July 11, 2020 12 min to read

Python - Classes - Methods in classes

This article is a part of the Python - 101 series, you can access the full version of the series here:

• Coding 101

  • Introduction
  • Coding 101 - exercises

• Writing functions in Python

  • Basic
  • Scope of variables
  • More on arguments and lambda functions
  • Best practices

• Control flows in Python

  • Looping
  • Conditions

• Errors and exceptions

  • Type of errors
  • Exceptions

• Classes

  • Basic
  • Methods in classes (you are here!)

• Last words and further tips

Welcome to the twelfth article of the Python - Coding 101 series, which is also the second part of Python Classes. After reading this article, you will learn:

• What is special method in Python
• Most common types of special methods in Python class: __init__, __str__, etc.

In the last article on Python classes - basic, you have learnt how to define classes and methods in classes using class and def. There also exists special methods that add magic to your class.

• 1. What is special methods in classes?
• 2. The two most common special methods
  • 2.1. The __init__ method
  • 2.2. The __str__ method
• 3. Other important methods in Python

1. What is special methods in classes?

Magic methods are not meant to be invoked directly by you, but the invocation happens internally from the class on a certain action. For example, when you add two numbers using the + operator, internally, the __add__() method will be called.

Built-in classes in Python define many magic methods. Use the dir() function to see the number of magic methods inherited by a class. For example, the following lists all the attributes and methods defined in the int class.

>>> dir(int)
['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getnewargs__', '__gt__', '__hash__', '__index__', '__init__', '__init_subclass__', '__int__', '__invert__', '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

As you can see above, the int class includes various magic methods surrounded by double underscores. For example, the __add__ method is a magic method which gets called when we add two numbers using the + operator. Consider the following example:

>>> num=10
>>> num + 5
15
>>> num.__add__(5)
15

Magic methods are most frequently used to define overloaded behaviours of predefined operators in Python. For instance, arithmetic operators by default operate upon numeric operands. This means that numeric objects must be used along with operators like +, -, *, /, etc. The + operator is also defined as a concatenation operator in string, list and tuple classes. We can say that the + operator is overloaded.

To prepare to learn more about special methods, first we define a new class called Time:

>>> class Time:
...   """Represents the time of day."""
...   def print_time(self):
...     print('%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second))

2. The two most common special methods

2.1. The __init__ method

The init method (short for “initialization”) is a special method that gets invoked when an object is instantiated. Its full name is __init__ (two underscore characters, followed by init, and then two more underscores). An init method for the Time class might look like this:

>>> # inside class Time:
...   def __init__(self, hour=0, minute=0, second=0):
...     self.hour = hour
...     self.minute = minute
...     self.second = second

It is common for the parameters of __init__ to have the same names as the attributes. The statement self.hour = hour stores the value of the parameter hour as an attribute of self. The parameters are optional, so if you call Time with no arguments, you get the default values:

>>> time = Time()
>>> time.print_time()
00:00:00

If you provide one argument, it overrides hour:

>>> time = Time (9)
>>> time.print_time()
09:00:00

If you provide two arguments, they override hour and minute:

>>> time = Time(9, 45)
>>> time.print_time()
09:45:00

And if you provide three arguments, they override all three default values.

2.2. The __str__ method

__str__ is a special method, like __init__, that is supposed to return a string representation of an object.

For example, here is a str method for Time objects:

>>> # inside class Time:
...   def __str__(self):
...     return '%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second)

When you print an object, Python invokes the str method:

>>> time = Time(9, 45)
>>> print(time)
09:45:00

When I write a new class, I almost always start by writing __init__, which makes it easier to instantiate objects, and __str__, which is useful for debugging.

3. Other important methods in Python