1. Department of Computing Science
Computing Science 1P
Lecture 16: Friday 16th February
Simon Gay
Department of Computing Science
University of Glasgow
2006/07

2. Variables, objects, parameters, functions…
We are going to pause in our introduction of Python features
and programming techniques, and try to gain a more detailed
understanding of the way in which programs are executed.
We will return to several points in the book which we have
skimmed over or ignored up to now.
Understanding these points is essential for a thorough mastery
of programming.
2006/07
Computing Science 1P Lecture 16 - Simon Gay

3. Computing Science 1P Lecture 16 - Simon Gay
Variables
The most basic way of understanding a variable is that it is a
box in which a value is stored. x
x = 2 2
y = 3 x y 2 3
print x+y
>>> 5
2006/07
Computing Science 1P Lecture 16 - Simon Gay

4. Computing Science 1P Lecture 16 - Simon Gay
Variables
The most basic way of understanding a variable is that it is a
box in which a value is stored.
x = 2
x  2
state diagrams
page 8
y = 3
x  2
y  3
print x+y
>>> 5
2006/07
Computing Science 1P Lecture 16 - Simon Gay

5. What about local variables?
Simple state diagrams are not adequate to explain the
behaviour of the following program:
def test():
x = 1
print x
x = 2
test()
2006/07
Computing Science 1P Lecture 16 - Simon Gay

6. What does the program print?
2 then 1
1 then 2
2 then 2
1 then 1
Something else
Don't know
2006/07
Computing Science 1P Lecture 16 - Simon Gay

7. What about local variables?
Simple state diagrams are not adequate to explain the
behaviour of the following program:
def test():
x = 1
print x
x = 2
test()
these do not refer to
the same "box"
2006/07
Computing Science 1P Lecture 16 - Simon Gay

8. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
2006/07
Computing Science 1P Lecture 16 - Simon Gay

9. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

10. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
test
a new stack frame
2006/07
Computing Science 1P Lecture 16 - Simon Gay

11. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
test
x  1
2006/07
Computing Science 1P Lecture 16 - Simon Gay

12. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
test
x  1
2006/07
Computing Science 1P Lecture 16 - Simon Gay

13. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

14. Computing Science 1P Lecture 16 - Simon Gay
Stack diagrams (page 21)
def test():
x = 1
print x
x = 2
test()
__main__
x  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

15. Computing Science 1P Lecture 16 - Simon Gay
What about parameters?
A function's parameters are similar to local variables, whose
initial values are the parameter values supplied by the
function call.
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
2006/07
Computing Science 1P Lecture 16 - Simon Gay

16. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
2006/07
Computing Science 1P Lecture 16 - Simon Gay

17. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
2006/07
Computing Science 1P Lecture 16 - Simon Gay

18. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

19. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

20. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

21. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

22. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

23. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

24. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

25. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

26. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

27. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
z  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

28. Stack diagram with function parameters
def calc(x):
y = x + x
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
z  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

29. Parameters really are local
Stack diagrams also allow us to see that assignments to the
parameters, within a function, have no permanent effect.
2006/07
Computing Science 1P Lecture 16 - Simon Gay

30. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
2006/07
Computing Science 1P Lecture 16 - Simon Gay

31. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
2006/07
Computing Science 1P Lecture 16 - Simon Gay

32. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

33. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

34. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

35. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

36. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

37. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

38. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

39. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  2
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

40. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  5
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

41. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  5
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

42. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
calc
x  5
y  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

43. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
z  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

44. Stack diagram with function parameters
def calc(x):
y = x + x
x = 5
return y
x = 1
y = 2
z = calc(y)
print z
__main__
x  1
y  2
z  4
2006/07
Computing Science 1P Lecture 16 - Simon Gay

45. Objects behave differently
We have already mentioned that lists and dictionaries are
known as objects. The state diagrams and stack diagrams we
have used so far are not adequate to explain the behaviour of
objects.
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
Example 1
Example 2
2006/07
Computing Science 1P Lecture 16 - Simon Gay

46. What does Example 1 print?
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
2006/07
Computing Science 1P Lecture 16 - Simon Gay

47. Computing Science 1P Lecture 16 - Simon Gay
What does it print?
[ 1 ]
[ 2 ]
Something else
Don't know
2006/07
Computing Science 1P Lecture 16 - Simon Gay

48. What does Example 2 print?
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
2006/07
Computing Science 1P Lecture 16 - Simon Gay

49. Computing Science 1P Lecture 16 - Simon Gay
What does it print?
[ 1 ]
[ 2 ]
Something else
Don't know
2006/07
Computing Science 1P Lecture 16 - Simon Gay

50. Computing Science 1P Lecture 16 - Simon Gay
Example 2
What happens if we try to understand this program in the way
we have just seen?
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
2006/07
Computing Science 1P Lecture 16 - Simon Gay

51. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
2006/07
Computing Science 1P Lecture 16 - Simon Gay

52. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
2006/07
Computing Science 1P Lecture 16 - Simon Gay

53. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
test
x  [ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

54. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
test
x  [ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

55. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
test
x  [ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

56. Computing Science 1P Lecture 16 - Simon Gay
Example 2
__main__
y  [ 1 ]
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
But this is not what happens if we run the program!
2006/07
Computing Science 1P Lecture 16 - Simon Gay

57. Computing Science 1P Lecture 16 - Simon Gay
Objects
We have to think of objects (including lists and dictionaries)
in a different way.
To represent the state arising from x = [ 1,2,3 ] :
instead of a state diagram like this:
x  [ 1,2,3 ]
use:
x 
[ 1, 2, 3 ]
The value of x is a reference to an object which has an
independent existence somewhere in the computer's memory.
2006/07
Computing Science 1P Lecture 16 - Simon Gay

58. State diagrams with lists
Now we can look at our previous examples, with lists instead
of simple values, to see how our new picture looks.
2006/07
Computing Science 1P Lecture 16 - Simon Gay

59. State diagrams with lists (1)
x = [ 1,2 ]
x 
[ 1, 2 ]
y = [ 3 ]
x 
y 
[ 1, 2 ]
[ 3 ]
x = [ 4 ]
[ 1, 2 ]
x 
y 
[ 4 ]
[ 3 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

60. State diagrams with lists (1)
x = [ 1,2 ]
x 
[ 1, 2 ]
y = [ 3 ]
x 
y 
[ 1, 2 ]
[ 3 ]
garbage
x = [ 4 ]
[ 1, 2 ]
x 
y 
[ 4 ]
[ 3 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

61. State diagrams with lists (2)
x = [ 1,2 ]
x 
[ 1, 2 ]
x[0] = 3
x 
[ 3, 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

62. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
2006/07
Computing Science 1P Lecture 16 - Simon Gay

63. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

64. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
test
a new stack frame
2006/07
Computing Science 1P Lecture 16 - Simon Gay

65. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
test
x 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

66. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
test
x 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

67. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

68. Stack diagrams with lists
def test():
x = [ 1 ]
print x
x = [ 2 ]
test()
__main__
x 
[ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

69. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

70. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
__main__
y 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

71. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
__main__
y 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

72. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
__main__
y 
[ 1 ]
test
x 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

73. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
__main__
y 
[ 1 ]
test
x 
[ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

74. Stack diagram: Example 1
def test(x):
x = [ 2 ]
y = [ 1 ]
test(y)
print y
>>> [ 1 ]
__main__
y 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

75. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

76. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
__main__
y 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

77. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
__main__
y 
[ 1 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

78. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
__main__
y 
[ 1 ]
test
x 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

79. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
__main__
y 
[ 2 ]
test
x 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

80. Stack diagram: Example 2
def test(x):
x[0] = 2
y = [ 1 ]
test(y)
print y
>>> [ 2 ]
__main__
y 
[ 2 ]
2006/07
Computing Science 1P Lecture 16 - Simon Gay

81. Computing Science 1P Lecture 16 - Simon Gay
Aliasing (page 60)
The previous example also illustrated that it is possible to have
two (or more) references to the same object.
x = [ 1,2,3 ]
x 
[ 1,2,3 ]
y = x
x 
[ 1,2,3 ]
x and y are aliases for the same list
y 
x 
[ 5,2,3 ]
x[0] = 5
y 
2006/07
Computing Science 1P Lecture 16 - Simon Gay

82. Computing Science 1P Lecture 16 - Simon Gay
Copying lists
What if we really want to make a copy of a list?
x = [ 1,2,3 ]
x 
[ 1,2,3 ]
y = x + []
x 
[ 1,2,3 ]
y 
[ 1,2,3 ]
x 
[ 5,2,3 ]
x[0] = 5
y 
[ 1,2,3 ]
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Computing Science 1P Lecture 16 - Simon Gay

83. Computing Science 1P Lecture 16 - Simon Gay
There is more to say…
This is not the full story yet, and we will return to it later.
If you want to know more, look up shallow copying and
deep copying.
Also, you could find out what the append method of a list does.
2006/07
Computing Science 1P Lecture 16 - Simon Gay