1. Fundamentals of Programming I More Data Modeling
Computer Science 111
Fundamentals of Programming I
More Data Modeling

2. More Modeling: a Bank Manages a set of accounts Operations
Add a new account
Remove an account
Access an account for deposits and withdrawals
Compute the interest on all accounts

3. Class Diagram Describes the relationship between two classes
Bank *
SavingsAccount
Describes the relationship between two classes
A Bank object contains zero or more SavingsAccount objects

4. The Interface of the Bank Class
Bank() # Returns a new bank
add(account) # Adds account to bank
remove(name, pin) # Removes an account from bank
get(name, pin) # Returns an account or None
computeInterest() # Computes the interest and
# deposits it in each account
str(aBank) # Returns string rep of bank

5. Example Use of the Bank Class
from bank import Bank
wellsFargo = Bank()
for pinNumber in range(1000, 1005):
wellsFargo.add(SavingsAccount('Ken', str(pinNumber)))
print(wellsFargo)
account = wellsFargo.get('Ken', '1001')
account.deposit(100)
account.withdraw(20)
print(wellsFargo.computeInterest())

6. Defining a Bank Class Set up a data structure for the data
Define methods to
Add a new account
Remove an account
Access an account for deposits and withdrawals
Compute the interest on all accounts

7. Defining the Bank Class
class Bank(object):
"""This class represents a bank."""
def __init__(self):
self.accounts = {}
# Other methods go here
Use a dictionary for the accounts, keyed by the name + pin

8. Defining the Bank Class
class Bank(object):
"""This class represents a bank."""
def __init__(self):
self.accounts = {}
def __str__(self):
result = ""
for account in self.accounts.values():
result += str(account) + "\n"
return result
Always define __init__ and __str__ first

9. Defining the Bank Class
class Bank(object):
"""This class represents a bank."""
def __init__(self):
self.accounts = {}
def __str__(self):
return "\n".join(map(str, self.accounts.values()))
Simplify, simplify!

10. Adding an Account
class Bank(object):
"""This class represents a bank."""
def __init__(self):
self.accounts = {}
def add(self, account):
key = account.name + account.pin
self.accounts[key] = account
An account is a value in the dictionary, keyed by its name and pin
The remaining methods are similar

11. Class Variables
An instance variable refers to storage owned by a single instance
A class variable refers to storage owned by the class, and thus available to all of its instances
For example, each savings account owns a separate balance, so that should be an instance variable
But all savings accounts have the same interest rate, so that should be a class variable

12. Class and Instance Variables
from account import SavingsAccount
a1 = SavingsAccount('Ken', '3322', )
a2 = SavingsAccount('Catherine', '3321', )
print(SavingsAccount.RATE)
SavingsAccount
SavingsAccount.RATE a1 a2
self.balance
self.balance

13. Using Class Variables account = SavingsAccount('Ken', '3322', 1000.00)
print(account.computeInterest())
SavingsAccount.RATE = 0.06

14. Defining a Class Variable
class SavingsAccount(object):
"""This class represents a savings account."""
RATE = # Class variable
def __init__(self, name, pin, balance = 0.0):
self.name = name
self.pin = pin
self.balance = balance
def computeInterest(self):
interest = SavingsAccount.RATE * self.balance
self.deposit(interest)
return interest
Class variables are picked out using the class name as a prefix
Their names are usually in uppercase

15. Design patterns with classes
For Friday
Design patterns with classes