Simple BMI calculator in python but while loop does not execute for incorrect input

Question

# Welcoming message
print('Welcome to the bmi calculator')
user = str(input("What's your name?:"))
print('\n')
# Declaring unit as empty
unit = ''
while not unit:
    unit = str(input("Chose 1 for imperial and 2 for metric units(1/2):"))
    if unit in ("12"):
        if unit == "1":
            print('You have chosen imperial units')
            heighti = float(input('Please enter your height in inches:'))
            weighti = float(input('Now please enter your weight in lbs:'))
            bmi = ((weighti / (heighti**2) * 703))
        elif unit == "2":
            print('You have chosen metric units')
            heightm = float(input('Please enter your height in metres:'))
            weightm = float(input('Please enter your weight in kg:'))
            bmi = (weightm / (heightm**2))
        else:
            unit = ''

        print(f'Your BMI is {bmi}')
        print('\n')
        print('your bmi results show that you are:')
        if bmi > 24:
            print('Overweight')
        elif bmi < 19:
            print('Underweight')
        else:
            print('In The ideal range for human')
    else:
        pass
else:
    print('Sorry,invalid choice,must be either 1 for imperial or 2 for metric')

Project runs smoothly once 1 or 2 is typed by the user, however if an input like 3 is put, the error is:

Sorry,invalid choice,must be either 1 for imperial or 2 for metric

***Repl Closed***

I have edited the code such that the 3rd if statement encapsulates the rest of the code. My situation is that the program works well if 1 or 2 is inputted by the user, however does not start over to ask for user's input if the first input is not 1 or 2.

Answer 1

Your IF statement if unit in ("12"): needs an additional else statement. It will accept 1 or 2 but without an else case to catch numbers outside of that scope, you're code wont move to the nested if statements.

ie:

while not unit:
    unit = str(input("Chose 1 for imperial and 2 for metric units(1/2):"))
    if unit in ("12"):
        if unit == "1":
            print('You have chosen imperial units')
            heighti = float(input('Please enter your height in inches:'))
            weighti = float(input('Now please enter your weight in lbs:'))
            bmi = ((weighti / (heighti**2) * 703), 5)
        elif unit == "2":
            print('You have chosen metric units')
            heightm = float(input('Please enter your height in metres:'))
            weightm = float(input('Please enter your weight in kg:'))
            bmi = (weightm / (heightm**2), 2)
        else:
            unit = ''
        print(f'Your BMI is {bmi}')
        print('\n')
        print('your bmi results show that you are:')
        if bmi > 24:
                  print('Overweight')
        elif bmi < 19:
                  print('Underweight')
        else:
            print('In The ideal range for human')

   else: 
        print('Please choose either 1 for imperial or 2 for metric')