__add__
- Python course
- Intermediate
- Lesson 31/116
- 5 min
Learn operator overloading with __add__.
The __add__ method
__add__(self, other) defines the + operator. It usually returns a new instance; it does not mutate self. This lets you do math-like operations on class instances.
Example: class Vector:\n def __add__(self, other):\n return Vector(self.x + other.x, self.y + other.y). Writing v1 + v2 returns a new Vector. self.x and other.x are summed; the result goes into a new instance. This gives immutable-style composition.
Return a new instance
__add__ does not mutate self; it returns a new instance. This keeps the original instances intact. After the operation, v1 and v2 stay the same.
Example: after v3 = v1 + v2, v1 and v2 are unchanged; v3 is a new instance. This is the expected behavior for mathematical vectors. If we mutated self, we would have unexpected side effects beyond v1 += v2.
Practice Add __add__ to the Vector class
Define the Vector class. Add def __init__(self, x, y): with self.x = x and self.y = y in the body. Add def __add__(self, other):; in the body write return Vector(self.x + other.x, self.y + other.y). Then call v1 = Vector(1, 2), v2 = Vector(3, 4), v3 = v1 + v2 and print(v3.x).
Starter code
# Define the Vector classSample solution
# Define the Vector class
class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
def __add__(self, other):
return Vector(self.x + other.x, self.y + other.y)
v1 = Vector(1, 2)
v2 = Vector(3, 4)
v3 = v1 + v2
print(v3.x)Expected output
4
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the __add__ body
class Vector:
def __add__(self, other):
return Vector(self.x + ____, self.y + other.y)class Vector:
def __add__(self, other):
return Vector(self.x + other.x, self.y + other.y)Explanation: other.x is added to self.x.
2 Which method does v1 + v2 call?
- __mul__
- __add__ Correct answer
- __sub__
- __radd__
Explanation: The + operator calls the __add__ method.
3 Match the __add__ concepts
| Term | Meaning |
|---|---|
__add__ |
+ operator |
return Vector(...) |
New instance |
v1 + v2 |
Operation call |