using try/except & while loop for calculator in python

Question

I am trying to create a calculator and use defensive programming to avoid unexpected events and user inputs.

I included a try/except block for user input & for zero division.

When I input the division operator to calculate a zero division, my loop is not taking me back to input the number. It's taking me back to input the operator.

I also, struggle to re`loop the calculator for the user to start a new calculation.

Many thanks for the help.

while True:

    try:
        
        num1 = float(input("Please enter a number:"))
        num2 = float(input("Please enter another number: "))
        break
        
    except ValueError:
        print("Wrong key entered. Please key in a number")

operator = input("Please enter the operator (+, -, *, /): ")        
while True: 
        
    if operator == "+":
        calculation = num1 + num2
        entry = ("{} + {} = {}".format(num1,num2,calculation))  
        print(entry) 
        
    elif operator == "-":
        calculation = num1 - num2
        entry = ("{} - {} = {}".format(num1,num2,calculation)) 
        print(entry)
        
    elif operator == "*":
        calculation = num1 * num2
        entry = ("{} * {} = {}".format(num1,num2, calculation)) 
        print(entry)   
        
    elif operator == "/":
        try:    
            calculation = num1 / num2
            entry = ("{} / {} = {}".format(num1, num2, calculation))
            print(entry)
            break
            
        except ZeroDivisionError:
            print("You cannot divide by zero. Please try again!") 
    
    choice = input("Continue - Y / No - N: ")
    if choice == "N":
            break
    else:   
        print("You have not entered a valid operator, please try again.")

Answer 1

Put the calculation code inside your first while True: loop, not a separate loop. Then you can go back to the prompts for the numbers when there's an arithmetic error or the user says to continue.

while True:
    try:
        num1 = float(input("Please enter a number:"))
        num2 = float(input("Please enter another number: "))

        operator = input("Please enter the operator (+, -, *, /): ")        

        if operator == "+":
            calculation = num1 + num2
            entry = ("{} + {} = {}".format(num1,num2,calculation))  
            print(entry) 

        elif operator == "-":
            calculation = num1 - num2
            entry = ("{} - {} = {}".format(num1,num2,calculation)) 
            print(entry)

        elif operator == "*":
            calculation = num1 * num2
            entry = ("{} * {} = {}".format(num1,num2, calculation)) 
            print(entry)   

        elif operator == "/":
            calculation = num1 / num2
            entry = ("{} / {} = {}".format(num1, num2, calculation))
            print(entry)

        else:   
            print("You have not entered a valid operator, please try again.")
            continue

        choice = input("Continue - Y / No - N: ")
        if choice == "N":
            break

    except ValueError:
        print("Wrong key entered. Please key in a number")

    except ZeroDivisionError:
        print("You cannot divide by zero. Please try again!") 

Answer 2

If you want to loop the whole calculator and be able to control the whole process, you can move operations to methods and add them into one while loop

def choose_numbers():
    try:
        num1 = float(input("Please enter a number:"))
        num2 = float(input("Please enter another number: "))
        return {"num1":num1,"num2":num2}
    except ValueError:
        print("Wrong key entered. Please key in a number")

def choose_operator(num1, num2):

    operator = input("Please enter the operator (+, -, *, /): ")

    if operator == "+":
        calculation = num1 + num2
        entry = ("{} + {} = {}".format(num1,num2,calculation))
        print(entry)

    elif operator == "-":
        calculation = num1 - num2
        entry = ("{} - {} = {}".format(num1,num2,calculation))
        print(entry)

    elif operator == "*":
        calculation = num1 * num2
        entry = ("{} * {} = {}".format(num1,num2, calculation))
        print(entry)

    elif operator == "/":
        calculation = num1 / num2
        entry = ("{} / {} = {}".format(num1, num2, calculation))
        print(entry)




if __name__ == '__main__':
    while True:
        number = choose_numbers()
        try:
            choose_operator(number["num1"], number["num2"])
        except ZeroDivisionError:
            print("You cannot divide by zero. Please try again!")

Answer 3

I would say that you're missing an external while to maintain the loop until the program reaches the answer N for the choice.

Additionally, you could segregate the tasks in your code using functions, to turn it more understandable and maintainable. Something like that, at main flow:

while True
    get_numbers()
    get_operation()
    do_calculation()
    get_choice()

At get_numbers() accept the numbers with first while loop for validation.

At get_operation() accept the operation. I suggest implement a similar while to validate the operations allowed.

At do_calculation() maintain just the if/elif structure, with the try for preventing division by zero error (while doesn't aply here)

At get_choice() accept the decision to continue with more calculations or quit, but remember to validate both options Y or N, and maybe convert input to upper. In the same way, a while is welcome to guarantees that user enter a valid choice.