Input validation loop in Python

Users type unexpected things: words, decimals, empty lines, numbers that are too big. This program keeps asking until the answer is a whole number from 1 to 10, and it explains each mistake.

The program

def ask_whole_number(prompt, low, high):
    while True:
        text = input(prompt).strip()
        if text == "":
            print("Please type something.")
            continue
        try:
            number = int(text)
        except ValueError:
            print(f"{text!r} is not a whole number.")
            continue
        if low <= number <= high:
            return number
        print(f"Please choose a number from {low} to {high}.")


rating = ask_whole_number("Rate the film from 1 to 10: ", 1, 10)
print(f"Thank you. You gave it {rating} out of 10.")

Typed in this example: (just Enter), seven, 7.5, 15, -3, 8. Press Run to type your own.

Rate the film from 1 to 10: 
Please type something.
Rate the film from 1 to 10: seven
'seven' is not a whole number.
Rate the film from 1 to 10: 7.5
'7.5' is not a whole number.
Rate the film from 1 to 10: 15
Please choose a number from 1 to 10.
Rate the film from 1 to 10: -3
Please choose a number from 1 to 10.
Rate the film from 1 to 10: 8
Thank you. You gave it 8 out of 10.

How it works

• while True repeats until a return ends the function. return only runs when the number passes both checks.
• int(text) raises a ValueError for seven and for 7.5, because int() only accepts text that looks like a whole number.
• low <= number <= high is a chained comparison. It means low <= number and number <= high.
• .strip() removes spaces around the answer. Without it, an answer of only spaces would pass the empty check and then fail in int() with a less helpful message.
• {text!r} shows the text with quotes, so the user can see exactly what they typed.

Because the checks are inside a function, you can use them for any question, for example age = ask_whole_number("Age: ", 0, 120).

The program does not trust the input at any point. This is the habit to keep for all user input, and also for data from files and websites. The number guessing game uses the same pattern inside a game loop.