无法添加到空白列表

Question

I am working on a program that can be used to add a given value say a car dealership with the cars it owns:

for example

car_storage = []

def add_cars(dealership, car):
        for items in car_storage:
            for values in items:
                if dealership in items:

                   #Adds car whether it exists or not in list
                   items.append(car)
                   return
                    
                #If the dealership does not exist it is created and added to the list
                else:

                    items.append([dealership, car])
                    return

add_cars("Manchester", "Mini")
add_cars("London", "Toyota")
add_cars("London", "BMW")
add_cars("London", "BMW")

#Desired output
[["Nottingham", "Audi"],["Manchester","Mini"],["London", "Lorry", "BMW", "BMW"]]

However, my code never adds to the list, where am I going wrong?

Answer 1

The issue with your code as @juanpa.arrivillaga said is that the before you add the first item the list car_dealership is empty so nothing in your function executes.

Your approach however is almost correct, Python supports else keyword with for loops. What it means is, if the for loop ran without breaking (no break statements) the else statement is called. For example

for i in range(1, 4):
    if i==2:
        break
else: # Not executed as there is a break
      # if we remove the break the else will run
    print("No Break")

Now similarly in your code. you can use the same concept.

def add_cars(dealership, car):
        for items in car_storage:
            for values in items:
                if dealership in items:

                   #Adds car whether it exists or not in list
                   items.append(car)
                   return
                    
        #If the dealership does not exist it is created and added to the list
        else:
            car_storage.append([dealership, car])

Which is very similar to your original code except the indentation level on the else part.

Now in this case since you're returning and not breaking you can even get away with removing the else entirely since the function will never reach there

def add_cars(dealership, car):
        for items in car_storage:
            for values in items:
                if dealership in items:

                   #Adds car whether it exists or not in list
                   items.append(car)
                   return
                    
        #If the dealership does not exist it is created and added to the list
        car_storage.append([dealership, car])