Python script that checks if a given number is prime and handles invalid input?

[avagoji]

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How do you create a Python script that checks if a given number is prime and handles invalid input?

[queenofcorgis]

Thanks for posting in the GitHub Community, @avagoji and apologies for the delay! I was reviewing our backlog and your discussion was in the wrong category.

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[shakibbinkabir]

Here is an example code with comments. If you don't understand any part, let me know.

import math

def is_prime(number):
    """
    Checks if a given number is a prime number.

    A prime number is a natural number greater than 1 that has no positive
    divisors other than 1 and itself.

    Args:
        number (int): The integer to be checked.

    Returns:
        bool: True if the number is prime, False otherwise.
    """
    # Prime numbers are greater than 1.
    if number < 2:
        return False
    # 2 is the only even prime number.
    if number == 2:
        return True
    # All other even numbers are not prime.
    if number % 2 == 0:
        return False

    # Check for divisibility from 3 up to the square root of the number,
    # only checking odd numbers to optimize.
    # We only need to check up to the square root because if a number `n`
    # has a divisor `d > sqrt(n)`, then it must also have a divisor
    # `n/d < sqrt(n)`.
    for i in range(3, int(math.sqrt(number)) + 1, 2):
        if number % i == 0:
            return False
    return True

def main():
    """
    Main function to get user input, validate it, and check for primality.
    Allows the user to check multiple numbers until they choose to exit.
    """
    print("--- Prime Number Checker ---")
    while True:
        user_input = input("Enter an integer (or type 'exit' to quit): ").strip()

        if user_input.lower() == 'exit':
            print("Exiting Prime Number Checker. Goodbye!")
            break

        try:
            num = int(user_input)
            if is_prime(num):
                print(f"'{num}' is a prime number.")
            else:
                print(f"'{num}' is not a prime number.")
        except ValueError:
            # Catch errors if the input cannot be converted to an integer.
            print(f"Invalid input: '{user_input}'. Please enter a valid integer.")
        except Exception as e:
            # Catch any other unexpected errors.
            print(f"An unexpected error occurred: {e}")
        print("-" * 30) # Separator for better readability

if __name__ == "__main__":
    # Ensures that main() is called only when the script is executed directly,
    # not when imported as a module.
    main()

[hasnaintypes]

Here’s a simple script I used when I was learning:

def is_prime(n):
    if not isinstance(n, int) or n < 2:
        return False
    for i in range(2, int(n**0.5)+1):
        if n % i == 0:
            return False
    return True

try:
    num = int(input("Enter a number: "))
    print("Prime!" if is_prime(num) else "Not prime.")
except ValueError:
    print("Please enter a valid integer.")

It checks for invalid input and handles edge cases too. Clean and works well for small numbers.