A class serves as a template for all objects whose type is that class. Every object is an instance of some particular class. The objects we have used so far all have built-in classes, but new user-defined classes can be created as well. A class definition specifies the attributes and methods shared among objects of that class. We will introduce the class statement by revisiting the example of a bank account.

When introducing local state, we saw that bank accounts are naturally modeled as mutable values that have a balance. A bank account object should have a withdraw method that updates the account balance and returns the requested amount, if it is available. To complete the abstraction: a bank account should be able to return its current balance, return the name of the account holder, and an amount for deposit.

An Account class allows us to create multiple instances of bank accounts. The act of creating a new object instance is known as instantiating the class. The syntax in Python for instantiating a class is identical to the syntax of calling a function. In this case, we call Account with the argument 'Kirk', the account holder's name.

>>> a = Account('Kirk')

An attribute of an object is a name-value pair associated with the object, which is accessible via dot notation. The attributes specific to a particular object, as opposed to all objects of a class, are called instance attributes. Each Account has its own balance and account holder name, which are examples of instance attributes. In the broader programming community, instance attributes may also be called fields, properties, or instance variables.

>>> a.holder
'Kirk'
>>> a.balance
0

Functions that operate on the object or perform object-specific computations are called methods. The return values and side effects of a method can depend upon and change other attributes of the object. For example, deposit is a method of our Account object a. It takes one argument, the amount to deposit, changes the balance attribute of the object, and returns the resulting balance.

>>> a.deposit(15)
15

We say that methods are invoked on a particular object. As a result of invoking the withdraw method, either the withdrawal is approved and the amount is deducted, or the request is declined and the method returns an error message.

>>> a.withdraw(10)  # The withdraw method returns the balance after withdrawal
5
>>> a.balance       # The balance attribute has changed
5
>>> a.withdraw(10)
'Insufficient funds'

As illustrated above, the behavior of a method can depend upon the changing attributes of the object. Two calls to withdraw with the same argument return different results.