The Class Statement

Motivating Classes

• Classes are used to create new objects
• A class defines a set of attributes that are associated with a collection of objects
• This collection of objects is known as instances
• In practice, an object is synonymous with an instance

• A class is most commonly a collection of:

  • Functions, known as methods
  • Variables, known as class variables
  • Computed attributes, known as properties

Relating Classes to Dictionaries

• Dictionaries can be thought of as:

  • Some data storage

• Classes can be thought of as:

  • Some data storage
  • Logic applied to that data storage
• This logic is represented using methods
• In other words, we may prefer to use a dictionary (or another data structure) if we only need to store some data
• However, we may prefer a class if we need operations performed on that data
• These operations should be closely related to the data and almost form a kind of entity

• To summarize:

  • Just start using a dictionary when we have related data
  • If we realize we need logic for the data, use a class

Defining a Class

• A class is defined using the class statement
• The methods and variables associated with a class are executed during the class definition
• The following is an example:

>>> class Account(object):
...     num_accounts = 0
...     def __init__(self, name, balance):
...         self.name = name
...         self.balance = balance
...         Account.num_accounts += 1
...     def __del__(self):
...         Account.num_accounts -= 1
...     def deposit(self, amt):
...         self.balance = self.balance + amt
...     def withdraw(self, amt):
...         self.balance = self.balance - amt
...     def inquiry(self):
...         return self.balance

• The namespace of this class object is the following:

  • Account.num_accounts
  • Account.__init__
  • Account.__del__
  • Account.deposit
  • Account.withdraw
  • Account.inquiry

• The following are considered as instance methods:

  • deposit()
  • withdraw()
  • inquiry()

• The following are considered as properties:

  • Account.name
  • Account.balance

• The following is considered as a class variable:

  • Account.num_accounts

References

• Python Essential Reference
• Comparing Classes to Dictionaries
• Describing Class Attributes