Python Object

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

1. Stephen Finucane Software Developer, OpenStack Team Intel Shannon
2. What is Object Orientated design, and why do we care? What is an object?
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.
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
6. class MyClass(object): def __init__(self, *args, **kwargs): pass def do_stuff(self): print('doing stuff')
7. object is Python being explicit You don’t need it in Python 2, but you should use it to get “new style classes”
8. self is also Python being explicit Think of it like the ‘this’ keyword in Java
9. public class MyClass { String name; ... public void setName(String name) { this.name = name; } } MyClass.java: The ‘this’ keyword
10. class MyClass(object): name = None ... def set_name(self, name): self.name = name self._example.py: The ‘self’ keyword
12. public class MyClass { public MyClass(String name) { ... } public MyClass(String name, int age) { ... } } MyClass.java: Constructors in Java
13. class MyClass(object): def __init__(name, age=None): ... constructors.py: Constructors in Python
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
15. class MyClass(object) { def display(self): print('Hello world') class MySubClass(MyClass): def display(self): print('Testing 123') constructors.py: Constructors in Python
16. public class MySubClass extends MyClass { public void display() { super.display(); System.out.println("Testing 123"); } } MyClass.java, MySubClass.java: Inheritance in Java
17. class MySubClass(MyClass): def display(self): super(MySubClass, self).display(self) print('Testing 123') superclasses.py: Accessing the Super Class
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
20. class Mammal: def eat(self): print('Mammal eats') ... class Reptile: def eat(self): print('Reptile eats') ... animals.py: Duck Typing in Python
22. >> stephens_boat = Boat(...) >> ankurs_boat = Boat(...) >> stephens_boat > ankurs_boat True The data model in practice
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
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
25. >>> stephens_boat = Boat(100, 'red') >>> stephens_boat.__doc__ Model a boat. >>> stephens_boat.__dict__ {'color': 'red', 'length': 100} >>> stephens_boat.__class__ <class '__main__.Boat'> The data model in practice
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
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