Python Object

How objects work in Python. A talk I gave to OSIC members, October 2015.

Transcript

1. Stephen Finucane
   Software Developer, OpenStack Team
   Intel Shannon
   

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2. What is Object Orientated design, and
   why do we care?
   What is an object?
   

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3. Know Java, and know Python is not Java
   These slides compare the two, assuming
   Java knowledge for fair comparison. Don’t
   try to write Python like Java, though.
   

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5. class MyClass(object):
   def __init__(self, *args, **kwargs):
   pass
   def do_stuff(self):
   print('doing stuff')
   >> instance = MyClass()
   >> instance.do_stuff()
   doing stuff
   classes.py: An example of classes in Python
   

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6. class MyClass(object):
   def __init__(self, *args, **kwargs):
   pass
   def do_stuff(self):
   print('doing stuff')
   

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7. object is Python being explicit
   You don’t need it in Python 2, but you
   should use it to get “new style classes”
   

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8. self is also Python being explicit
   Think of it like the ‘this’ keyword in Java
   

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9. public class MyClass {
   String name;
   ...
   public void setName(String name) {
   this.name = name;
   }
   }
   MyClass.java: The ‘this’ keyword
   

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10. class MyClass(object):
    name = None
    ...
    def set_name(self, name):
    self.name = name
    self._example.py: The ‘self’ keyword
    

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12. public class MyClass {
    public MyClass(String name) {
    ...
    }
    public MyClass(String name, int age) {
    ...
    }
    }
    MyClass.java: Constructors in Java
    

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13. class MyClass(object):
    def __init__(name, age=None):
    ...
    constructors.py: Constructors in Python
    

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14. public class MyClass {
    public void display() {
    System.out.println("Hello, world!");
    }
    }
    public class MySubClass extends MyClass {
    public void display() {
    System.out.println("Testing 123");
    }
    }
    MyClass.java, MySubClass.java: Inheritance in Java
    

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15. class MyClass(object) {
    def display(self):
    print('Hello world')
    class MySubClass(MyClass):
    def display(self):
    print('Testing 123')
    constructors.py: Constructors in Python
    

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16. public class MySubClass extends MyClass {
    public void display() {
    super.display();
    System.out.println("Testing 123");
    }
    }
    MyClass.java, MySubClass.java: Inheritance in Java
    

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17. class MySubClass(MyClass):
    def display(self):
    super(MySubClass, self).display(self)
    print('Testing 123')
    superclasses.py: Accessing the Super Class
    

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19. public interface Animal {
    public void eat();
    public void travel();
    }
    public class Mammal implements Animal {
    public void eat() {
    System.out.println("Mammal eats");
    ...
    Animal.java, Mammal.java: Interfaces in Java
    

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20. class Mammal:
    def eat(self):
    print('Mammal eats')
    ...
    class Reptile:
    def eat(self):
    print('Reptile eats')
    ...
    animals.py: Duck Typing in Python
    

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22. >> stephens_boat = Boat(...)
    >> ankurs_boat = Boat(...)
    >> stephens_boat > ankurs_boat
    True
    The data model in practice
    

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23. class Boat(object):
    """Model a boat."""
    length = 0
    color = None
    def __init__(self, length, color):
    ...
    def __gt__(self, other):
    return self.length > other.length:
    boat.py: Using the data model
    

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24. __lt__ - less than
    __le__ - less than or equal
    __eq__ - equal
    __ne__ - not equal
    __gt__ - greater than
    __ge__ - greater than or equal
    __cmp__ - fallback, if above not implemented
    boat.py: Using the data model
    

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25. >>> stephens_boat = Boat(100, 'red')
    >>> stephens_boat.__doc__
    Model a boat.
    >>> stephens_boat.__dict__
    {'color': 'red', 'length': 100}
    >>> stephens_boat.__class__
    
    The data model in practice
    

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27. class Person(object):
    weight_kg = 0
    @property
    def weight_lb(self):
    return self.weight_kg * 2.2
    @weight_lb.setter
    def weight_lb(self, value):
    self.weight_kg = value / 2.2
    boat.py: Using the data model
    

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29. • https://docs.python.org/2/tutorial/classes.html
    • https://en.wikipedia.org/wiki/Duck_typing
    • https://stackoverflow.com/questions/222877/how-to-use-super-
    in-python
    • https://docs.python.org/2/reference/datamodel.html
    

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