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