f"Hello my name is {name} and I am {age} years old."
.format
"Hello my name is {name} and I am {age} years old.".format(name='Carter', age=19) "Hello my name is {1} and I am {0} years old.".format(19, 'Carter') "Hello my name is {} and I am {} years old.".format('Carter', 19)
Objects
Classes
A class serves as a template for its instances.
Each object serves as an instance of some class.
The Class Statement
A class statement creates a new class and binds that class to <name> in the first frame of the current environment.
Assignment and def statements in <suite>create attributes of the class (not names in frames)
Object Construction
When a class is called:
A new instance of that class is created
The __init__ method of the class is called with the new object as its first argument (named self), along with any additional arguments provided in the call expression
Classes and Objects
Class is a template (blueprint, factory, prototype) for creating objects.
Object (instance of a class): basic unit of Object Oriented Programming and represents the real life entities.
Constructor: a special kind of method that Python calls when it instantiates (creates) an object using the definitions found in your class.
Object Identity
Every object that is an instance of a user-defined class has a unique identity
Methods
Methods
Methods are functions defined in the suite of a class statement
Invoking Methods
All invoked methods have access to the object via the self parameter, and so they can all access and manipulate the object’s state.
Attributes
data that is stored within either an instance or the class itself
@classmethod
@staticmethod - cannot access class or instance data, they cannot modify the class state
Methods and Functions
Python distinguishes between
Functions, which we have been using since the beginning of the course
Bound methods, which couple together a function and the object on which that method will be invoked
Object + Function = Bound Method
Class Attributes
Class attributes are “shared” across all instances of a class because they are attributes of a class, not the instance.
Looking up attributes by name
<expression>.<name>
To evaluate a dot expression:
Evaluate the <expression> to the left of the dot, which yields the object of the dot expression
<name> is matched against the instance attributes of that object; if an attribute with that name exists, its value is returned
If not, <name> is looked up in the class, which yields a class attribute value
That value is returned unless it is a function, in which case a bound method is returned instead
Assignment to Attributes
Assignment statements with a dot expression on their left-hand side affect attributes for the object of that dot expression
If the object is an instance, then assignment sets an instance attribute
If the object is a class, then assignment sets a class attribute
Inheritance
Inheritance Terminology
Parent class
Super class
Base class
Child class
Subclass
Derived class
Inheritance
Inheritance is a technique for relating classes together
A common use: Two similar classes differ in their degree of specialization
The specialized class may have the same attributes as the general class, along with some special-case behavior
Conceptually, the new subclass (child class) inherits (shares) attributes of its base (parent, super) class
The subclass may override certain inherited attributes
Using inheritance, we implement a subclass by specifying its differences from the base class
Looking Up Attribute Names on Classes
Base class attributes aren’t copied into subclasses!
To look up a name in a class:
If it names an attribute in the class, return the attribute value
Otherwise, look up the name in the base class, if there is one
Object-Oriented Design
Designing for Inheritance
Don’t repeat yourself; use existing implementations
Attributes that have been overwritten are still accessible via class objects
Loop up attributes on instances whenver possible
Inheritance and Composition
Composition: when one object has another object as an attribute
Inheritance is best for representing is-a relationships
Composition is best for representing has-a relationships
Multiple Inheritance
One child class has multiple parent classes
A class may inherit from multiple base classes in Python