cs_lec_12

#r/lecture

Lecture notes: link

Lecture: 10/7

print("Hello world i am cool")

Mutable values

![[WhatsApp Image 2022-10-07 at 1.18.23 PM.jpeg]]

![[pythonw_5kBuKRGa2t.png]]

Object-oriented programming A method for organizing programs

Data abstraction
Bundling together information and related behaviour A metaphor for computation using distributed state
Each object has its own local state
Each object also knows how to manage its own local state, based on method calls
Method calls (can be thought of) as messages passed between objects
Several objects may all be instances of a common type
Different types may relate to each other.

Specialised syntax and vocabulary to support this metaphor ![[WhatsApp Image 2022-10-07 at 1.25.30 PM.jpeg]]

Classes

A class describes the general behaviour of its instances Idea: All bank accounts have a balance and an account holder; the Account class should add those attributes to each newly created instance.

>>>a = Account('John')
>>>a.holder
'John'
>>>a.balance

Idea: All bank accounts should have withdraw and deposit behaviours that all work in the same way.

>>>a.deposit(15)
15
>>>a.withdraw(10)
5
>>>a.balance
5

Better Idea: All bank accounts share a withdraw method and a deposit method

The class Statement

class name:
    suite

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 creates attributes of the class (not names in frames)

Object Construction Idea: All bank accounts have a balance and an account holder; The Account class should add those attributes to each of its instances.

![[WhatsApp Image 2022-10-07 at 1.32.40 PM.jpeg]]

Q: Is init kinda setting up defaults? A: Not rlly, its kinda like setting up the object to prepare it to be used.

Can't write def constructor, must write def init

The init function is always called whenever the constructor is called.

Object Identity Every object that is an instance of a user-defined class has a unique identity: ![[chrome_HX8HwQpibp.png]]

Methods Methods are functions defined in the suite of a class statement ![[chrome_hlpwVyZaMp.png]]

Invoking Methods All invoked methods have access to the objet via the self parameter, and so they can all access and manipulate the object's state

Dot Expressions Objects receive messages via dot notation

Dot notation accesses attributes of the instance or its class $<expression>,<name>$

The $<expression>$ can be any valid Python Expression The $<name>$ must be a simple name

![[chrome_bqWGMOpvCG.png]] Above expression evaluates to the value of the attribute looked up by $<name>$ in the object that is the value of the $<expression>$

![[WhatsApp Image 2022-10-07 at 1.52.10 PM.jpeg]]

Can also assign methods to instance. Assume we have that $a$ is an Account object

Account.deposit(a, amount) #would work
a.deposit(amount) #would also work

Accessing Attributes Using getattr, we can look up an attribute

Methods and Functions Python distinguishes between:

Functions, which we have been creating since the beginning of the ccourse, and
Bound methods, which couple together a function and the object on which that method will be invoked
- Object + Function = Bound Method

Looking Up Attributes by Name expression, name

To evaluate a dot expression:

Evlaluate the expression to the left of the dot, which yields the object of the dot expression
name ...
...

Class Attributes