Week 8 Lecture: Programming

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1 Week 8 Lecture: Introduction to Object Oriented Programming Introduction to Programming for GIS & Remote Sensing GEO GEO A

2 Objects: You ve Been Exposed! Modularization, you know about it Methods Procedures Obj t Thi k f th d l i i Objects: Think of them as modularizing modules

3 First, Objects in Action Object: Instance of a class Class: Defines the possible operations and states of an object Each instance of a class (each object) is ( j ) independent of all others!

4 Other Object Oriented Definitions Encapsulation: Each hinstance manages its own state tt Polymorphism: Same method call can invoke one of many possible methods, dependent only on object type Inheritance: Define new classes by extending old ones Reflection: Programs can inspect themselves (specific to og a s ca spec e se es(spec c o certain languages)

5 Classes in Python A Python script may define many classes: Start code block using class keyword Classcontents areindented: Pyt tho on

6 Classes Contain Methods and Variables Create instance variables by assignment to self.varname within a method No declaration, just use! And yes, this means you can add members any time you want With great power comes great responsibility!

7 Instance Variables Explained self refers to one particular instance of an object, so self.value can be different for every instance of Counter: Pyt tho on

8 An Instance in Practice

9 In Addition to Instance Variables, Classes Have Class Variables Defined at the class level Does not use the self keyword because self implies object or instance relevance Pyt tho on

10 Instance or Class Variables Can Be Objects!!! In Python (and this is key) everything is an object! So it should come as no surprise that objects and instances of classes can contain references to other objects.

11 Inheritance You can extend a parent class to create a child by putting parents name in paren s To get all of the functionality of the parent constructor, init (), you must call it! Pyt tho on

12 Overriding the Parent Stepper has all the functions and instance variables defined by Counter But it also overrides the step() method to decrement instead of increment, in addition to adding the tripper() function. Pyt tho on

13 Relationships Between Classes There s a basic distinction to understand: Is a vs. Has a

14 Take simple 2D shapes for example: Classdefinition for a point The Point instances will have two instance variables: self.x self.y Pyt tho on

15 Take simple 2D shapes for example: Classdefinition for a line The Line instances will have point objects as instance variables Pyt tho on

16 Visualizing Objectsand Class Relationships UML is a has a composed of

17 Not just about init (), Other Double Underscore Functions Specially named methods for every arithmetic operator: add for + mul for * So if x is an object, x+2 is really x. add (2) Operators also have right hand methods (e.g. radd ()): So 2+x is x. radd (2) Execution order for a+b is: If a has a method add, call a. add (b) If b has a method radd, call b. radd (a) Else use Python s built in in default

18 So imagine overriding the + operator for our Point object: We can now add points because we ve defined a method for the + operator when Point objects are involved: Pyt tho on

19 So imagine overriding the + operator for our Point object: + isn t defined yet We defined add (), now + works Pyt tho on

20 In Lab This Week We ll continue exploring different methods for modularization through h the use of basic object oriented programming Supplemental reading: org/2/tutorial/classes html p// py _ oriented_ framework/index.html

Week 8 Lecture: Getting Things Done

Week 8 Lecture: Getting Things Done Week 8 Lecture: Input and Output, and Moving to Getting Things Done Introduction to Programming for GIS & Remote Sensing GEO6938-4172 GEO4938-4166 4166 Where We Are To this point we ve covered: Basics