Classes and OOP I

The str Method

When you print a list, Python shows its contents neatly. When you print your own object, you get something like <__main__.Dog object>. The __str__ method lets you control what appears.

When you print a list, Python shows its contents. When you print one of your own objects, you get something unhelpful. Let's fix that.

Look what happens when you print a custom object with no special setup:

class Dog:
	def __init__(self, name):
		self.name = name
 
rex = Dog("Rex")
print(rex)

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Output

Not useful. Python is showing the default representation: the class name and a memory address. You can change this by defining a __str__ method.

The __str__ method must return a string. Python calls it automatically when you use print() or str():

class Dog:
	def __init__(self, name):
		self.name = name
 
	def __str__(self):
		return f"Dog: {self.name}"

Python

Now printing the object gives a readable result:

class Dog:
	def __init__(self, name):
		self.name = name
 
	def __str__(self):
		return f"Dog: {self.name}"
 
rex = Dog("Rex")
print(rex)

Python

Output

What will be the output?

class Cat:
	def __init__(self, name):
		self.name = name
 
	def __str__(self):
		return self.name
 
c = Cat("Luna")
print(c)

Python

Remember f-strings from the Strings II series? They are perfect for building readable representations with multiple attributes.

Combine multiple attributes into a single string:

class Product:
	def __init__(self, name, price):
		self.name = name
		self.price = price
 
	def __str__(self):
		return f"{self.name}: ${self.price}"
 
p = Product("Book", 12)
print(p)

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Output

You can also use format specifiers inside the f-string. You learned these in Strings II.

Use :.2f to show exactly two decimal places:

class Product:
	def __init__(self, name, price):
		self.name = name
		self.price = price
 
	def __str__(self):
		return f"{self.name}: ${self.price:.2f}"
 
p = Product("Pen", 1.5)
print(p)

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Output

What will be the output?

class Temp:
	def __init__(self, c):
		self.c = c
 
	def __str__(self):
		return f"{self.c:.1f} C"
 
t = Temp(36.567)
print(t)

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Calling str() on an object also triggers __str__. That means you can use the result as a normal string.

Convert to string and use it in other expressions:

class Color:
	def __init__(self, r, g, b):
		self.r = r
		self.g = g
		self.b = b
 
	def __str__(self):
		return f"rgb({self.r}, {self.g}, {self.b})"
 
c = Color(255, 128, 0)
text = "Color is " + str(c)
print(text)

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Output

What will be the output?

class Point:
	def __init__(self, x, y):
		self.x = x
		self.y = y
 
	def __str__(self):
		return f"({self.x}, {self.y})"
 
p = Point(3, 7)
print(str(p))

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What will be the output?

class Coin:
	def __init__(self, value):
		self.value = value
 
	def __str__(self):
		return f"{self.value} cents"
 
c = Coin(25)
print(f"I have a {c}")

Python

What will be the output?

class Box:
	def __init__(self, w, h):
		self.w = w
		self.h = h
 
	def __str__(self):
		return f"{self.w}x{self.h}"
 
b = Box(10, 5)
result = str(b)
print(len(result))

Python

The __str__ Method

What will be the output?

What will be the output?

What will be the output?

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What will be the output?

The str Method