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Object-Oriented Programming in Python

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Object

  • = state + bahaviour
  • treat class as blue print, outlining states and bahaviours
  • state <-> attributes
  • behaviour <-> methods

list all attrs and methods

dir(object)

def __init__(self, attr1, attr2):
# is a constructor that is called every time an instance of a class is created
    self.attr1 = value
    self.attr2 = value

Best practice

  • CamelCase for class name and lower_snake_case for methods and attributes.
  • use docstring

Core Principles of OOP

Instance Level Data

  • instance attrs created when initializing an instance of a class.
  • self binds to an instance

Class Level Data

  • Data shared between all instance of a class
  • Define the class attr in the body of a class
  • Not using self but class name when refering to it
class X:
    MIN_VALUE = value
    def __init__(self, attr):
        ...
        X.MIN_VALUE

Class methods

  • Methods are already shared: same code for every instance
  • Class methods can't use instance-level data

But why a class method?

  • Alternative way to a constructor (init())

Use class method to create an object return cls() will call __init__

def X:
    @classmethod
    def this_method(cls, args...):
        name = value
        return cls(name)
        
X.this_method(args)

Class Inheritance

class Child(Parent):
    # Both Child and Parent are defined classes
    def __init__(self, attr1, attr2)
        Parent.__init__(self, attr1)
        self.attr2 = value

Operator Overloading: Comparison

Overloading eq()

def __eq__(self, other): is called when 2 objects of a class are compared using == return (self.id == other.id) and (self.attr1 == other.attr1)

Operator Overloading: String Representation

return the printable presentation of an object but not pointer.

  • __str__()

    • print(obj), str(obj)
    • informal, for end user
    • _str_ing representation

  • __repr__()

    • printing in console without calling print explicitly
    • formal, for developer
    • _repr_oducible _repr_esentation
    • fall back for print
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