To support these operations we will need a bank account ID (for the transfer operation) and the current balance.

We can create a class to support that:

class BalanceError(Exception): 
      value = "Sorry you only have $%6.2f in your account"

class BankAccount:
    def __init__(self, initialAmount):
       self.balance = initialAmount
       print "Account created with balance %5.2f" % self.balance

    def deposit(self, amount):
       self.balance = self.balance + amount

    def withdraw(self, amount):
       if self.balance >= amount:
          self.balance = self.balance - amount
       else:
          raise BalanceError, BalanceError.value % self.balance

    def checkBalance(self):
       return self.balance
       
    def transfer(self, amount, account):
       try: 
          self.withdraw(amount)
          account.deposit(amount)
       except BalanceError:
          print BalanceError.value % self.balance

Note 1: We check the balance before withdrawing and also use an exception to handle errors. Of course there is no Python error type BalanceError so we needed to create one of our own - it's simply an subclass of the standard Exception class with a string value. The string value is defined as an attribute of the exception class purely as a convenience, it ensures that we can generate standard error messages every time we raise an error. When we raise BalanceError we pass the internal format string value filled in with the current value of the object's balance. Notice that we didn't use self when defining the value in BalanceError, that's because value is a shared attribute across all instances, it is defined at the class level and known as a class variable. We access it by using the class name followed by a dot: BalanceError.value as seen above. Now, when the error generates its traceback it concludes by printing out the formatted error string showing the current balance.

Note 2: The transfer method uses the BankAccount's withdraw/deposit member functions or methods to do the transfer. This is very common in OO and is known as self messaging. It means that derived classes can implement their own versions of deposit/withdraw but the transfer method can remain the same for all account types.

The InterestAccount class

Now we use inheritance to provide an account that adds interest (we'll assume 3%) on every deposit. It will be identical to the standard BankAccount class except for the deposit method. So we simply override that:

class InterestAccount(BankAccount):
   def deposit(self, amount):
       BankAccount.deposit(self,amount)
       self.balance = self.balance * 1.03
       

And that's it. We begin to see the power of OOP, all the other methods have been inherited from BankAccount (by putting BankAccount inside the parentheses after the new class name). Notice also that deposit called the superclass's deposit method rather than copying the code. Now if we modify the BankAccount deposit to include some kind of error checking the sub-class will gain those changes automatically.

The ChargingAccount class

This account is again identical to a standard BankAccount class except that this time it charges $3 for every withdrawal. As for the InterestAccount we can create a class inheriting from BankAccount and modifying the withdraw method.

class ChargingAccount(BankAccount):
    def __init__(self, initialAmount):
        BankAccount.__init__(self, initialAmount)
        self.fee = 3
        
    def withdraw(self, amount):
        BankAccount.withdraw(self, amount+self.fee)

Note 1: We store the fee as an instance variable so that we can change it later if necessary. Notice that we can call the inherited __init__ just like any other method.

Note 2: We simply add the fee to the requested withdrawal and call the BankAccount withdraw method to do the real work.

Note 3: We introduce a side effect here in that a charge is automatically levied on transfers too, but that's probably what we want, so is OK.

Testing our system

To check that it all works try executing the following piece of code (either at the Python prompt or by creating a separate test file).

from bankaccount import *

# First a standard BankAccount
a = BankAccount(500)
b = BankAccount(200)
a.withdraw(100)
# a.withdraw(1000)
a.transfer(100,b)
print "A = ", a.checkBalance()
print "B = ", b.checkBalance()


# Now an InterestAccount
c = InterestAccount(1000)
c.deposit(100)
print "C = ", c.checkBalance()


# Then a ChargingAccount
d = ChargingAccount(300)
d.deposit(200)
print "D = ", d.checkBalance()
d.withdraw(50)
print "D = ", d.checkBalance()
d.transfer(100,a)
print "A = ", a.checkBalance()
print "D = ", d.checkBalance()


# Finally transfer from charging account to the interest one
# The charging one should charge and the interest one add
# interest
print "C = ", c.checkBalance()
print "D = ", d.checkBalance()
d.transfer(20,c)
print "C = ", c.checkBalance()
print "D = ", d.checkBalance()

Now uncomment the line a.withdraw(1000) to see the exception at work.

That's it. A reasonably straightforward example but it shows how inheritance can be used to quickly extend a basic framework with powerful new features.

We've seen how we can build up the example in stages and how we can put together a test program to check it works. Our tests were not complete in that we didn't cover every case and there are more checks we could have included - like what to do if an account is created with a negative amount...

Collections of Objects

One problem that might have occurred to you is how we deal with lots of objects. Or how to manage objects which we create at runtime. It's all very well creating Bank Accounts statically as we did above:

acc1 = BankAccount(...)
acc2 = BankAccount(...)
acc3 = BankAccount(...)
etc...

But in the real world we don't know in advance how many accounts we need to create. How do we deal with this? Lets consider the problem in more detail:

We need some kind of 'database' that allows us to find a given bank account by its owners name (or more likely their bank account number - since one person can have many accounts and several persons can have the same name...)

Finding something in a collection given a unique key....hmmm, sounds like a dictionary! Lets see how we'd use a Python dictionary to hold dynamically created objects:

from bankaccount import *
import time

# Create new function to generate unique id numbers
def getNextID():
    ok = raw_input("Create account[y/n]? ")
    if ok[0] in 'yY':  # check valid input
       id = time.time() # use current time as basis of ID
       id = int(id) % 10000 # convert to int and shorten to 4 digits
    else: id = -1  # which will stop the loop
    return id
    
# Let's create some accounts and store them in a dictionary
accountData = {}  # new dictionary
while 1:          # loop forever
   id = getNextID()
   if id == -1: 
      break       # break forces an exit from the while loop
   bal = float(raw_input("Opening Balance? "))  # convert string to float  
   accountData[id] = BankAccount(bal) # use id to create new dictionary entry
   print "New account created, Number: %04d, Balance %0.2f" % (id,bal)

# Now let's access the accounts
for id in accountData.keys():
    print "%04d\t%0.2f" % (id,accountData[id].checkBalance())

# and find a particular one
# Enter non-number to force exception and end program
while 1:
   id = int(raw_input("Which account number? "))
   if id in accountData.keys():
      print "Balance = %0.2d" % accountData[id].checkBalance()
   else: print "Invalid ID"

Of course the key you use for the dictionary can be anything that uniquely identifies the object, it could be one of its attributes, like balance say (except that balance would not be a very good unique key!). Anything at all that is unique. You might find it worthwhile going back to the raw materials chapter and reading the dictionary section again, they really are very useful containers.

Saving Your Objects

One snag with all of this is that you lose your data when the program ends. You need some way of saving objects too. As you get more advanced you will learn how to use databases to do that but we will look at using a simple text file to save and retrieve objects. (If you are using Python there are a couple of modules called Pickle and Shelve) that do this much more effectively but as usual I'll try to show you the generic way to do it that will work in any language. Incidentally the technical term for the ability to save and restore objects is Persistence.

The generic way to do this is to create save and restore methods at the highest level object and override in each class, such that they call the inherited version and then add their locally defined attributes:

class A:
   def __init__(self,x,y):
     self.x = x
     self.y = y

   def save(self,fn):
     f = open(fn,"w")
     f.write(str(self.x)+ '\n') # convert to a string and add newline
     f.write(str(self.y)+'\n')
     return f             # for child objects to use

   def restore(self, fn):
     f = open(fn)
     self.x = int(f.readline()) # convert back to original type
     self.y = int(f.readline())
     return f
     
class B(A):
   def __init__(self,x,y,z):
     A.__init__(self,x,y)
     self.z = z
   
   def save(self,fn):
     f = A.save(self,fn)  # call parent save
     f.write(str(self.z)+'\n')
     return f         # in case further children exist
   
   def restore(self, fn):
     f = A.restore(self,fn)
     self.z = int(f.readline())
     return f

# create instances
a = A(1,2)
b = B(3,4,5)

# save the instances
a.save('a.txt').close() # remember to close the file
b.save('b.txt').close()

# retrieve instances
newA = A(5,6)
newA.restore('a.txt').close() # remember to close the file
newB = B(7,8,9)
newB.restore('b.txt').close()
print "A: ",newA.x,newA.y
print "B: ",newB.x,newB.y,newB.z

Note: The values printed out are the restored values not the ones we used to create the instances.

The key thing is to override the save/restore methods in each class and to call the parent method as the first step. Then in the child class only deal with child class attributes. Obviously how you turn an attribute into a string and save it is up to you the programmer but it must be output on a single line. When restoring you simply reverse the storing process.

Mixing Classes and Modules

Modules and classes both provide mechanisms for controlling the complexity of a program. It seems reasonable that as programs get bigger we would want to combine these features by putting classes into modules. Some authorities recommend putting each class into a separate module but I find this simply creates an explosion of modules and increases rather than decreases complexity. Instead I group classes together and put the group into a module. Thus in our example above I might put all the bank account class definitions in one module, bankaccount, say, and then create a separate module for the application code that uses the module. A simplified representation of that would be:

# File: bankaccount.py
#
# Implements a set of bank account classes
###################

class BankAccount: ....

class InterestAccount: ...

class ChargingAccount: ...

And then to use it:

import bankaccount

newAccount = bankaccount.BankAccount(50)
newChrgAcct = bankaccount.ChargingAccount(200)


# now do stuff

But what happens when we have two classes in different modules that need to access each others details? The simplest way is to import both modules, create local instances of the classes we need and pass the instances of one class to the other instances methods. Passing whole objects around is what makes it object oriented programming. You don't need to extract the attributes out of one object and pass them into another, just pass the entire object. Now if the receiving object uses a polymorphic message to get at the information it needs then the method will work with any kind of object that supports the message.

Let's make that more concrete by looking at an example. Let's create a short module called logger that contains two classes. The first logs activity in a file. This logger will have a single method log() which has a "loggable object" as a parameter. The other class in our module is a Loggable class that can be inherited by other classes to work with the logger. It looks like this:

# File: logger.py
#
# Create Loggable and Logger classes for logging activities 
# of objects
############

class Loggable:
   def activity(self):
       return "This needs to be overridden locally"

class Logger:
   def __init__(self, logfilename = "logger.dat"):
       self._log = open(logfilename,"a")
       
   def log(self, loggedObj):
       self._log.write(loggedObj.activity() + '\n')

   def __del__(self):
       self._log.close()

Note that we have provided a destructor method (__del__) to close the file when the logger object is deleted or garbage collected. This is another "magic method" in Python (as shown by the double '_' characters) similar in many ways to __init__()

Also notice that we've called the log attribute _log with a '_' character in front of the name. This is another common naming convention in Python, like using capitalized words for class names. A single underscore indicates that the attribute is not intended to be accessed directly, but only via the methods of the class.

Now before we can use our module we will create a new module which defines loggable versions of our bank account classes:

# File: loggablebankaccount.py
#
# Extend Bank account classes to work with logger module.
###############################

import bankaccount, logger

class LoggableBankAccount(bankaccount.BankAccount, logger.Loggable):
    def activity(self):
       return "Account balance = %d" % self.checkBalance()

class LoggableInterestAccount(bankaccount.InterestAccount,
                              logger.Loggable):
    def activity(self):
       return "Account balance = %d" % self.checkBalance()

class LoggableChargingAccount(bankaccount.ChargingAccount,
                              logger.Loggable):
    def activity(self):
       return "Account balance = %d" % self.checkBalance()

Notice we are using a feature called multiple inheritance, where we inherit not one but two parent classes. This isn't strictly needed in Python since we could just have added an activity() method to our original classes and achieved the same effect but in statically typed OOP languages such as Java or C++ this technique would be necessary so I will show you the technique here for future reference.

The sharp eyed amongst you may have noticed that the activity() method in all three classes is identical. That means we could save ourselves some typing by creating an intermediate type of loggable account class that inherits Loggable and only has an activity method. We can then create our three different loggable account types by inheriting from that new class as well as from the vanilla Loggable class. Like this:

class LoggableAccount(logger.Loggable):
     def activity(self):
         return "Account balance = %d" % self.checkBalance()

class LoggableBankAccount(bankaccount.BankAccount, LoggableAccount):
     pass

class LoggableInterestAccount(bankaccount.InterestAccount, LoggableAccount):
     pass

class LoggableChargingAccount(bankaccount.ChargingAccount, LoggableAccount):
     pass

It doesn't save a lot of code but it does mean we only have one method definition to test and maintain instead of three identical methods. This type of programming where we introduce a superclass with shared functionality is sometimes called mixin programming and the minimal class is called a mixin class. It is a common outcome of this style that the final class definitions have little or no body but a long list of inherited classes, just as we see here. It's also quite common that mixin classes do not themselves inherit from anything, although in this case we did. In essence it's just a way of adding a common method (or set of methods) to a class or set of classes via the power of inheritance. (The term mixin originates in the world of ice cream parlours where different flavours of ice cream are added (or mixed in) to vanilla to produce a new flavour. The first language to support this style was called Flavors which was a popular dialect of Lisp for a while.)

Now we come to the point of this excercise which is to show our application code creating a logger object and some bank accounts and passing the accounts to the logger, even though they are all defined in different modules!

# Test logging and loggable bank accounts.
#############

import logger
import loggablebankaccount as lba

log = logger.Logger()

ba = lba.LoggableBankAccount(100)
ba.deposit(700)
log.log(ba)

intacc = lba.LoggableInterestAccount(200)
intacc.deposit(500)
log.log(intacc)

Note the use of the as keyword to create a shortcut name when importing loggablebankaccount

Note also that once we have created the local instances we no longer need to use the module prefix and because there is no direct access from one object to the other, it is all via messages, there is no need for the two class definition modules to directly refer to each other either. Finally notice also that the Logger works with instances of both LoggableBankAccount and LoggableInterestAccount because they both support the Loggable interface. Compatibility of object interfaces via polymorphism is the foundation upon which all OOP programs are built.

I should point out that a much more sophisticated logging system is included in the standard library logging module, this one was purely to demonstrate some techniques. If you want logging facilities in your own programmes investigate the standard logging module first of all.

Hopefully this has given you a taste of Object Oriented Programming and you can move on to some of the other online tutorials, or read one of the books mentioned at the beginning for more information and examples. Now we will briefly look at how OOP is done in VBScript and JavaScript.

OOP in VBScript

VBScript supports the concept of objects and allows us to define classes and create instances, however it does not support the concepts of inheritance or polymorphism. VBScript is therefore what is known as Object Based rather than fully Object Oriented. Nonetheless the concepts of combining data and function in a single object remain useful, and a limited form of inheritance is possible using a technique called delegation which we discuss below.

Defining classes

A class is defined in VBScript using the Class statement, like this:

<script type=text/VBScript>
Class MyClass
   Private anAttribute
   Public Sub aMethodWithNoReturnValue()
       MsgBox "MyClass.aMethodWithNoReturnValue"
   End Sub
   Public Function aMethodWithReturnValue()
       MsgBox "MyClass.aMethodWithReturnValue"
       aMethodWithReturnValue = 42
   End Function
End Class
</script>

This defines a new class called MyClass with an attribute called anAttribute which is only visible to the methods inside the class, as indicated by the keyword Private. It is conventional to declare data attributes to be Private and most methods to be Public. This is known as data hiding and has the advantage of allowing us to control access to the data by forcing methods to be used and the methods can do data quality checks on the values being passed in and out of the object. Python provides its own mechanism for achieving this but it is beyond the scope of this tutorial.

Creating Instances

We create instances in VBScript with a combination of the Set and New keywords. The variable to which the new instance is assigned must also have been declared with the Dim keyword as is the usual VBScript style.

<script type=text/VBScript>
Dim anInstance
Set anInstance = New MyClass
</script>

This creates an instance of the class declared in the previous section and assigns it to the anInstance variable.

Sending Messages

Messages are sent to instances using the same dot notation used by Python.

<script type=text/VBScript>
Dim aValue
anInstance.aMethodWithNoReturnValue()
aValue = anInstance.aMethodWithReturnValue()
MsgBox "aValue = " & aValue
</script>

The two methods declared in the class definition are called, in the first case there is no return value, in the second we assign the return to the variable aValue. There is nothing unusual here apart from the fact that the subroutine and function are preceded by the instance name.

Inheritance and Polymorphism

VBScript as a language does not provide any inheritance mechanism nor any mechanism for polymorphism. However we can fake it to some degree by using a technique called delegation. This simply means that we define an attribute of the sub class to be an instance of the theoretical parent class. We then define a method for all of the "inherited" methods which simply calls (or delegates to), in turn, the method of the parent instance. Let's subclass MyClass as defined above:

<script type=text/VBScript>
Class SubClass
   Private parent
   Private Sub Class_Initialize()
      Set parent = New MyClass
   End Sub
   Public Sub aMethodWithNoReturnValue()
      parent.aMethodWithNoREturnVAlue
   End Sub
   Public Function aMethodWithReturnValue()
      aMethodWithReturnValue = parent.aMethodWithReturnValue
   End Function
   Public Sub aNewMethod
      MsgBox "This is unique to the sub class"
   End Sub
End Class

Dim inst,aValue
Set inst = New SubClass
inst.aMethodWithNoReturnVAlue
aValue = inst.aMethodWithReturnValue
inst.aNewMethod
MsgBox "aValue = " & CStr(aValue)
</script>

The key points to note here are the use of the private attribute parent and the special, private method Class_Initialise. The former is the superclass delegate attribute and the latter is the equivalent of Pythons __init__ method for initializing instances when they are created, it is the VBScript constructor in other words.

OOP in JavaScript

JavaScript supports objects using a technique called prototyping. This means that there is no explicit class construct in JavaScript and instead we can define a class in terms of a set of functions or a dictionary like concept known as an initializer.

Defining classes

The most common way to define a JavaScript "class" is to create a function with the same name as the class, effectively this is the constructor, but is not contained within any other construct. It looks like this:

<script type=text/JavaScript>
function MyClass(theAttribute)
{
   this.anAttribute = theAttribute;
};
</script>

You might notice the keyword this which is used in the same way as Python's self as a placeholder reference to the current instance.

We can add new attributes to the class later using the built in prototype attribute like this:

<script type=text/JavaScript>
MyClass.prototype.newAttribute = null;
</script>

This defines a new attribute of MyClass called newAttribute.

Methods are added by defining a normal function then assigning the function name to a new attribute with the name of the method. Normally the method and function have the same name, but there is nothing to stop you calling the methods something different, as illustrated below:

<script type=text/JavaScript>
function oneMethod(){
    return this.anAttribute;
}
MyClass.prototype.getAttribute = oneMethod;
function printIt(){
    document.write(this.anAttribute + "<BR>");
};
MyClass.prototype.printIt = printIt;
</script>

Of course it would be more convenient to define the functions first then finish up with the constructor and assign the methods inside the constructor and this is in fact the normal approach, so that the full class definition looks like this:

<script type=text/JavaScript>
function oneMethod(){
    return this.anAttribute;
};

function printIt(){
    document.write(this.anAttribute + "<BR>");
};

function MyClass(theAttribute)
{
   this.anAttribute = theAttribute;
   this.getAttribute = oneMethod;
   this.printIt = printIt;
};
</script>

Creating Instances

We create instances of classes using the keyword new, like this:

<script type=text/JavaScript>
var anInstance = new MyClass(42);
</script>

Which creates a new instance called anInstance.

Sending Messages

Sending messages in JavaScript is no different to our other languages, we use the familiar dot notation.

<script type=text/JavaScript>
document.write("The attribute of anInstance is: <BR>");
anInstance.printIt();
</script>

Inheritance and Polymorphism

Unlike VBScript it is possible to use JavaScript's prototyping mechanism to inherit from another class. It is rather more complex than the Python technique but is not completely unmanageable, but it is, in my experience, a relatively uncommon technique among JavaScript programmers.

The key to inheritance in JavaScript is the prototype keyword (we used it in passing in the code above). By using prototype we can effectively add features to an object after it has been defined. We can see this in action here:

<script type="text/javascript">
function Message(text){
   this.text = text;
   this.say = function(){
   	document.write(this.text + '<br>');
	};
};

msg1 = new Message('This is the first');
msg1.say();

Message.prototype.shout = function(){
    alert(this.text);
    };

msg2 = new Message('This gets the new feature');
msg2.shout();

/* But so did msg1...*/
msg1.shout();

</script>

Note 1: We added the new alert method using prototype after creating instance msg1 of the class but the feature was available to the existing instance as well as to the instance, msg2 created after the addition. That is, the new feature gets added to all instances of Message both existing and new.

Note 2: We used function in a new way here. It effectively is used to create a function object which is assigned to the object property. That is:

obj.func = function(){...};

is equivalent to saying:

function f(){....};
obj.func = f;

We will see a similar concept in Python when we get to the Functional Programming topic.

This prototyping feature gives rise to the interesting capability to change the behavior of built-in JavaScript objects, either adding new features or changing the way existing features function! Use this capability with great care if you don't want to spend your time grappling with really confusing bugs.

This use of prototype as a mechanism for adding functionality to existing classes has the disadvantage that it alters the existing instance behaviors and changes the original class definition.

More conventional style inheritance is available too, as shown below:

<script type="text/javascript">
function Parent(){
   this.name = 'Parent';
   this.basemethod = function(){
       alert('This is the parent');
       };
};

function Child(){
   this.parent = Parent;
   this.parent();
   this.submethod = function(){
       alert('This from the child');
       };
};

var aParent = new Parent();
var aChild = new Child();

aParent.basemethod();
aChild.submethod();
aChild.basemethod();

</script>

The key point to note here is that the Child object has access to the basemethod without it being explicitly granted, it has inherited it from the parent class by virtue of the assignment/call pair of lines:

   this.parent = Parent;
   this.parent();

within the Child class definition. And thus we have inherited the basemethod from the Parent class!

We can, of course, use the same delegation trick we used with VBScript. Here is the VBScript example translated into JavaScript:

<script type=text/JavaScript>
function noReturn(){
   this.parent.printIt();
};

function returnValue(){
      return this.parent.getAttribute();
};

function newMethod(){
      document.write("This is unique to the sub class<BR>");
};

function SubClass(){
   this.parent = new MyClass(27);
   this.aMethodWithNoReturnValue = noReturn; 
   this.aMethodWithReturnValue = returnValue;
   this.aNewMethod = newMethod;
};

var inst, aValue;
inst = new SubClass(); // define superclass 
document.write("The sub class value is:<BR>");
inst.aMethodWithNoReturnValue();
aValue = inst.aMethodWithReturnValue();
inst.aNewMethod();
document.write("aValue = " + aValue);
</script>

We will see classes and objects being used in the following topics and case studies. It is not always obvious to a beginner how this, apparently complex, construct can make programs easier to write and understand but hopefully as you see classes being used in real programs it will become clearer. One thing I would like to say is that, for very small programs they do not really help and almost certainly will make the program longer. However as your programs start to get bigger - over about 100 lines say - then you will find that classes and objects can help to keep things organized and even reduce the amount of code you write.

If you are one of those who finds the whole OOP concept confusing don't panic, many people have programmed for their whole lives without ever creating a single class! On the other hand, if you can get to grips with objects it does open up some powerful new techniques.