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Objects in PythonDamian Gordon

The Point Classclass MyFirstClass:pass# END Class

The Point Classclass MyFirstClass:pass# END Class“move along, nothingto see here”

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

>>> a = MyFirstClass()>>> print(a)<__main__.MyFirstClass object at 0x02D60B10>>>> b = a>>> print(b)<__main__.MyFirstClass object at 0x02D60B10>>>> b = MyFirstClass()>>> print(b)<__main__.MyFirstClass object at 0x02D60B30>

The Point Classclass Point:pass# END Classp1 = Point()p2 = Point()

The Point Classclass Point:pass# END Classp1 = Point()p2 = Point()Creating a class

The Point Classclass Point:pass# END Classp1 = Point()p2 = Point()Creating a classCreating objectsof that class

The Point Classp1.x = 5p1.y = 4p2.x = 3p2.y = 6print("P1-x, P1-y is: ", p1.x, p1.y);print("P2-x, P2-y is: ", p2.x, p2.y);

The Point Classp1.x = 5p1.y = 4p2.x = 3p2.y = 6print("P1-x, P1-y is: ", p1.x, p1.y);print("P2-x, P2-y is: ", p2.x, p2.y);Adding Attributes:This is all you need todo, just declare them

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

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

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

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

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

The Point Classp = Point()p.x = 5p.y = 4print("P-x, P-y is: ", p.x, p.y);p.reset()print("P-x, P-y is: ", p.x, p.y);5 40 0

The Point Classp = Point()p.x = 5p.y = 4print("P-x, P-y is: ", p.x, p.y);p.reset()print("P-x, P-y is: ", p.x, p.y);We can also say:Point.reset(p)

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

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

The Point Classclass Point:def move(self,a,b):self.x = aself.y = b# END Movedef reset(self):self.move(0,0)# END Reset# END ClassDeclare a new methodcalled “move” that writesvalues into the object.

The Point Classclass Point:def move(self,a,b):self.x = aself.y = b# END Movedef reset(self):self.move(0,0)# END Reset# END ClassDeclare a new methodcalled “move” that writesvalues into the object.Move the values 0 and 0into the class to reset.

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

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

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

The Point Classimport mathclass 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 Classp1 = 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))p1p2

Initialising an Object• What if we did the following:

Initialising an Objectp1 = Point()p1.x = 5print("P1-x, P1-y is: ", p1.x, p1.y);

>>>Traceback (most recent call last):File "C:/Users/damian.gordon/AppData/Local/Programs/Python/Python35-32/Point-error.py",line 11, in <module>print("P1-x, P1-y is: ", p1.x, p1.y);AttributeError: 'Point' object has no attribute 'y‘>>>

Initialising an Object• So what can we do?

Initialising an Object• So what can we do?• We need to create a method that forces the programmers toinitialize the attributes of the class to some starting value, justso that we don’t have this problem.

Initialising an Object• So what can we do?• We need to create a method that forces the programmers toinitialize the attributes of the class to some starting value, justso that we don’t have this problem.• This is called an initialization method.

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

class Point:def __init__(self,x,y):self.move(x,y)# END Initdef move(self,a,b):self.x = aself.y = b# END Movedef reset(self):self.move(0,0)# END Reset# END ClassInitialising an Object

Initialising an Objectclass Point:def __init__(self,x,y):self.move(x,y)# END Initdef move(self,a,b):self.x = aself.y = b# END Movedef reset(self):self.move(0,0)# END Reset# END ClassWhen you create an object fromthis class, you are going to have todeclare initial values for X and Y.

Initialising an Object• 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)

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

Initialising an Object• 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", line21, in <module>p = Point()TypeError: __init__() missing 2 requiredpositional arguments: 'x' and 'y'

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

Initialising an Objectdef __init__(self, x=0, y=0):self.move(x,y)# END Init

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

Initialising an Object• And then we can do:– p1 = Point()– p2 = Point(2,2)If we don’t supply any values, theinitialization method will set thevalues to 0,0.

Initialising an Object• And then we can do:– p1 = Point()– p2 = Point(2,2)If we don’t supply any values, theinitialization method will set thevalues to 0,0.But we can also supply the values,and the object is created with thesedefault values.

Documenting the Methods• Python is considered one of the most easyprogramming languages, but nonetheless a vital partof object-orientated programming is to explain whateach class and method does to help promote objectreuse.

Documenting the Methods• Python supports this through the use ofdocstrings.• These are strings enclosed in either quotes(‘) ordoublequotes(“) just after the class or methoddeclaration.

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

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

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

Initialising an Object• 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| ----------------------------------------------

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