How to Validate User Input in Python

Validating user input is crucial for creating robust and reliable Python applications. It helps prevent errors, ensures data integrity, and improves the user experience.

This guide explores various techniques for validating user input, covering numerical and string data, as well as multiple validation conditions.

Basic User Input Validation with Loops

The core concept of user input validation involves using a loop to repeatedly prompt the user until valid input is received. We can use while loops combined with if/else statements and try/except blocks to achieve this.

1. Validating Integer Input

Here's how to validate integer user input to ensure it's within a specific range:

num = 0
while True:
    try:
        num = int(input("Enter an integer 1-10: "))
    except ValueError:
        print("Please enter a valid integer 1-10")
        continue
    if 1 <= num <= 10:
        print(f'You entered: {num}')
        break
    else:
        print('The integer must be in the range 1-10')

Explanation:

• The while True creates an infinite loop that will continue until explicitly broken.
• The try block attempts to convert the user's input to an integer using int().
• If the user enters a non-numeric string, a ValueError is raised, which is caught by the except block and prints a helpful error message. The continue statement restarts the loop, prompting the user again.
• The if 1 <= num <= 10 checks if the converted integer falls within the range of 1 to 10 (inclusive). If it does, a confirmation message is printed and the break statement exits the loop.
• If the integer is not within the specified range, the else statement runs, providing an error message, and the loop continues.

2. Validating String Input

Here's how to validate string input based on length:

password = ''
while True:
    password = input('Enter your password: ')
    if len(password) < 5:
        print('Password too short')
        continue
    else:
        print(f'You entered {password}')
        break
print(password)

Explanation:

• The loop continues until the user enters a password of at least 5 characters.
• If the password length is less than 5, an error message is displayed, and the continue statement goes back to the beginning of the loop for the next iteration.
• If the length is 5 or greater, a confirmation message is printed, and the break statement exits the loop.

Validating with Multiple Conditions (OR)

You can use the boolean or operator to check if at least one of multiple conditions is met. Here's how to validate a password that is either too short or is a commonly used password:

password = ''
common_passwords = ['abcde', 'asdfg']
while True:
    password = input('Enter your password: ')
    if len(password) < 5 or password in common_passwords:
        print('Pick a strong password')
        continue
    else:
        print(f'You entered {password}')
        break
print(password)

Explanation:

• If the password length is less than 5 or if the password is present in the list of common_passwords using the in operator, the if statement condition evaluates to True, and an error message is printed.
• The continue statement then skips the rest of the code block and continues with the next iteration of the while loop.

Validating with Multiple Conditions (AND)

You can use the boolean and operator to check if all conditions are met. Here, we require both conditions be true, the password length has to be greater than 5 characters, and the password can not be a commonly used password.

password = ''
common_passwords = ['abcde', 'asdfg']
while True:
    password = input('Enter your password: ')
    if len(password) > 5 and password not in common_passwords:
        print(f'You entered {password}')
        break
    else:
        print('Pick a strong password')
        continue
print(password)

Explanation:

• The if statement uses the and keyword, so both conditions have to evaluate to True for the code in the if block to execute. The password has to have more than 5 characters, and has to not be in common_passwords using the not in operator.

Accepting Input Until Enter is Pressed

Sometimes, you need to take input until the user presses Enter without providing any text. This is often used for collecting a variable number of inputs from the user.

1. String Input

Here's how to collect string input until an empty Enter is pressed:

my_list = []
while True:
    user_input = input('Enter a string: ')
    if user_input == '':
        print('User pressed enter')
        break
    my_list.append(user_input)
print(my_list)

Explanation:

• The while True loop keeps running until the break statement is executed.
• If the user types in any text and then presses enter, the string will be stored in the user_input variable, otherwise it will be an empty string.
• If the user_input is an empty string (meaning that the user pressed enter without typing any text), then break is used to exit the loop, thus stopping the collection of input.
• If a non-empty string was typed in, it is appended to the my_list list using append().

2. Integer Input

Here's how to collect integer input until an empty Enter is pressed, handling ValueError exceptions:

my_list = []
while True:
    user_input = input('Enter a number: ')
    if user_input == '':
        print('User pressed enter')
        break
    try:
        my_list.append(int(user_input))
    except ValueError:
        print('Invalid number.')
        continue
print(my_list)

Explanation:

• This code is similar to the previous example, but it tries to convert each input to an integer.
• The try/except block handles the ValueError exception, if the user types in a non-numeric character.