object.__add__(self, other) method returns a new object that represents the sum of two objects. It implements the addition operator
+ in Python.
We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.
In the following example, you create a custom class
Data and overwrite the
__add__() method so that creates a new
Data object with the value being the sum of the values of the two operands
b of type
class Data: def __init__(self, value): self.value = value def __add__(self, other): return Data(self.value + other.value) a = Data(40) b = Data(2) c = a + b print(c.value) # 42
You have defined the dunder method so that the resulting sum of two
Data objects is a
Data object itself:
print(type(c)) # <class '__main__.Data'>
If you hadn’t defined the
__add__() method, Python would’ve raised a
How to Resolve TypeError: unsupported operand type(s) for +
Consider the following code snippet where you try to add two custom objects without defining the dunder method
class Data: def __init__(self, value): self.value = value a = Data(40) b = Data(2) c = a + b print(c.value)
Running this leads to the following error message on my computer:
Traceback (most recent call last): File "C:\Users\xcent\Desktop\code.py", line 11, in <module> print(a + b) TypeError: unsupported operand type(s) for +: 'Data' and 'Data'
The reason for this error is that the
__add__() dunder method has never been defined—and it is not defined for a custom object by default. So, to resolve the
TypeError: unsupported operand type(s) for +, you need to provide the
__add__(self, other) method in your class definition as shown previously:
class Data: def __init__(self, value): self.value = value def __add__(self, other): return Data(self.value + other.value)
Advanced Example of Adding Lists in a Custom Class
To use the addition operator on custom objects, you need to define the
__add__() dunder method that takes two arguments:
other and returns the result of
self + other. To obtain the result for a custom object, you can use the attributes (data) maintained in this object.
In the following code, you add two baskets together by combining their contents:
class Basket: def __init__(self, contents): self.contents = contents def __add__(self, other): return Basket(self.contents + other.contents) my_basket = Basket(['banana', 'apple', 'juice']) your_basket = Basket(['bred', 'butter']) our_basket = my_basket + your_basket print(our_basket.contents)
The output of this code snippet is the combined basket:
['banana', 'apple', 'juice', 'bred', 'butter']
The code consists of the following steps:
- Create the class
Basketthat holds the list contents to store some goods.
- Define the magic method
__add__that creates a new Basket by combining the list of goods (
contents) from the two operand baskets. Note that we rely on the already implemented addition operator on lists, i.e., list concatenation, to actually implement the addition operator for baskets.
- We create two baskets
your_basket, and add them together to a new basket
our_basketusing the defined addition operation.
Python __add__ vs __iadd__
Python provides the operator
x += y to add two objects in-place by calculating the sum
x + y and assigning the result to the first operands variable name
x. You can set up the in-place addition behavior for your own class by overriding the magic “dunder” method
__iadd__(self, other) in your class definition.
>>> x = 1 >>> x += 2 >>> x 3
x += y is syntactical sugar for the longer-form
x = x + y:
>>> x = 1 >>> x = x + 2 >>> x 3
So, here’s the difference between Python
__add__ and Python
If you overwrite the
__add__ magic method, you’ll define the semantics of simple Python addition
x + y. And if you overwrite the
__iadd__ magic method, you’ll define the semantics of in-place Python addition
x += y changing the original object
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.