Classes and OOP II

Magic Methods

You wrote __str__ in OOP I to control what print() shows. Python has a whole family of these dunder methods. They let your objects work with ==, +, and len().

You wrote __str__ in OOP I to control what print() shows. There is a whole family of these dunder methods that hook your objects into Python operators and built-in functions.

Without a custom dunder the default repr just shows the type and memory address — not very useful:

class Point:
	def __init__(self, x, y):
		self.x = x
		self.y = y
 
print(repr(Point(1, 2)))
# <__main__.Point object at 0x...>

Python

__repr__ is the developer-facing version of __str__. It is what you see in the REPL or when an object appears inside a list.

Without __repr__ you get the default ugly form. With it, you control the output:

class Point:
	def __init__(self, x, y):
		self.x = x
		self.y = y
 
	def __repr__(self):
		return f"Point({self.x}, {self.y})"
 
p = Point(1, 2)
print(repr(p))

Python

Output

What will be the output?

class Item:
	def __init__(self, n):
		self.n = n
 
	def __repr__(self):
		return f"Item:{self.n}"
 
print(repr(Item(7)))

Python

__eq__ controls what == means for your class. By default two objects are equal only if they are the same instance.

Two coins with the same value should compare equal. Define __eq__ to make that happen:

class Coin:
	def __init__(self, value):
		self.value = value
 
	def __eq__(self, other):
		return self.value == other.value
 
a = Coin(25)
b = Coin(25)
print(a == b)

Python

Output

What will be the output?

class Box:
	def __init__(self, n):
		self.n = n
 
a = Box(5)
b = Box(5)
print(a == b)

Python

__add__ teaches Python what + means for your class. The result is whatever the method returns.

Adding two vectors returns a new Vector with summed parts:

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)
 
v = Vector(1, 2) + Vector(3, 4)
print(v.x, v.y)

Python

Output

What will be the output?

class N:
	def __init__(self, v):
		self.v = v
 
	def __add__(self, other):
		return N(self.v + other.v)
 
r = N(2) + N(3)
print(r.v)

Python

__len__ tells the built-in len() what to return for your object. It must return an integer.

A Playlist holds a list of songs. len() reports how many:

class Playlist:
	def __init__(self, songs):
		self.songs = songs
 
	def __len__(self):
		return len(self.songs)
 
p = Playlist(["a", "b", "c"])
print(len(p))

Python

Output

What will be the output?

class Bag:
	def __init__(self, items):
		self.items = items
 
	def __len__(self):
		return len(self.items)
 
b = Bag([10, 20])
print(len(b))

Python

What will be the output?

class P:
	def __init__(self, x):
		self.x = x
 
	def __eq__(self, other):
		return self.x == other.x
 
print(P(7) == P(7))

Python

What will be the output?

class V:
	def __init__(self, n):
		self.n = n
 
	def __add__(self, other):
		return V(self.n + other.n)
 
r = V(10) + V(5)
print(r.n)

Python

What will be the output?

class List2:
	def __init__(self, items):
		self.items = items
 
	def __len__(self):
		return len(self.items) * 2
 
l = List2([1, 2, 3])
print(len(l))

Python

What will be the output?

class T:
	def __init__(self, n):
		self.n = n
 
	def __repr__(self):
		return f"T<{self.n}>"
 
print([T(1), T(2)])

Python