1. Objects (again)
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Objects (again)

2. Variables, Lists, and Objects
Objects — review
Reading assignment:– Chapter 4
Needed for HW #3
And everything else we do in this course!
Object: computational abstraction that includes
Data
Methods (a. k. a. Functions)
P. 81:–
Objects “know” stuff
Objects “do” stuff
Objects can refer to other objects
Objects can interact with other objects
An object may be assigned to a variable
Or, equivalently, to an element of a list
CS-1004, A-Term 2014
Variables, Lists, and Objects

3. Objects — review (continued)
Objects are organized into Classes
All objects in a class have the same kind of information
All objects in a class have the same set of methods (a.k.a. functions)
Classes are discussed in Chapter 10
Including how to define new classes
How to construct objects of a class
How to define methods of a class
How to define data elements of a class object
Probably will not get to this chapter in this course
CS-1004, A-Term 2014
Variables, Lists, and Objects

4. Object Notation — review
If X is an object of class C, …
… and if class C has a method/function named f, …
… then the notation
X.f()
means “Apply the function f (and its arguments) to the object X”
I.e., f may cause the object X to DO something
Or f may cause the object X to change what it knows
CS-1004, A-Term 2014
Variables, Lists, and Objects

5. Creating Objects — review
If C is a class, then the constructor function for that class is also named C
V = C(arg1, arg2, …) creates a new object of class C and assigns it to variable V
All of the methods associated with class C are now available for object V
Examples
CS-1004, A-Term 2014
Variables, Lists, and Objects

6. Assigning Objects — review
V = SomeClass(<arguments> )
W = V causes W and V to now refer to the very same object
Not to copies of each other, but exactly the same object
CS-1004, A-Term 2014
Variables, Lists, and Objects

7. Questions?
CS-1004, A-Term 2014
Objects (again)

8. How do we create new classes?
See Chapter 10
Keyword class followed by class name
… followed by method functions
… all indented under class
… the first parameter representing object
CS-1004, A-Term 2014
Objects (again)

9. Textbook example Multi-sided die class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
def roll(self):
self.value = randrange(1, self.sides + 1)
def getValue(self):
return self.value
def setValue(self, value):
self.value = value
CS-1004, A-Term 2014
Objects (again)

10. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
def roll(self):
self.value = randrange(1, self.sides + 1)
def getValue(self):
return self.value
def setValue(self, value):
self.value = value
Note colon after class name
Note indentation of method definitions
Note 2nd indentation of method bodies
CS-1004, A-Term 2014
Objects (again)

11. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
Name __init__() is function called to create object of class
I.e., when application says MSDie(6) …
… Python sets aside memory for one MSDie object
… and invokes __init__ with self referring to that memory
Arguments of className assigned to additional parameters of __init__
Special function name
CS-1004, A-Term 2014
Objects (again)

12. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
'Self' could be any name
Refers to the object
Remember: Parameters act variables assigned at time function/method is called
All class methods must have first parameter referring to object
‘self’ is just a parameter name
CS-1004, A-Term 2014
Objects (again)

13. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
def roll(self):
self.value = randrange(1, self.sides + 1)
def getValue(self):
return self.value
def setValue(self, value):
self.value = value
All class methods must have first parameter referring to object
Other parameters
CS-1004, A-Term 2014
Objects (again)

14. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
They make it possible of object “to know stuff”
Called “fields” in Java
Called “members” in C/C++
Part of every object of this class
“Dot” notation to access with methods
These are ‘instance variables’
CS-1004, A-Term 2014
Objects (again)

15. Textbook example (continued)
Multi-sided die
class MSDie:
def __init__(self, sides):
self.sides = sides self.value = 1
def roll(self):
self.value = randrange(1, self.sides + 1)
def getValue(self):
return self.value
def setValue(self, value):
self.value = value
__init__ initializes instance variables
Other methods use and/or manipulate them
CS-1004, A-Term 2014
Objects (again)

16. Questions?
CS-1004, A-Term 2014
Objects (again)

17. Example
Game of Life
CS-1004, A-Term 2014
Objects (again)