While loop still continues

When the user inputs anything but a number for the first input, the program should quit. But instead it just keeps going.

I tried to put many if … break statements in the main function after the "a" to break it, but nothing helped.

def is_number(str):
    try:
        float(str)
        return True
    except ValueError:
        return False
       

def is_valid_operator(operator):
    operators = ('+', '-', '*', '/')
    return operator in operators



def ask_for_a_number(force_valid_input):
    while force_valid_input is True:
        a = input('Please provide a number: ')
        if is_number(a):
            return float(a)
        else:
            if not force_valid_input:
                return None
            print("This doesn't look like a number, try again!")

def ask_for_an_operator(force_valid_input):
    while force_valid_input is True:
        operator = input('Please provide an operator: ')
        if is_valid_operator(operator):
            return operator
        else:
            if not force_valid_input:
                return None
            print('Unknown Operator')



def calc(operator, a, b):
    if not is_number(a) or not is_valid_operator(operator) or not is_number(b):
        return None
    result = 0
    if operator == '+':
        result = a + b
    elif operator == '-':
        result = a - b
    elif operator == '*':
        result = a * b
    elif operator == '/':
        if b != 0:
            result = a / b
        else:
            print('Error: Divide by zero is not allowed')
            return None

    return result

def main():
    while True:
        a = ask_for_a_number(force_valid_input = True)
        operator = ask_for_an_operator(force_valid_input = True)
        b = ask_for_a_number(force_valid_input = True)
        result = calc(operator, a, b)
        if result is not None:
            print(f'The result is {result}')
    
    


if __name__ == '__main__':
    main()

>Solution :

You check whether a is a number or not, but then don’t do anything if it isn’t one. So check the value of a

a = ask_for_a_number(force_valid_input = True)
if not a:
    return