Python - Magic or Dunder Methods

Magic methods in Python are the special methods that start and end with the double underscores. They are also called dunder methods. 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.

print(dir(int))

#output:
#['__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
res = num.__add__(5) 
print(res)

As you can see, when you do num+10, the + operator calls the __add__(10) method. You can also call num.__add__(5) directly which will give the same result. However, as mentioned before, magic methods are not meant to be called directly, but internally, through some other methods or actions.

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.

In order to make the overloaded behaviour available in your own custom class, the corresponding magic method should be overridden. For example, in order to use the + operator with objects of a user-defined class, it should include the __add__() method.

Let's see how to implement and use some of the important magic methods.

__new__() method

Languages such as Java and C# use the new operator to create a new instance of a class. In Python the __new__() magic method is implicitly called before the __init__() method. The __new__() method returns a new object, which is then initialized by __init__().

class Employee:
    def __new__(cls):
        print ("__new__ magic method is called")
        inst = object.__new__(cls)
                return inst
    def __init__(self):
        print ("__init__ magic method is called")
        self.name='Satya'

The above example will produce the following output when you create an instance of the Employee class. The __new__() method is called before the __init__() method.

__str__() method

Another useful magic method is __str__(). It is overridden to return a printable string representation of any user defined class. We have seen str() built-in function which returns a string from the object parameter. For example, str(12) returns '12'. When invoked, it calls the __str__() method in the int class.

num=12
val = int.__str__(num)
print(type(val))

Let us now override the __str__() method in the employee class to return a string representation of its object.

class employee:
  def __init__(self):
    self.name='Swati'
    self.salary=10000
  def __str__(self):
    return 'name='+self.name+', salary=$'+str(self.salary)
    
    
e1 = employee()
print(e1)

The str() function internally calls the __str__() method defined in the above employee class. This is why it is called a magic method!

__add__() method

In following example, a class named distance is defined with two instance attributes - ft and inch. The addition of these two distance objects is desired to be performed using the overloading + operator.

To achieve this, the magic method __add__() is overridden, which performs the addition of the ft and inch attributes of the two objects. The __str__() method returns the object's string representation.

class distance:
  def __init__(self, x=None,y=None):
    self.ft=x
    self.inch=y
  def __add__(self,x):
    temp=distance()
    temp.ft=self.ft+x.ft
    temp.inch=self.inch+x.inch
    if temp.inch>=12:
        temp.ft+=1
        temp.inch-=12
        return temp
  def __str__(self):
    return 'ft:'+str(self.ft)+' in: '+str(self.inch)
    
    
d1=distance(3,10)
d2=distance(4,4)
print("d1= {} d2={}".format(d1, d2))

d3=d1+d2
print(d3)

The example overloades the __add__() method which will be called when adding two objects using the + operator.

__ge__() method

The following method is added in the distance class to overload the >= operator.

class distance:
  def __init__(self, x=None,y=None):
      self.ft=x
      self.inch=y
  def __ge__(self, x):
      val1=self.ft*12+self.inch
      val2=x.ft*12+x.inch
      if val1>=val2:
          return True
      else:
          return False
          
          
d1=distance(2,1)
d2=distance(4,10)
print(d1>=d2)

The __ge__() method gets invoked when the >= operator is used and returns True or False. Accordingly, the appropriate message can be displayed.

Important Magic Methods

The following tables list important magic methods in Python 3.

Thus, you can use the appropriate magic methods to add various functionalities in your custom class.