Python magic or Dunder method

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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 .Python magic method is defined as the special method which adds “magic” to a class. It starts and ends with double underscores, for example, _init_ or _str_.

The built-in classes define many magic methods. The dir() function can be used to see the number of magic methods inherited by a class. It has two prefixes, and suffix underscores in the method name.

It is most frequently used to define the overloaded behaviors of predefined operators.

The _init_ method is called after the instance of the class has been created but before it returned to the caller. It is invoked without any call, when an instance of the class is created like constructors in other programming languages such as C++, Java, C#, PHP, etc. These methods are also known as initialize and are called after _new_. Its where you should initialize the instance variables.

__str__

This function computes “informal” or a nicely printable string representation of an object and must return a string object.

__repr__

This function is called by the repr() built-in function to compute the “official” string representation of an object and returns a machine-readable representation of a type. The goal of the _repr_ is to be unambiguous.

__len__

This function should return the length of an object.

__call__

We can make an object callable by adding the _call_ magic method, and it is another method that is not needed quite as often is _call_.

If defined in a class, then that class can be called. But if it was a function, instance itself rather than modifying.

__del__

Just as _init_, which is a constructor method, _del_ is like a destructor. If you have opened a file in _init _, then _del_ can close it.

__bytes__

It offers to compute a byte-string representation of an object and should return a string object.

__ge__

This method gets invoked when >= operator is used and returns True or False.

__neg__

This function gets called for the unary operator.

__ipow__

This function gets called on the exponents with arguments. e.g. a**=b.

__le__

This function gets called on comparison using <= operator.

_nonzero_

This function returns the Boolean value of the object. It gets invoked when the bool (self) function is 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

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.https://imasdk.googleapis.com/js/core/bridge3.516.0_en.html#goog_383774773

__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__().Example: __new__() Copy

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.>>> emp = Employee()
__new__ magic method is called
__init__ magic method is called

Thus, 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
>>> str(num)
’12’
>>> #This is equivalent to
>>> int.__str__(num)
’12’

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

class Employee:
    def __init__(self):
        self.name='Swati'
        self.salary=10000
    def __str__(self):
        return 'name='+self.name+' salary=$'+str(self.salary)

See how the str() function internally calls the __str__() method defined in the Employee class. This is why it is called a magic method!>>> e1=Employee()
>>> print(e1)
name=Swati salary=$10000

__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.Example: Override __add__() Copy

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)

Run the above Python script to verify the overloaded operation of the + operator.>>> d1=distance(3,10)
>>> d2=distance(4,4)
>>> print(“d1= {} d2={}”.format(d1, d2))
d1= ft:3 in: 10 d2=ft:4 in: 4
>>> d3=d1+d2
>>> print(d3)
ft:8 in: 2

__ge__() method

The following method is added in the distance class to overload the >= operator.Example: __ge__() Copy

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

This method gets invoked when the >= operator is used and returns True or False. Accordingly, the appropriate message can be displayed>>> d1=distance(2,1)
>>> d2=distance(4,10)
>>> d1>=d2
False

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.