CS61A: Lecture 17

Objects

Review - Mutability

• What would be the final values of the names defined within this code segment?

def func1(lst1):
    lst1 = lst1[0:-1]
    return lambda lst: lst + lst1

def func2(lst1):
    lst1[0] = lst1.pop()

lst1 = [1, 19, 2022]
lst2 = [5, 10, 2022]

res1 = func1(lst1)
res2 = res1(lst2)
res3 = func2(lst2)

# lst1 = [1, 19, 2022]
# lst2 = [2022, 10]
# res1 = lambda
# res2 =  [5, 10, 2022, 1, 19]
# res3 = None

[2022, 10]

• Arithmetic operators do not mutate the list, but rather returns a new list. If we wanted to mutate the original list to add new elements, use extend.

Class Statements

• A class describes the behavior of its instances
  • Ex: All bank accounts have a balance and an account holder; the Account class should add those attributes to each newly created instance.
• The class is a blueprint for creating objects/instances of that class.
  • Every class has attributes and methods.
  • Attributes are like variables, they store information.
  • Methods are like functions, they perform operations to the Object.
  • We access attributes and metjods using the dot notation.

The Account Class

• The keyword “self” refers to the instance of the class itself.
• The __init__ method is a special method name for the function that constructs an Account instance.
• The Class can be thought of as a new type of an object.
  • Everything in python belongs to a class
• Methods that include the self arguemnt are instance methods, without self, they are NOT instance methods and do not affect the contents of each instance.
• Other than instance attributes, we may also have class variables that can be accessed freely.
  • With self: instance
  • Without self: class

class Account:
    company = "CS61A Bank" # This is a class variable
    def __init__(self, account_holder):
        self.balance = 0 # This is an instance variable
        self.holder = account_holder
    
    def deposit(self, ammount): # This is an instance method
        self.balance = self.balance + ammount
        return self.balance
    
    def withdraw(self, amount):
        if amount > self.balance:
            return "Insufficient funds"
        self.balance = self.balance - amount
        return self.balance
    
    def foo(x): # This is a class method
        return Account.company
    
    def transfer(self, into, amount):
        result = self.withdraw(amount)
        if type(result) == str:
            return result
        else:
            into.deposit(amount)
            return "Transfer Successful"

a = Account("Alex")
a.deposit(50000)

b = Account("Bobby")
b.deposit(10000)

a.transfer(b, 20000)
print(a.balance) # 30000
print(b.balance) # 30000

30000
30000