Calculating total Price of groceries with execptions in Python

You can define try/except structures like below.

user_input = input("Enter grocery names: ")
user_input = user_input.split(',')
my_dictionary={"Potato":12, "Onion":16, "Ginger":15, "Garlic":12, "Tomato":15}
total_price = 0

try:
    for items in user_input:
        total_price = total_price + my_dictionary[items]
    print(total_price)
except:
    print("Some groceries are not avaiable in the dictionary.")

If there's no key in my_dictionary, it makes an exception so it will print your error message.

Here's the result.

Enter grocery names: Potato,Onion,Ginger
43

Enter grocery names: Potato,Onion,Ginger,Beef
Some groceries are not avaiable in the dictionary.

Your approach is very close, but the issue is that the following line prevents your try-block from raising an exception, so you never trigger the except block since the if statement causes missing groceries to be skipped, without raising an exception.

if items in my_dictionary.keys():

There are two ways you can alter to get to your desired behavior.

1 Removing 'if statement' for try block

You actally also need to check for 'KeyError' since this is what will be raised when getting value for a non-existent dictionary key.

def get_groceries_price(user_input):
    groceries = user_input.split(',')
    my_dictionary={"Potato":12, "Onion":16, "Ginger":15, "Garlic":12, "Tomato":15}
    total_price = 0
    try:
        for items in groceries:
            total_price = total_price + my_dictionary[items]    
        print(total_price)

    except KeyError:
        for items in user_input:
            if items not in my_dictionary:
                print("Some groceries are not avaiable in the dictionary.")
                break

get_grocery_price_1('Potato,Onion,Ginger')
# 42
get_grocery_price_1('Potato,Onion,Ginger,Beef')
# 'Some groceries are not avaiable in the dictionary.'

Here in the case when 'Beef' is not found in the groceries dictionary, a KeyError is raised, which is caught by your except block, printing your message.

2 Moving 'groceries not avialible' message into else statement

def get_groceries_price_2(user_input):
    groceries = user_input.split(',')
    my_dictionary={"Potato":12, "Onion":16, "Ginger":15, "Garlic":12, "Tomato":15}
    total_price = 0
    all_groceries_flag = True
    for item in groceries:
        if item in my_dictionary.keys():
            total_price = total_price + my_dictionary[item]
        else:
            print("Some groceries are not avaiable in the dictionary.")
            all_groceries_flag = False
            break
    if all_groceries_flag:
        print(total_price)
get_grocery_price_2('Potato,Onion,Ginger')
# 42
get_grocery_price_2('Potato,Onion,Ginger,Beef')
# 'Some groceries are not avaiable in the dictionary.'

Here when a grocery is not a key of 'my_dictionary', a flag is changed,and the code breaks out of the for-loop adding up grocery prices.

So in summary, you need to use either if/else, or try/except logic to achieve your desired behaviour, but not both.

Also regarding your question of checking whether total_price is initialised, this can also be done using try/except or if/else.

Try except

try:
    total_price
    # rest of groceries logic
except NameError:
    print('You do not have total_price variable')

Referencing an undefined variable will trigger a NameError, which you can catch in your except-block

If else

if 'total_price' in locals():
    # rest of groceries logic
    pass
else:
    print('You do not have total_price variable')

Locally defined variables are stored in locals(), you can check if 'total_price' is defined in here.

You are defining if items in user_input, but not else part of it. In else part, you can manually raise exception:

for items in user_input:
    if items in my_dictionary.keys():
        total_price = total_price + my_dictionary[items]
     else:
        raise ValueError
print(total_price)