CS61A(16): Objects

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This is the lecture note of CS61A - Lecture 16.

OOP (Object-Oriented Programming)

Objects are organized according to classes.

  • A class combines (and abstracts) the storing of information data and functionality on the data (data + methods).
  • An object is an instantiation of a class.

Class Statements

Class statements let you create any type of data you want.

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>>> a = Account('Jim')
>>> b = Account('Jack')
>>> a is b
False
>>> a is not b
True

Constructors,

  • allocate memory for an object
  • initialize an object with values
  • return address of the object

Methods

Objects receive messages via dot notation. Dot notation accesses attributes of the instance or its class.

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<expression>.<attribute_name>

"""
expression can be any valid Python expression
attribute can be data or methods;
"""

Attributes

Class attributes are "shared" across all instances of a class because they are attributes of the class, not the instance.

Code

Code of today's lecture:

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class Account:
    """An account has a balance and a holder.
    All accounts share a common interest rate.

    >>> a = Account('John')
    >>> a.holder
    'John'
    >>> a.deposit(100)
    100
    >>> a.withdraw(90)
    10
    >>> a.withdraw(90)
    'Insufficient funds'
    >>> a.balance
    10
    >>> a.interest
    0.02
    >>> Account.interest = 0.04
    >>> a.interest
    0.04
    """

    interest = 0.02  # A class attribute

    def __init__(self, account_holder):
        self.holder = account_holder
        self.balance = 0

    def deposit(self, amount):
        """Add amount to balance."""
        self.balance = self.balance + amount
        return self.balance

    def withdraw(self, amount):
        """Subtract amount from balance if funds are available."""
        if amount > self.balance:
            return 'Insufficient funds'
        self.balance = self.balance - amount
        return self.balance