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Chapter 08 | A Little Bit of Class | Abstracting Behavior and State

  • Classes let us bundle code behavior and the state together.
  • Python borrows its programming approaches from these
    • Object-Oriented
    • Functional
    • Procedural

Object Oriented Primer

  • Python with its Object-Oriented Paradigm supports
    • Inheritance
    • Polymorphism
    • Encapsulation
  • A class bundles behavior and state.
    • Behavior means function.
    • State means variables.
  • A class in Python has methods and attributes.
    • Methods is the OOP name given to a function defined inside a Class.
    • Attributes is the name given to variables inside a Class.

Object Instantiation

  • To use a class, we have to create an object from it. This process is called Object Instantiation.
  • A class is created in this way
    • class CountFromBy:
    • The important things are the use of the keyword class
    • The Class name starts capital letters which is a convention.
  • We create an object of the above Class by using
    • a = CountFromBy()
    • The type(a) = <class '__main__.CountFromBy'>
    • Creating a object is same as calling a function.
  • There is well established convention in the Python Community in the difference between naming of function and classes.
    • Functions are always named using lowercase letter, with underscore for emphasis.
    • CamelCase if used for Class names.
    • Both the above rule is not enforced by the interpreter.
  • Each object created from a class shares the behaviors (methods), but maintains its own copy of any state (the attribute).

Method Invocation

  • A Object can invoke a method from a class in this way.
    • c.increase()
  • This is interpreted as
    • CountFromBy.increase(c) by the interpreter. The object is passed as a parameters to the method.
  • The first argument of a methods is called self by convention in Python.
  • The interpreter supplies the first argument to a method, we do not have to pass it explicitly.
  • Not adding self to a method definition cause to raise TypeError.
  • Each Object maintains it's own copy of attributes, which is possible the use of self.

Variable scope

  • A variable defined inside a function's suite, exists while the function runs.
  • Outside the function suite these variable's give NameError:
  • When we attach a variable with self inside a class method, it's scope is preserved outside the scope of the method.
  • The value in self is an alias that points back to the object invoking the method.

Class Init

  • In Python, we have to initialize variables with a starting value, before we can use them in expression.
  • In Python Class, the magic method to initialize if __init__()
  • The methods are called dunders, and it provide hooks into every class standard behavior.
  • The __init__ standard behavior is implemented in a class called object.
  • All Python Class automatically inherits from object class.
  • There are certain times when we would like to override the dunder methods.
    • __init__ : when initializing a class
    • __eq__ : when we want to make sure our class reacts to == operator.

init

  • We can pass arguments to __init__.
  • To pass value to __init__, we have to pass arguments while calling the class constructor.
    • CountFromBy(100, 10)
  • When creating objects, any argument values provided to the class are passed to the __init__ method.
  • We can access the attribute values by combining the object's name with the attribute name.
    • a.val
  • When using the object name on its own, the interpreter spits back a cryptic message.
    • <__main__.CountFromBy object at 0x104194150>

Object Reference

  • When we print the value of a object, we get this value
    • <__main__.CountFromBy object at 0x10aad9150>
  • This value is combination of the output from type
    • <class '__main__.CountFromBy'>
  • The value of id in hex
    • 0x10aad9150
  • This is the default representation by the object class of the method __repr__

Bullet Points

  • Python Classes let you share behavior and state.
  • Methods are functions and State are variable.
  • The class keyword introduces a new class in code.
  • Creating a new object from a class looks very like a function call.
  • When an object is created from a class, the object shares the class code.
  • The object shares the code but maintains own copy of attributes.
  • self is the object reference inside a class.
  • __init__ method is one of the many magic methods in Python.
  • Attributes are initialized by the __init__ method.
  • __repr__ is another magic method which allows to control the object representation.