Python and object-oriented programming

(Programação orientada a objetos com Python)

Daniel Moser

Feb 14th, 2017

2nd IAG Python Boot Camp

What is OOP?

OOP is a programming paradigm. It is usually compared with the traditional procedural programming.

The procedural programming is based on procedures (i.e., routines and sub-routines), where the procedures and data variables are independent. Examples: Cobol, C, Fortran...

program Fibonacci;

function fib(n: Integer): Integer;
var a: Integer = 1;
    b: Integer = 1;
    f: Integer;
    i: Integer;
begin
  if (n = 1) or (n = 2) then
     fib := 1
  else
    begin
      for i := 3 to n do
      begin
         f := a + b;
         b := a;
         a := f;
      end;
      fib := f;
    end;
end;

begin
  WriteLn(fib(6));
end.

OOP is based in objects, a variable that combines methods (procedures) and attributes (data).

Examples of OOP compatible languages: C#, C++, Java, Lisp, Python, Ruby...

class Fibonacci:
   def __init__(self):
       self.cache = {}
   def fib(self, n):
       if self.cache.has_key(n):
           return self.cache[n]
       if n == 1 or n == 2:
            return 1
       else:
           a = 1
           b = 1
           for i in range(2, n):
               f = a + b;
               b = a;
               a = f;
           self.cache[n] = f;
           return f;


fibonaccyCounter = Fibonacci()
# print(fibonaccyCounter.fib(600))

# %time fibonaccyCounter.fib(600)
# %time fibonaccyCounter.fib(600)

Why OOP compatible? Because usually in a OOP language one can still work as a procedural language...

Examples of pure OOP languages: Smalltalk and Self.

OOP Concepts: abstraction, class and object

abstraction is the process of defining only the essential aspects in a specific context, ignoring non-relevant characteristics.
class is an abstraction that defines a type of object and what objects of that type have within them (their attributes) and also defines what kind of actions that type of object is able to perform (methods).

"Class has no life, it's just a concept."

object (or class instance) is a living variable, created according to a (conceptual) class.
constructor has a special function: it serves to initialize attributes and runs automatically whenever you create a new object.

Examples

class testing:
    """This is a IAG Python Boot Camp class definition"""

    def __init__(self, init_value):
        """This is a constructor"""
        self.some_value = init_value
        return

    def double_attr(self):
        """This is a method that double the ``some_value`` atribute"""
        self.some_value *= 2
        return

OOP Concepts: heritage

heritage is the mechanism by which one class can be extended to another one. The inheritance mechanism allows a class (subclass) to share the source code of another class (superclass), taking advantage of its behaviors (methods) and possible variables (attributes).

class Pets(object):
    owner = "Python Boot Camp"

    def __init__(self, name, age, condition='healthy'):
        print("A new pet entry!")
        self.name = name
        self.age = age
        self.condition = condition

    def __repr__(self):
        return "Pet '{0}', {1} year(s)".format(self.name, self.age)

class Dog(Pets):
    def bark(self):
        print(" Woof-woof!!")
        return

class Monkey(Pets):
    def __init__(self, name, age, condition=''):
        """Overwriting the Pets() constructor"""
        condition = 'custom'
        Pets.__init__(self, name, age, condition)

    def gibber(self):
        print(" (gibber)!!")
        return

class Horse(Pets):
    def __init__(self, name, age):
        super(Horse, self).__init__(name, age)

    def neigh(self):
        print(" Neigh!!")
        return

class Cat(Pets):
    def __init__(self):
        super(Pets, self).__init__()

    def meow(self):
        print(" Meow!!")
        return

specialization is the inheritance process in which a subclass is created from existing superclass(es).
generalization is the process in which a superclass is created from existing subclasses.
Python supports multiple inheritance:

class Toys:
    color = 'light blue'

class Lassie(Pets, Toys):
    pass

Must read: Python’s super() considered super!

Ways of combining classes and their properties

association is the mechanism by which one object uses the resources (attributes) of another.

class A(object):
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

    def addNums():
        self.b + self.c

class B(object):
    def __init__(self, d, e):
        self.d = d
        self.e = e

    def addAllNums(self, Ab, Ac):
        x = self.d + self.e + Ab + Ac
        return x

ting = A("yo", 2, 6)
ling = B(5, 9)

print ling.addAllNums(ting.b, ting.c)

coupling or aggregation: is the process where a (sub)class is (integrally) incorporated as an attribute of another one.

class A(object):
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

    def addNums():
        self.b + self.c

class B(object):
    def __init__(self, d, e, A):
        self.d = d
        self.e = e
        self.A = A

    def addAllNums(self):
        x = self.d + self.e + self.A.b + self.A.c
        return x

ting = A("yo", 2, 6)
ling = B(5, 9, ting)

print ling.addAllNums()

composition: is similar to the aggregation, but only incorporating a specific version of the subclass.

class A(object):
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

    def addNums():
        self.b + self.c

class B(object):
    def __init__(self, d, e):
        self.d = d
        self.e = e
        self.A = A("yo", 2, 6)

    def addAllNums(self):
        x = self.d + self.e + self.A.b + self.A.c
        return x

ling = B(5, 9)

print ling.addAllNums()

Remember: those "association/combination" names may vary in the literature.

Encapsulation and polymorphism

encapsulation: In an object oriented program, one can restrict access to methods and attributes.

class C(object):
    def __init__(self):
        self.a = 1    # OK to access directly
        self._a = 2   # should be considered private
        self.__a = 3  # considered private, name Disfigured

Warning! Some people say that *encapsulation* can prevent the data from being modified. This is **not** the case in Python (and even Java[!]. Other methods need to used to avoid modification).

polymorphism: is the principle by which two or more classes can invoke methods that have the same identification (signature) but distinct, specialized behaviors for each one.

class Person(object):
    def pay_bill():
        raise NotImplementedError

class Millionare(Person):
    def pay_bill():
        print "Here you go! Keep the change!"

class GradStudent(Person):
    def pay_bill():
        print "Can I owe you ten bucks or do the dishes?"

interface: is a contract between the class and the outside world. When a class contemplates/implements an interface, it is committed to providing the behavior expected by the interface.
packages (or namespaces) are references for class/interface organization.

import scipy

scipy.interpolate
scipy.interpolate.quad

scipy.integrate

Other OOP Concepts

ducktyping (or duck typing)

As far as the function in_the_forest is concerned, the Person object is a duck:

class Duck:
    def quack(self):
        print("Quaaaaaack!")
    def feathers(self):
        print("The duck has white and gray feathers.")

class Person:
    def quack(self):
        print("The person imitates a duck.")
    def feathers(self):
        print("The person takes a feather from the ground and shows it.")
    def name(self):
        print("John Smith")

def in_the_forest(duck):
    duck.quack()
    duck.feathers()

def game():
    donald = Duck()
    john = Person()
    in_the_forest(donald)
    in_the_forest(john)

game()

message (passing) is the selection of the code to execute in response to a method call, typically by looking up the method in a table associated with the object. This feature is known as dynamic dispatch, and distinguishes an object from an abstract data type, which has a fixed (static) implementation of the operations for all instances. [Not covered here]

OOP pro et contra

Advantages of OOP:

A more logical and better encapsulated code division.
This makes maintaining and extending the code easier and with less risk of inserting bugs.
It is also easier to reuse the code.
It is easier to manage the development of this type of software when we have a large team.

Disadvantages of OOP

Learning of the object-oriented programming paradigm is more complicated. In traditional procedural programming, just decorate a few dozen commands and you can already make a simple program.
Hardly an object-oriented language will be able to run over non-object-oriented languages.

Extra

UML (Unified Modeling Language) is a general-purpose, developmental, modeling language in the field of software engineering, that is intended to provide a standard way to visualize the design of a system.
SysML (Systems Modeling Language) can be seen as a system view of the application, or as an extension of subsets of UML.

This kind of tools can be used, for example, together with abstraction level concepts for automatic programming (or automatic coding)!

Exercises

What dir() do?
What hasattr(), getattr(), setattr() and delattr() do?
What is happening here?

class test:
    def __init__(self):
        print "init 1"

    def __init__(self):
        print "init 2"

How can you change a class (global) attribute?
What isinstance(variable, class) do?
Is Lassie an instance of Pets or Toys?
What instance.__base__ do?
About interfaces. Understand what is happening here:

"""http://python-3-patterns-idioms-test.readthedocs.io/en/latest/ChangeInterface.html"""

class WhatIHave:
    def g(self): pass
    def h(self): pass

class WhatIWant:
    def f(self): pass

class ProxyAdapter(WhatIWant):
    def __init__(self, whatIHave):
        self.whatIHave = whatIHave

    def f(self):
        # Implement behavior using
        # methods in WhatIHave:
        self.whatIHave.g()
        self.whatIHave.h()

class WhatIUse:
    def op(self, whatIWant):
        whatIWant.f()

# Approach 2: build adapter use into op():
class WhatIUse2(WhatIUse):
    def op(self, whatIHave):
        ProxyAdapter(whatIHave).f()

# Approach 3: build adapter into WhatIHave:
class WhatIHave2(WhatIHave, WhatIWant):
    def f(self):
        self.g()
        self.h()

# Approach 4: use an inner class:
class WhatIHave3(WhatIHave):
    class InnerAdapter(WhatIWant):
        def __init__(self, outer):
            self.outer = outer
        def f(self):
            self.outer.g()
            self.outer.h()

    def whatIWant(self):
        return WhatIHave3.InnerAdapter(self)

whatIUse = WhatIUse()
whatIHave = WhatIHave()
adapt= ProxyAdapter(whatIHave)
whatIUse2 = WhatIUse2()
whatIHave2 = WhatIHave2()
whatIHave3 = WhatIHave3()
whatIUse.op(adapt)
# Approach 2:
whatIUse2.op(whatIHave)
# Approach 3:
whatIUse.op(whatIHave2)
# Approach 4:
whatIUse.op(whatIHave3.whatIWant())