Bound to It

Objectives

Kratos, angry developer, wanted to track the count of how many instances are created out of the class "Car". This is the code he wrote:

count = 0 
         
         
class Car:
    def __init__(self, color):
        count += 1
        self.color = color
         
         
print(f"Before: {count}")
car = Car("Red")
print(f"After: {count}")

1. What's the problem with the code? What exception will it raise?
2. What would you suggest Kratos to fix it?

Solution

1. In Python a variable inside a function is local. So when we try to increment count inside __init__ it fails since count isn't assigned with a value. It is NOT the count defined above the class Car. The exception raised is UnboundLocalError. In other words, you have a local variable that isn't assigned with a variable and you try to access the value of that variable anyway.

2. Use class attributes:

class Car:
    count = 0

    def __init__(self, color):
        count += 1
        self.color = color
         
         
print(f"Before: {count}")
car = Car("Red")
print(f"After: {count}")

Another common solution is to use global this way

def __init__(self, color):
        global count
        count += 1
        self.color = color

But this is not recommended and it's better to avoid it.