Asking the user for input until they give a valid response

python

prompt-engineering

functions

validation

byNikita Barsukov·Jan 5, 2025

Here's a quick recipe! Use a while True loop combined with input(), and cook up some error handling using try-except. This will keep pestering the user until they dish out a correct input.

while True:
    try:
        num = int(input("Enter a number: ")) # Don't let them give you any ol' thing!
        print("You entered:", num) # They did good, give'em some feedback
        break  # All good, let's roll
    except ValueError: # Uh-oh. Houston, we have a problem.
        print("That's a no-no! Let's try again.") # More polite than "RTFM", right?

This is a nifty pattern for enforcing valid user input, only moving on when they finally get it right.

Function: The double-edged sword

Functions are like double-edged swords - they can be your best friend or worst nightmare. Use them wisely. Here, we encapsulate our validation logic into a reusable function and check it inside the while loop to ensure all user inputs meet the criteria.

def validate_age(user_input): # One 'Function Sword' coming right up!
    if user_input.isdigit() and int(user_input) > 0: # We don't want a Benjamin Button, do we?
        return True
    return False

while True:
    age_input = input("How far are you from being a fossil? Tell us your age: ")
    if validate_age(age_input):
        print("Congratulations! You're officially old!") # Who am I to judge? ;)
        break
    print("Sorry, we couldn't process your age. Are you sure you're from Earth?")

Taking control with maximum tries

Imagine having that friend who just won't stop talking. We don't want our application to be like that, right? That's why, like all polite conversations, we set a maximum tolerated limit for both parties' sanity.

max_attempts = 10 # Let's not turn this into an interrogation.
attempts = 0 # First time? Wish you all the best buddy.

while attempts < max_attempts:
    # Whatever You do, "I am your Father" is not a valid input.
    # Just like a password Strength meter, huh?
    if attempts == max_attempts - 1: 
        print("One last chance. Muster your strength, young Padawan.")
    data = input("Enter the sacred text: ")
    if data == 'valid': # Replace 'valid' with your validation logic
        print("Bingo! The force is strong with this one.")
        break
    attempts += 1 # Count the sin.
    print("Oops! That wasn't the droids we were looking for. Try again.")
else:
    print("Our scanners indicate too many failed attempts. Cool down period activated.")

Advanced techniques

If you ask me, every programmer should know his tools like a Jedi knows his lightsaber. That's where libraries like Click come to the rescue. They offer a universe of ready-to-use validation and prompt utilities.

import click

number = click.prompt('Please enter a number', type=click.IntRange(0, 100))
password = click.prompt('Please enter your password', hide_input=True) # Move along, this is not the password you're looking for

Potential pitfalls

Each of these problems is as annoying as an Ewok, and twice as hard to shake off. Here's how to tackle them:

Infinite Loops: A simple break condition in your loop can avoid the infinite loop black hole.
Vague Prompts: A clearer prompt guarantees fewer SNAFUs.
Python Version Compatibility: A universe divided against itself cannot stand. Make sure your Python 3 code can run on Python 2.

# Sample Sudo code
data = input("Tell me your secret: ") # Don't worry, I'm a professional!

from __future__ import braces

Wouldn't you like to prevent these issues before they make your life difficult?

Streamlining your prompt flow

A Jedi doesn't rely on one tool, he uses the Force. When dealing with complex inputs, mixing functools and itertools can offer a clear path through the mist.

from itertools import repeat

for attempts in repeat(None, 3):
    password = input("Tell me your password, you must: ")
    if password == "correct": # Use the force...
        print("Welcome, Master Jedi!")
        break
    print("Incorrect password, more training you need.")