Computer Science 111

Fundamentals of Programming IPersistent Data Models

Object Serialization

from bank import Bank

def createBank(numAccounts = 1): """Returns a new bank with the given number of accounts.""" bank = Bank() upperPin = numAccounts + 1000 for pinNumber in range(1000, upperPin): bank.add(SavingsAccount('Ken', str(pinNumber))) return bank

Helper Function to Create a Bank

bank = createBank(5)

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

File storageData model

save

load

Saving a Savings Account

def save(self, fileObj): # In SavingsAccount fileObj.write(self.name + '\n') fileObj.write(self.pin + '\n') fileObj.write(str(self.balance) + '\n')

Include a method for writing an account’s info to a text file:

>>> a = SavingsAccount('Ken', '8809', 500.00)

>>> fileObj = open('account.txt', 'w')

>>> a.save(fileObj)

>>> fileObj.close()

Run the method in the shell and then inspect the file:

Saving an Entire Bank

def save(self, fileName): # In Bank fileObj = open(fileName, 'w') for account in self.accounts.values(): account.save(fileObj) fileObj.close()

Include a method for writing an bank’s info to a text file:

>>> bank = createBank(5)

>>> bank.save('bank.txt')

Run the method in the shell and then inspect the file:

Class Methods vs Instance Methods

class MyClass:

def instanceMethod(self): return 'I am an instance method.' @classmethod def classMethod(cls): return 'I am a class method.'

Define a class method and an instance 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:

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

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:

Loading a Savings Account

@classmethoddef 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))

Include a class method to read an account’s info from a text file and return a new instance with that info:

>>> fileObj = open('account.txt', 'r')

>>> a = SavingsAccount.load(fileObj)

>>> print(a)

Run the method in the shell and then inspect the account:

Loading an Entire Bank

@classmethoddef 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

Include a class method to load accounts and create a bank:

>>> bank = Bank.load('bank.txt')

>>> print(bank)

Test the method in the shell and then inspect the account:

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

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

File storageData model

pickling

unpickling

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

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”

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!

Using pickle with Accounts

>>> import pickle

>>> fileObj = open('test.dat', 'wb')

>>> account = SavingsAccount('Ken', '8809', 500.00)

>>> pickle.dump(account, fileObj)

>>> fileobj.close()

>>> fileObj = open('test.dat', 'rb')

>>> account = pickle.load(fileObj)

>>> print(account)Blah blah blah

Saving an Entire Bank (Text)

def save(self, fileName): # In Bank fileObj = open(fileName, 'w') for account in self.accounts.values(): account.save(fileObj) fileObj.close()

Include a method for writing an bank’s info to a text file:

>>> bank = createBank(5)

>>> bank.save('bank.txt')

Run the method in the shell and then inspect the file:

Saving an Entire Bank (Pickled)

def save(self, fileName): # In Bank fileObj = open(fileName, 'wb') for account in self.accounts.values(): pickle.dump(account, fileObj) fileObj.close()

Include a method for writing an bank’s info to a text file:

>>> bank = createBank(5)

>>> bank.save('bank.dat')

Run the method in the shell and then inspect the file:

Loading an Entire Bank (Text)

@classmethoddef 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

Include a class method to load accounts and create a bank:

>>> bank = Bank.load('bank.txt')

>>> print(bank)

Test the method in the shell and then inspect the account:

Loading an Entire Bank (Pickled)

@classmethoddef 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

Include a class method to load accounts and create a bank:

>>> bank = Bank.load('bank.dat')

>>> print(bank)

Test the method in the shell and then inspect the account: