Simple BMI calculator in python but while loop does not execute for incorrect input - python-3.8

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')