(Python) Basic way on how to end code if incorrect user input?

Question

this is my first time trying code. I viewed a basics on the foundations of python and I decided to start with something simple. I'm making a Fahrenheit to Celsius calculator and vise versa. As you can see in the code below, I want the program to stop running if the user inputs the incorrect variable at the two times they are asked for user input. I used an if, elif, and else statement if the user doesn't display an A or B, however, anything that I tried handling the integer asked always ends the code in a ValueError. Basically, if the user inputs a letter instead of a number I want the code to say "please try again!"

```
# printing all conversion options
print("Conversion Options")
print("A. Celcius to Fahrenheit")
print("B. Fahrenheit to Celcius")

# Selecting the options
option = input("Select Option A or B: ")

# Value of Degree in conversion
value = float(input("Enter Degree Amount: "))

# Conversion and option
if option == 'A':
new_value = (value * 1.8) + 32

elif option == 'B':
    new_value = (value - 32) * 5/9

else:
    print("You did not select A or B. Try again!")
exit()

# Enjoy your results
print("Conversion is now complete!")
print(new_value,"degrees")
```

Side note: your use case seems better fit for `sys.exit` (requires `import sys` line) rather than `exit` see further discussion https://stackoverflow.com/questions/6501121/difference-between-exit-and-sys-exit-in-python — Daweo, May 26 '22 at 07:58

score 4 · Answer 1 · answered May 26 '22 at 07:50

Basically, if the user inputs a letter instead of a number I want the code to say "please try again!"

The best way to do this is to isolate that in a function:

def ask_user(prompt, verify, convert, reprompt="please try again!", limit=5):
    value = input(prompt)
    if verify(value):
        return convert(value)
    for i in range(limit):
        print(reprompt)
        value = input(prompt)
        if verify(value):
            return convert(value)
    raise RuntimeError('Exceeded input attempt limit')

Really just a small suggestion for improvement: Replace the iteration variable `i` with an underscore since it is not used. — colidyre, May 26 '22 at 08:24

testfile · Accepted Answer · 2022-05-26T08:13:20.607

You need input validation. I've gone with the try except method of doing this but you can also capture the input first and then check if its a numbers instead of relying on the exception. I've also added validation to option so that users don't have to input a number if the option is wrong from the start.

# printing all conversion options
print("Conversion Options")
print("A. Celcius to Fahrenheit")
print("B. Fahrenheit to Celcius")

# Selecting the options
option = input("Select Option A or B: ")
if option not in ['A', 'B']:
    print('Not a valid option. Try again')
    exit(1)

# Value of Degree in conversion
try:
    value = float(input("Enter Degree Amount: "))
except ValueError as e:
    print('Not a valid number. Try again')
    exit(1)

# Conversion and option
if option == 'A':
    new_value = (value * 1.8) + 32
elif option == 'B':
    new_value = (value - 32) * 5/9
else:
    print("You did not select A or B. Try again!")
    exit()

# Enjoy your results
print("Conversion is now complete!")
print(new_value, "degrees")

Some questions in the comments:

ValueError as e: assigns the exception to the variable e. We can then handle the exception and do some logging or do something else with the exception:

# Value of Degree in conversion
try:
    value = float(input("Enter Degree Amount: "))
except ValueError as e:
    print('Exception message: ')
    print(e)
    print('Not a valid number. Try again')
    exit(1)

output:

Enter Degree Amount: t
Exception message: 
could not convert string to float: 't'
Not a valid number. Try again

I like the validation for the option, thank you. Can you clarify on why the first two exits contain a 1 in the parenthesis or what 'as e' stands for? — Yuki mB, May 26 '22 at 07:59
the param to `exit()` is the return code of the application. exit code 0 is success. As in everything worked fine. exit code that is non 0 means something broke. Its just a nice way to show the running app that something broke. In this case its because the input was invalid. — testfile, May 26 '22 at 08:08
the `except ValueError as e:` means catch the exception of type `ValueError` and assign the exception to the variable e. Ive updated the answer with some info. — testfile, May 26 '22 at 08:13

(Python) Basic way on how to end code if incorrect user input?

2 Answers2