Unable to access parent class' global variable with Child class' object

Question

I want to update the counter of available cars whenever someone rents the car. When I try to access this variable through child class, it shows an error as "UnboundLocalError: local variable 'available_cars' referenced before assignment"

PS: Still working on it, that's why I haven't completed all the method codes.

Parent Class

available_cars = 1000

class Car_rental():

def __init__(self):
    pass

def display_available_cars(self):
    print("Total avaialable cars for renting is:", available_cars)

def rent_hourly(self, cars_rented):
    print("Our hourly rate is $100/hr.")

    if cars_rented > available_cars:
        print("Sorry! We currently do not have the number of cars requested. You can have {} cars for now if you want.".format(
            available_cars))
    elif cars_rented < available_cars:
        print("Thank you for renting {} cars from Swift car renting portal. Enjoy you ride.".format(
            cars_rented))

    elif cars_rented < 0:
        print ("Please provide a valid number of cars.")

def rent_weekly(self):
    pass

def rent_monthly(self):
    pass

def bill(self):
    pass

def update_invetory(self, cars_rented):
    available_cars = available_cars - cars_rented

Child Class

from carRental import *

class Customer(Car_rental): def init(self): Car_rental.init(self)

def rent_cars(self):

    mode = int(input(
        "Please select the mode of renting the car:\n1. Hourly\n2. Weekly\n3. Monthly\n"))
    if mode == 1 or mode == 'hourly' or mode == 'Hourly':
        cars_rented = int(input("How many cars do you wish to rent?"))
        self.rent_hourly(cars_rented)
        self.update_invetory(cars_rented)

    elif mode == 2 or mode == 'weekly' or mode == 'Weekly':
        self.rent_weekly()
    elif mode == 3 or mode == 'monthly' or mode == 'Monthly':
        self.rent_monthly()
    else:
        print("Please provide appropriate input.")

def return_cars(self):
    pass

Answer 1

So if I follow your question correctly, you seem to have objectives with your code.

1. You want to define a Car_Rental class with a class-level variable available_cars which is initially set to 1000, and then as customers rent vehicles decrement the number of available cars.

2. You also want a second Customer class which inherits the Car_Rental class which implements the billing functionality.

3. While I am unsure of this, it seems that you would also like each of these class definitions to reside in their own separate python script file.

This is how I would address these issues.

# If desired could be placed in separate file Car_Rental.py
class Car_Rental():
    available_cars = 1000

    def display_available_cars(self):
        print("Total avaialable cars for renting is:", Car_Rental.available_cars)

    def isOkayToRent(self, nmbr):
        return nmbr <= Car_Rental.available_cars
    
    def rent_hourly(self, cars_rented):
        print("Our hourly rate is $100/hr.")

        if cars_rented < 0:
            print ("Please provide a valid number of cars.")
            
        else:
            if self.isOkayToRent(cars_rented):
                print("Thank you for renting {} cars from Swift car renting portal. Enjoy you ride.".format(
                cars_rented))
            else:
                print("Sorry! We currently do not have the number of cars requested. You can have {} cars for now if you want.".format(
                Car_Rental.available_cars))
                
    def rent_weekly(self):
        pass

    def rent_monthly(self):
        pass

    def bill(self):
        pass

    def update_inventory(self, nmbr):                       
        assert nmbr > 0
        Car_Rental.available_cars -= nmbr
        
    @property
    def cars(self):
        return Car_Rental.available_cars

# Can be placed in  second file. 

#Note include this next line if classes are stored in separate files 
#from Car_rental import Car_Rental 


class Customer(Car_Rental): 
    def __init__(self):
        Car_Rental.__init__(self)

    def rent_cars(self):

        mode = int(input(
            "Please select the mode of renting the car:\n1. Hourly\n2. Weekly\n3. Monthly\n"))
        if mode == 1:
            cars_rented = int(input("How many cars do you wish to rent?"))
            self.rent_hourly(cars_rented)
            self.update_inventory(cars_rented)

        elif mode == 2:
            self.rent_weekly()
        elif mode == 3:
            self.rent_monthly()
        else:
            print("Please provide appropriate input.")

    def return_cars(self):
        pass  

To use:

c1 = Customer()
c1.cars  

1000

c1.rent_cars()  
Please select the mode of renting the car:
1. Hourly
2. Weekly
3. Monthly
How many cars do you wish to rent? 2
Our hourly rate is $100/hr.
Thank you for renting 2 cars from Swift car renting portal. Enjoy you ride.  

To illustrate proper inventory reduction: c1.cars yields 998