Magic or Dunder Methods

Advanced Python - Magic or Dunder Methods

Python lets our classes inherit from built-in classes. An inheriting child class of a built-in shares all the same attributes, including methods as the built-in. We can take advantage of core built-in functionality, but customize selected operations through the use of magic methods.

(this article modifies the three magic methods which control list indexing)

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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']

Consider the following example.

>>> num=10

>>> num + 5

>>> num.__add__(5)

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.

Some important magic methods:

__new()__

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__().

(see article for example)

str() method

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

ANOTHER __add__ example:

__add__ magic method is used to add the attributes of the class instance. For example, let’s say object1 is an instance of a class A and object2 is an instance of class B and both of these classes have an attribute called ‘a’, that holds an integer. When the operation object1 + object2 is done the __add__ method implicitly adds the attributes of these instances like object1.a + object2.a, if defined so.

class One:

def __init__(self, a):

self.a = a

def __add__(self, object2):

return self.a + object2.a

class Two:

def __init__(self, a):

self.a = a

def __add__(self, object2):

return self.a + object2.a

a_instance = One(10)

b_instance = Two(20)

print(a_instance + b_instance)

# Output: 30

In the code above, classes One and Two possess an attribute called ‘a’ that holds an integer value. We define two instances a_instance and b_instances that contain 10 and 20 values in their attribute respectively. When we perform the operation a_instance + b_instance the __add__ method defined would convert the operation into a_instance.a + b_instance.a that results in output 30.

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

len magic method

__len__ magic method is used to find the length of the instance attributes. When we use len(instance), it returns the length of the instance attribute which is usually containers.

class LenExample:

def __init__(self, item):

self.item = item

def __len__(self):

return len(self.item)

len_instance = LenExample([1, 2, 3])

print(len(len_instance))

# Output: 3

When we invoke the len() method the length of the list named item is returned that is defined inside the __len__ method.

Important Magic Methods

The following tables list important magic methods in Python 3.

Initialization and Construction	Description
__new__(cls, other)	To get called in an object's instantiation.
__init__(self, other)	To get called by the __new__ method.
__del__(self)	Destructor method.

Unary operators and functions	Description
__pos__(self)	To get called for unary positive e.g. +someobject.
__neg__(self)	To get called for unary negative e.g. -someobject.
__abs__(self)	To get called by built-in abs() function.
__invert__(self)	To get called for inversion using the ~ operator.
__round__(self,n)	To get called by built-in round() function.
__floor__(self)	To get called by built-in math.floor() function.
__ceil__(self)	To get called by built-in math.ceil() function.
__trunc__(self)	To get called by built-in math.trunc() function.

Augmented Assignment	Description
__iadd__(self, other)	To get called on addition with assignment e.g. a +=b.
__isub__(self, other)	To get called on subtraction with assignment e.g. a -=b.
__imul__(self, other)	To get called on multiplication with assignment e.g. a *=b.
__ifloordiv__(self, other)	To get called on integer division with assignment e.g. a //=b.
__idiv__(self, other)	To get called on division with assignment e.g. a /=b.
__itruediv__(self, other)	To get called on true division with assignment
__imod__(self, other)	To get called on modulo with assignment e.g. a%=b.
__ipow__(self, other)	To get called on exponentswith assignment e.g. a **=b.
__ilshift__(self, other)	To get called on left bitwise shift with assignment e.g. a<<=b.
__irshift__(self, other)	To get called on right bitwise shift with assignment e.g. a >>=b.
__iand__(self, other)	To get called on bitwise AND with assignment e.g. a&=b.
__ior__(self, other)	To get called on bitwise OR with assignment e.g. a\|=b.
__ixor__(self, other)	To get called on bitwise XOR with assignment e.g. a ^=b.

Type Conversion Magic Methods	Description
__int__(self)	To get called by built-int int() method to convert a type to an int.
__float__(self)	To get called by built-int float() method to convert a type to float.
__complex__(self)	To get called by built-int complex() method to convert a type to complex.
__oct__(self)	To get called by built-int oct() method to convert a type to octal.
__hex__(self)	To get called by built-int hex() method to convert a type to hexadecimal.
__index__(self)	To get called on type conversion to an int when the object is used in a slice expression.
__trunc__(self)	To get called from math.trunc() method.

String Magic Methods	Description
__str__(self)	To get called by built-int str() method to return a string representation of a type.
__repr__(self)	To get called by built-int repr() method to return a machine readable representation of a type.
__unicode__(self)	To get called by built-int unicode() method to return an unicode string of a type.
__format__(self, formatstr)	To get called by built-int string.format() method to return a new style of string.
__hash__(self)	To get called by built-int hash() method to return an integer.
__nonzero__(self)	To get called by built-int bool() method to return True or False.
__dir__(self)	To get called by built-int dir() method to return a list of attributes of a class.
__sizeof__(self)	To get called by built-int sys.getsizeof() method to return the size of an object.

Attribute Magic Methods	Description
__getattr__(self, name)	Is called when the accessing attribute of a class that does not exist.
__setattr__(self, name, value)	Is called when assigning a value to the attribute of a class.
__delattr__(self, name)	Is called when deleting an attribute of a class.

Operator Magic Methods	Description
__add__(self, other)	To get called on add operation using + operator
__sub__(self, other)	To get called on subtraction operation using - operator.
__mul__(self, other)	To get called on multiplication operation using * operator.
__floordiv__(self, other)	To get called on floor division operation using // operator.
__truediv__(self, other)	To get called on division operation using / operator.
__mod__(self, other)	To get called on modulo operation using % operator.
__pow__(self, other[, modulo])	To get called on calculating the power using ** operator.
__lt__(self, other)	To get called on comparison using < operator.
__le__(self, other)	To get called on comparison using <= operator.
__eq__(self, other)	To get called on comparison using == operator.
__ne__(self, other)	To get called on comparison using != operator.
__ge__(self, other)	To get called on comparison using >= operator.

__str__() method

__add__() method

__ge__() method

__len__ magic method

Important Magic Methods

str() method

add() method

ge() method

len magic method