1].append(1)
grid
1 1 1
1
Python Aliasing

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

48. If aliasing can cause bugs, why allow it?
1. Some languages don't
Python Aliasing

49. If aliasing can cause bugs, why allow it?
1. Some languages don't
Or at lleeaasstt aappppeeaarr nnoott ttoo
Python Aliasing

50. If aliasing can cause bugs, why allow it?
1. Some languages don't
Or at lleeaasstt aappppeeaarr nnoott ttoo
2. Aliasing a million-element list is more efficient
than copying it
Python Aliasing

51. If aliasing can cause bugs, why allow it?
1. Some languages don't
Or at lleeaasstt aappppeeaarr nnoott ttoo
2. Aliasing a million-element list is more efficient
than copying it
3. Sometimes really do want to update a structure
in place
Python Aliasing

52. created by
Greg Wilson
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.