Object Oriented Python

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Object Oriented Python
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Object Oriented Programming

• Programming paradigm
• Uses software objects - discrete, reusable units
  of code
• Different from modular programming which focuses
  on the function of a module in that it also
  incorporates data
• Allows abstraction, polymorphism (same method
  does different things for objects of different
  classes), etc.

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Software Objects

• Objects have attributes (data) and behaviors
  (methods).
• A class is a larger category of object
• Class hierarchy
• Ex. Parent class Animal
• Subclass of animal Dog
• Subclass of Dog Chihuahua, Lab, Collie

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Dog Object

• Attributes (Data)
• Size
• Fur color
• Breed
• Behavior (Methods)
• Bark
• Sleep
• Eat
• Fetch

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Python Classes

• class dog
• abstraction of a dog
• def __init__(self, size, fur_color, breed)
• self.size size
• self.color fur_color
• self.breed breed
• def bark(self)
• code to execute
• def bark_twice(self)
• self.bark()
• self.bark()

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self, init

• __init__ is called immediately after the class is
  instantiated
• __init__ never returns a value
• The first argument of every method in a class is
  a reference to the instantiation of the class
  that is calling the method represented by self
• def methodname(self, arg1, arg2, arg3, )
• Do not use self when calling the method using the
  instantiation e.g. instance.methodname(arg1,arg2,
  arg3)

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Inheritance

• class mammal
• abstraction of a mammal
• def __init__(self, )
• class dog(mammal)
• abstraction of a dog
• def __init__(self, size, fur_color, breed)
• mammal.__init__(self)
• Multiple Inheritance
• class dog(vertebrate, mammal)

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Instantiation

• import dog
• lassie dog(medium,brown,collie)
• lassie.bark()
• lassie.save_timmy()
• size lassie.size
• Do not use self when instantiating an object,
  retrieving an attribute or calling a method.

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Special Methods

• Convert non-method-calling syntax into a method
  call
• Ex. With a dictionary dictkeyname uses
  __getitem__
• __setitem__
• __getitem__
• __repr__ - return string representation of object
• __cmp__ - called when you use
• __len__ - called when you use lenI()
• __delitem__ - called when you use del to delete
  an item from a dictionary

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Private Functions/Methods/Attributes

• Cannot be called outside of their modules/classes
• Determined by name
• Must start with 2 underscores but not end with 2
  underscores __private or __private_
• Calling private methods will raise an exception

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Good Practice

• Include docstrings in your class
• Initialize all attribute variables to something
  reasonable in the __init__ section of the class
• Uninitialized attributes will return an
  AttributeError exceptio if they are referenced

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Notes

• Python does not support function overloading
• Methods are defined solely by their name and
  cannot be distinguished by the number of input
  parameters they expect
• The __init__ statement of the subclass always
  overrides that of the parent class

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More Info

• Dive into Python

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Exercises

• Look over matrix.py on the course website
• Adapt this code to create a matrix class capable
  of handling 3D matrices as well as 2D
• Check out http//docs.python.org/tutorial/errors.h
  tml - look at the user defined exceptions section
• Use this information to design your own exception
  class. Have your class inherit from the existing
  python Exception class