Creating Objects in Python

Objects in Python
Damian Gordon

The Point Class
class MyFirstClass:
pass
# END Class

The Point Class
class MyFirstClass:
pass
# END Class
“move along, nothing
to see here”

The Point Class
class MyFirstClass:
pass
# END Class
class <ClassName>:
<Do stuff>
# END Class

>>> a = MyFirstClass()
>>> print(a)
<__main__.MyFirstClass object at 0x02D60B10>
>>> b = a
>>> print(b)
>>> b = MyFirstClass()
>>> print(b)

The Point Class
class Point:
pass
# END Class
p1 = Point()
p2 = Point()

The Point Class
class Point:
pass
# END Class
p1 = Point()
p2 = Point()
Creating a class

The Point Class
class Point:
pass
# END Class
p1 = Point()
p2 = Point()
Creating a class
Creating objects
of that class

The Point Class
p1.x = 5
p1.y = 4
p2.x = 3
p2.y = 6
print("P1-x, P1-y is: ", p1.x, p1.y);

The Point Class
p1.x = 5
p1.y = 4
p2.x = 3
p2.y = 6
Adding Attributes:
This is all you need to
do, just declare them

Python: Object Attributes
• In Python the general form of declaring an attribute is as
follows (we call this dot notation):
OBJECT. ATTRIBUTE = VALUE

The Point Class
class Point:
def reset(self):
self.x = 0
self.y = 0
# END Reset
# END Class

The Point Class
class Point:
def reset(self):
self.x = 0
self.y = 0
# END Reset
# END Class
Adding Methods:
This is all you need

The Point Class
p = Point()
p.x = 5
p.y = 4
print("P-x, P-y is: ", p.x, p.y);
p.reset()

The Point Class
p = Point()
p.x = 5
p.y = 4
p.reset()
5 4

The Point Class
p = Point()
p.x = 5
p.y = 4
p.reset()
5 4
0 0

The Point Class
p = Point()
p.x = 5
p.y = 4
p.reset()
We can also say:
Point.reset(p)

The Point Class
• We can do this in a slightly different way, as follows:

The Point Class
class Point:
def move(self,a,b):
self.x = a
self.y = b
# END Move
def reset(self):
self.move(0,0)
# END Reset
# END Class

The Point Class
class Point:
def move(self,a,b):
self.x = a
self.y = b
# END Move
def reset(self):
self.move(0,0)
# END Reset
# END Class
Declare a new method
called “move” that writes
values into the object.

The Point Class
class Point:
def move(self,a,b):
self.x = a
self.y = b
# END Move
def reset(self):
self.move(0,0)
# END Reset
# END Class
Declare a new method
called “move” that writes
values into the object.
Move the values 0 and 0
into the class to reset.

The Point Class
• The distance between two points is:
d
d = √(x2 – x1)2 + (y2 – y1) 2
d = √(6 – 2)2 + (5 – 2) 2
d = √(4)2 + (3)2
d = √16 + 9
d = √25
d = 5

The Point Class
• Let’s see what we have already:

The Point Class
• Now let’s add a new method in:

The Point Class
import math
class Point:
def calc_distance(self, other_point):
return math.sqrt(
(self.x – other_point.x)**2 +
(self.y – other_point.y)**2)
# END calc_distance
# END Class d = √(x2 – x1)2 + (y2 – y1)2

The Point Class
• Now let’s add some code to make it run:

The Point Class
p1 = Point()
p2 = Point()
p1.move(2,2)
p2.move(6,5)
print("P1-x, P1-y is: ", p1.x, p1.y)
print("P2-x, P2-y is: ", p2.x, p2.y)
print("Distance from P1 to P2 is:", p1.calc_distance(p2))
p1
p2

Initialising an Object
• What if we did the following:

p1 = Point()
p1.x = 5

>>>
Traceback (most recent call last):
File "C:/Users/damian.gordon/AppData/
Local/Programs/Python/Python35-32/Point-error.py",
line 11, in <module>
AttributeError: 'Point' object has no attribute 'y‘
>>>

• So what can we do?

• We need to create a method that forces the programmers to
initialize the attributes of the class to some starting value, just
so that we don’t have this problem.

• We need to create a method that forces the programmers to
initialize the attributes of the class to some starting value, just
so that we don’t have this problem.
• This is called an initialization method.

• Python has a special name it uses for initialization methods.
_ _ init _ _()

class Point:
def __init__(self,x,y):
self.move(x,y)
# END Init
def move(self,a,b):
self.x = a
self.y = b
# END Move
def reset(self):
self.move(0,0)
# END Reset
# END Class

class Point:
self.move(x,y)
# END Init
def move(self,a,b):
self.x = a
self.y = b
# END Move
def reset(self):
self.move(0,0)
# END Reset
# END Class
When you create an object from
this class, you are going to have to
declare initial values for X and Y.

• So without the initialization method we could do this:
– p1 = Point()
– p2 = Point()
• but with the initialization method we have to do this:
– p1 = Point(6,5)
– p2 = Point(2,2)

• And if we forget to include the values, what happens?

• And if we forget to include the values, what happens?
Traceback (most recent call last):
File "C:/Users/damian.gordon/AppData/Local/
Programs/Python/Python35-32/Point-init.py", line
21, in <module>
p = Point()
TypeError: __init__() missing 2 required
positional arguments: 'x' and 'y'

• But if we want to be lazy we can do the following:

def __init__(self, x=0, y=0):
self.move(x,y)
# END Init

• And then we can do:
– p1 = Point()
– p2 = Point(2,2)

– p1 = Point()
– p2 = Point(2,2)
If we don’t supply any values, the
initialization method will set the
values to 0,0.

– p1 = Point()
– p2 = Point(2,2)
If we don’t supply any values, the
initialization method will set the
values to 0,0.
But we can also supply the values,
and the object is created with these
default values.

Documenting the Methods
• Python is considered one of the most easy
programming languages, but nonetheless a vital part
of object-orientated programming is to explain what
each class and method does to help promote object
reuse.

• Python supports this through the use of
docstrings.
• These are strings enclosed in either quotes(‘) or
doublequotes(“) just after the class or method
declaration.

class Point:
“Represents a point in 2D space”
‘Initialise the position of a new point’
self.move(x,y)
# END Init

def move(self,a,b):
‘Move the point to a new location’
self.x = a
self.y = b
# END Move
def reset(self):
‘Reset the point back to the origin’
self.move(0,0)
# END Reset

• Now run the program, and then do:
>>>
>>> help (Point)

• And you’ll get:
Help on class Point in module __main__:
class Point(builtins.object)
| Represents a point in 2D space
|
| Methods defined here:
|
| calc_distance(self, other_point)
| Get the distance between two points
|
| move(self, a, b)
| Move the point to a new location
|
| reset(self)
| Reset the point back to the origin
| ----------------------------------------------

Creating Objects in Python

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