Python Language Introduction to classes


A class, functions as a template that defines the basic characteristics of a particular object. Here's an example:

class Person(object):
     """A simple class."""                            # docstring
     species = "Homo Sapiens"                         # class attribute

     def __init__(self, name):                        # special method
         """This is the initializer. It's a special
         method (see below).
         """ = name                             # instance attribute

     def __str__(self):                               # special method
         """This method is run when Python tries 
         to cast the object to a string. Return 
         this string when using print(), etc.

     def rename(self, renamed):                       # regular method
         """Reassign and print the name attribute.""" = renamed
         print("Now my name is {}".format(

There are a few things to note when looking at the above example.

  1. The class is made up of attributes (data) and methods (functions).
  2. Attributes and methods are simply defined as normal variables and functions.
  3. As noted in the corresponding docstring, the __init__() method is called the initializer. It's equivalent to the constructor in other object oriented languages, and is the method that is first run when you create a new object, or new instance of the class.
  4. Attributes that apply to the whole class are defined first, and are called class attributes.
  5. Attributes that apply to a specific instance of a class (an object) are called instance attributes. They are generally defined inside __init__(); this is not necessary, but it is recommended (since attributes defined outside of __init__() run the risk of being accessed before they are defined).
  6. Every method, included in the class definition passes the object in question as its first parameter. The word self is used for this parameter (usage of self is actually by convention, as the word self has no inherent meaning in Python, but this is one of Python's most respected conventions, and you should always follow it).
  7. Those used to object-oriented programming in other languages may be surprised by a few things. One is that Python has no real concept of private elements, so everything, by default, imitates the behavior of the C++/Java public keyword. For more information, see the "Private Class Members" example on this page.
  8. Some of the class's methods have the following form: __functionname__(self, other_stuff). All such methods are called "magic methods" and are an important part of classes in Python. For instance, operator overloading in Python is implemented with magic methods. For more information, see the relevant documentation.

Now let's make a few instances of our Person class!

>>> # Instances
>>> kelly = Person("Kelly")
>>> joseph = Person("Joseph")
>>> john_doe = Person("John Doe")

We currently have three Person objects, kelly, joseph, and john_doe.

We can access the attributes of the class from each instance using the dot operator . Note again the difference between class and instance attributes:

>>> # Attributes
>>> kelly.species
'Homo Sapiens'
>>> john_doe.species
'Homo Sapiens'
>>> joseph.species
'Homo Sapiens'

We can execute the methods of the class using the same dot operator .:

>>> # Methods
>>> john_doe.__str__()
'John Doe'
>>>  print(john_doe)
'John Doe'
>>>  john_doe.rename("John")
'Now my name is John'