Magic Methods
In Python methods that start with a double underscore (dunder) and end with a double underscore are known as magic methods or dunder methods.
These methods are not supposed to be used by the programmer, but they are used by Python for special operations on the class.
For example, when you add two numbers of type int, python invokes (calls) the __add__() method for the int class.
If you type dir(int) into the terminal you will get a list of all the method names defined in the int class:
['__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__', 'as_integer_ratio', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']
You can see that int has an __add__() method, this is the method that is actually invoked (called) when you use the + operator. e.g. 5 + 3.
We can even do this manually (try it in the terminal):
x = 5
x.__add__(3) # will return a new int object 8
What Python does is provide you with the + operator because it makes sense too, but really it is doing the above.
NOTE: You should never invoke (call) a magic method yourself, they are used by other methods or special symbols like +.
1. __init__() Method
We have already seen this method. This is the method that gets invoked when an instance of a class is created.
For example,
class Person:
""" A simple person class """
def __init__(self, name):
self.name = name
if __name__ == "__main__":
person_one = Person("Sarah") # Here we call __init__("Sarah") i.e. name = "Sarah"
Here Person("Sarah") creates an instance of the class by passing __init__() the value Sarah, this is then bound to name and in turn, this is then set as the instance attribute self.name.
2. __str__() Method
The __str__() method is the method called when we try and print an object. It returns a str (string).
class Person:
""" A simple person class """
def __init__(self, name):
self.name = name
if __name__ == "__main__":
person_one = Person("Sarah") # Here we call __init__("Sarah") i.e. name = "Sarah"
print(person_one) # prints something like <__main__.Person object at 0x7fdf15fe3640>
The default printing of an object is just its name and the memory location of the object.
Python knows to call the __str()__ method, but we can do it manually:
print(person_one.__str__()) # prints something like <__main__.Person object at 0x7fdf15fe3640>
We can choose to overwrite this to print something more sensible.
class Person:
""" A simple person class """
def __init__(self, name):
self.name = name
def __str__(self):
desc = "Instance of Person\n\n"
desc += f"Name: {self.name}"
return desc
if __name__ == "__main__":
person_one = Person("Sarah") # Here we call __init__("Sarah") i.e. name = "Sarah"
print(person_one)
Prints out:
Instance of Person
Name: Sarah
3. __eq__() Method
The __eq__() method is the method called when we use the == operator.
We can make use of this for our Person class. While this is a bit odd, it demonstrates how to use it. We will say an instance of a Person is equal to another instance if the name and age are the same in both instances.
class Person:
""" A simple person class """
def __init__(self, name, age):
self.name = name
self.age = age
# test if the name and age are the same.
def __eq__(self, other):
# test if this instance (self) has the same name and age as other.
if self.name == other.name and self.age == other.age:
return True
else:
return False
if __name__ == "__main__":
person_one = Person("Sarah", 27)
person_two = Person("Sarah", 29)
person_three = Person("Sarah", 27)
print(person_one == person_two) # prints False because age is different
print(person_one == person_three) # prints True because name and age are the same
4. __add()__ Method
The last method we will look at (we also started with it) is __add__() which is called when we use the + operator.
Again we can make use of this for our Person class. Again this is a bit odd, but it demonstrates how to use it. We will say that we add an instance of a Personwith another instance by adding the names and the ages and creating a new person.
Clearly this is nonsense, but it shows how we can add two instances of the same class and get a new instance of a the same class.
If we think about adding two ints, 1+2 will create a new int with value 3. This is essentially what we are doing here.
class Person:
""" A simple person class """
def __init__(self, name, age):
self.name = name
self.age = age
# add the ages of this instance and other
def __add__(self, other):
name = self.name + other.name
age = self.age + other.age
return Person(name, age) # clearly this is nonsense, but you are returning a new Person object by adding the other two person objects
if __name__ == "__main__":
person_one = Person("Sarah", 27)
person_two = Person("Bob", 35)
print(person_one + person_two) # prints 62
List of all Built-in Methods
You can find a comprehensive list of magic methods via the following link:
Tutorials Teacher - Magic Methods
=== TASK ===
Create a class called Point.
Point should have two attributes:
| Attribute Name | Description |
|---|---|
x | x-coordinate of the point |
y | y-cordinate of the point |
Make sure the attributes are named as per the table.
You should override the __init__(), __str__(), __add__(), __sub__() and __mul__() methods.
__init__() Method
You should be able to create a new point as follows.
point_one = Point(3,2) # creates a point with x=3, y=2
__str__() Method
You should be able to print out a point as follows.
point_one = Point(3,2) # creates a point with x=3, y=2
print(point_one)
Should print:
Instance of Point
x: 3
y: 2
__add__() Method
You should be able to add two points as follows. NOTE: This should return a new Point object.
point_one = Point(3,2) # creates a point with x=3, y=2
point_two = Point(5,3) # creates a point with x=5, y=3
point_three = point_one + point_two # creates a new instance of point (object) x=8, y=5, i.e. Point(8,5)
print(point_three)
Should print:
Instance of Point
x: 8
y: 5
__sub__() Method
You should be able to subtract two points as follows. NOTE: This should return a new Point object.
point_one = Point(3,2) # creates a point with x=3, y=2
point_two = Point(5,3) # creates a point with x=5, y=3
point_three = point_one - point_two # creates a new instance of point (object) x=-2, y=-1, i.e. Point(-2,-1)
print(point_three)
Should print:
Instance of Point
x: -2
y: -1
__mul__() Method
You should be able to multiply two points as follows. NOTE: This should return a new Point object.
point_one = Point(3,2) # creates a point with x=3, y=2
point_two = Point(5,3) # creates a point with x=5, y=3
point_three = point_one * point_two # creates a new instance of point (object) x=15, y=6, i.e. Point(15,6)
print(point_three)
Should print:
Instance of Point
x: 15
y: 6
__eq__() Method
You should be able to test if two points are equal as follows. NOTE: This should return a new bool object.
point_one = Point(3,2) # creates a point with x=3, y=2
point_two = Point(5,2) # creates a point with x=5, y=2
point_three = Point(3,2) # creates a point with x=3, y=2
print(point_one == point_two) # prints False because x is different
print(point_one == point_three) # prints True because x and y are the same
Getting Started
You can copy and paste the following into a new Python file to get started.
class Point:
""" A simple representation of a point in 2d space"""
pass
if __name__ == "__main__":
point_one = Point(3,2)
print(point_one)
print()
point_two = Point(5,3)
print(point_two)
print()
point_three = point_one + point_two
print(point_three)
print()
point_four = point_one - point_two
print(point_four)
print()
point_five = point_one * point_two
print(point_five)
print()
print(point_one == point_two)
If your class is implemented correctly it will print out the following.
Instance of Point
x: 3
y: 2
Instance of Point
x: 5
y: 3
Instance of Point
x: 8
y: 5
Instance of Point
x: -2
y: -1
Instance of Point
x: 15
y: 6
False