1. Computer Science 111 Fundamentals of Programming I
Persistent Data Models
Object Serialization

2. Helper Function to Create a Bank
from bank import Bank
def createBank(numAccounts = 1):
"""Returns a new bank with the given number of
accounts."""
bank = Bank()
upperPin = numAccounts
for pinNumber in range(1000, upperPin):
bank.add(SavingsAccount('Ken', str(pinNumber)))
return bank
bank = createBank(5) # Creates 5 accounts in bank

3. Transience and Persistence
Data in computer memory are transient and are lost when the program quits
Data become persistent when they can be saved to a file and reloaded from a file
save
Data model
File storage
load

4. Saving a Savings Account
Include a method for writing an account’s info to a text file:
def save(self, fileObj): # In SavingsAccount
fileObj.write(self.name + '\n')
fileObj.write(self.pin + '\n')
fileObj.write(str(self.balance) + '\n')
>>> a = SavingsAccount('Ken', '8809', )
>>> fileObj = open('account.txt', 'w')
>>> a.save(fileObj)
>>> fileObj.close()
Run the method in the shell and then inspect the file:

5. Saving an Entire Bank
Include a method for writing a bank’s info to a text file:
def save(self, fileName): # In Bank
fileObj = open(fileName, 'w')
for account in self.accounts.values():
account.save(fileObj)
fileObj.close()
>>> bank = createBank(5)
>>> bank.save('bank.txt')
Run the method in the shell and then inspect the file:

6. Class Methods vs Instance Methods
Define a class method and an instance method:
class MyClass:
def instanceMethod(self):
return 'I am an instance method.'
@classmethod
def classMethod(cls):
return 'I am a class method.'
>>> print(MyClass.classMethod())
I am a class method.
>>> obj = MyClass()
>>> print(obj.instanceMethod())
I am an instance method.
Try both methods in the shell:

7. Class Methods vs Instance Methods
Like a class variable, a class method is used with the class’s name
An instance method is always used with an instance
Class methods can see class variables but not instance variables
Instance methods can see both kinds of variables
>>> print(MyClass.classMethod())
I am a class method.
>>> print(obj.instanceMethod())
I am an instance method.

8. Why Use Class Methods?
Class methods are often used to create and return instances of those classes
Such methods are also called factory methods
>>> class MyClass:
@classmethod
def getInstance(cls): return MyClass()
>>> obj = MyClass.getInstance()
Example:

9. Loading a Savings Account
Include a class method to read an account’s info from a text file and return a new instance with that info:
@classmethod
def load(cls, fileObj): # In SavingsAccount
name = fileObj.readline().strip()
pin = fileObj.readline().strip()
balance = fileObj.readline().strip()
if not name: return None
else: return SavingsAccount(name, pin, float(balance))
>>> fileObj = open('account.txt', 'r')
>>> a = SavingsAccount.load(fileObj)
>>> print(a)
Run the method in the shell and then inspect the account:

10. Loading an Entire Bank
Include a class method to load accounts and create a bank:
@classmethod
def load(cls, fileName): # In Bank
fileObj = open(fileName, 'r')
bank = Bank()
while True:
a = SavingsAccount.load(fileObj)
if not a: break
else: bank.addAccount(a)
fileObj.close()
return bank
>>> bank = Bank.load('bank.txt')
>>> print(bank)
Test the method in the shell and then inspect the account:

11. Problems
Lots of manual labor to output and input the attributes of accounts as text
Must change this code every time we modify the structure of an account (add an address attribute, etc.)
A text file is insecure and inefficient

12. Solution: Pickle the Data
Pickling automates the process of converting an object to a form suitable for file storage
Unpickling automates the process of converting data from file storage back to the appropriate objects
pickling
Data model
File storage
unpickling

13. The pickle Module
dump(obj, fileObj) # Pickles obj and writes it to fileObj
load(fileObj) # Reads an object from fileObj,
# unpickles it, and returns it
# Raises an exception if the end of
# the file is reached

14. Using pickle for Output
>>> import pickle
>>> fileObj = open('test.dat', 'wb')
>>> pickle.dump('Hi there!', fileObj)
>>> fileobj.close()
Must use the argument “wb” or “rb” when opening a file for output or input
The “b” stands for “byte”

15. Using pickle for Input >>> import pickle
>>> fileObj = open('test.dat', 'wb')
>>> pickle.dump('Hi there!', fileObj)
>>> fileobj.close()
>>> fileObj = open('test.dat', 'rb')
>>> s = pickle.load(fileObj)
>>> print(s)
Hi there!

16. Using pickle with Accounts
>>> import pickle
>>> fileObj = open('test.dat', 'wb')
>>> account = SavingsAccount('Ken', '8809', )
>>> pickle.dump(account, fileObj)
>>> fileobj.close()
>>> fileObj = open('test.dat', 'rb')
>>> account = pickle.load(fileObj)
>>> print(account)
Blah blah blah

17. Saving an Entire Bank (Text)
Include a method for writing an bank’s info to a text file:
def save(self, fileName): # In Bank
fileObj = open(fileName, 'w')
for account in self.accounts.values():
account.save(fileObj)
fileObj.close()
Run the method in the shell and then inspect the file:
>>> bank = createBank(5)
>>> bank.save('bank.txt')

18. Saving an Entire Bank (Pickled)
Include a method for writing an bank’s info to a text file:
def save(self, fileName): # In Bank
fileObj = open(fileName, 'wb')
for account in self.accounts.values():
pickle.dump(account, fileObj)
fileObj.close()
Run the method in the shell and then inspect the file:
>>> bank = createBank(5)
>>> bank.save('bank.dat')

19. Loading an Entire Bank (Text)
Include a class method to load accounts and create a bank:
@classmethod
def load(cls, fileName): # In Bank
fileObj = open(fileName, 'r')
bank = Bank()
while True:
a = SavingsAccount.load(fileObj))
if not a: break
else: bank.addAccount(a)
fileObj.close()
return bank
>>> bank = Bank.load('bank.txt')
>>> print(bank)
Test the method in the shell and then inspect the account:

20. Loading an Entire Bank (Pickled)
Include a class method to load accounts and create a bank:
@classmethod
def load(cls, fileName): # In Bank
fileObj = open(fileName, 'rb')
bank = Bank()
try:
while True:
a = pickle.load(fileObj)
bank.addAccount(a)
except Exception(e): fileObj.close()
return bank
Test the method in the shell and then inspect the account:
>>> bank = Bank.load('bank.dat')
>>> print(bank)

21. For Wednesday
Search algorithms